Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing (Advanced Topics in Science and Technology in China, 69) 9819937752, 9789819937752

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Advanced Topics in Science and Technology in China

Yong Yu

Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing

Advanced Topics in Science and Technology in China Volume 69

Zhejiang University is one of the leading universities in China. In Advanced Topics in Science and Technology in China, Zhejiang University Press and Springer jointly publish monographs by Chinese scholars and professors, as well as invited authors and editors from abroad who are outstanding experts and scholars in their fields. This series will be of interest to researchers, lecturers, and graduate students alike. Advanced Topics in Science and Technology in China aims to present the latest and most cutting-edge theories, techniques, and methodologies in various research areas in China. It covers all disciplines in the fields of natural science and technology, including but not limited to, computer science, materials science, the life sciences, engineering, environmental sciences, mathematics, and physics. This book series is indexed by both the Scopus and Compendex databases. If you are interested in publishing your book in the series, please contact Violetta Xu (Email: [email protected]) and Mengchu Huang (Email: mengchu. [email protected]).

Yong Yu

Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing

Yong Yu College of Biosystems Engineering and Food Science Zhejiang University Hangzhou, Zhejiang, China

ISSN 1995-6819 ISSN 1995-6827 (electronic) Advanced Topics in Science and Technology in China ISBN 978-981-99-3775-2 ISBN 978-981-99-3776-9 (eBook) https://doi.org/10.1007/978-981-99-3776-9 Jointly published with Zhejiang University Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Zhejiang University Press. © Zhejiang University Press 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Since its inception, ultra-high-pressure (UHP) technology has focused on the sterilization and passivation of enzymes in agricultural and food products as its core research and application area. Since 2009, the author of this book has been engaged in research on UHP processing technologies for agricultural products and foodstuffs as well as the industrialization and promulgation of research results. Additionally, following theoretical guidance, the author has achieved many results with practical application value. As the breadth and depth of the author’s research within the field of UHP technology continues to increase, so has the author’s understanding of the UHP technologies described in this book. The author believes that UHP technology is, in fact, the artificial creation of an UHP environment and that changes in this pressure environment can alter some of the physicochemical properties of the treated objects in comparison to when they were exposed to the atmospheric environment while retaining their original basic characteristics. These changes in physicochemical properties can be reversible or irreversible. Some of these irreversible changes in the physicochemical properties of treated objects can potentially improve their quality. What researchers must do is clarify the quality improvement brought about by changes in irreversible physicochemical properties of different objects treated under an UHP environment and strengthen this improvement in quality through scientific research. After more than a decade of research, the author believes that traditional applications aimed at sterilization and enzyme passivation are not the most effective uses of UHP technology in agricultural and food production industries and that there are many more areas where UHP technology can be used to improve other quality aspects of agricultural and food products. The book begins with an overview of the basic principles of UHP technology. Nontraditional applications of UHP technology for agricultural products such as Chinese liquor, brown rice, and wood are then presented; these applications and the resulting quality improvement of the treated objects described are highly representative. Among the applications described, UHP treatment can have a significant effect on the main volatile flavor components of Chinese liquor. These changes and their trends are consistent, to a certain degree, with the natural aging process of Chinese liquor, which can be significantly accelerated via UHP treatment. The degree of v

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acceleration, however, is influenced by several UHP processing parameters and is limited by certain natural laws. In addition, UHP treatment can be used to improve the texture of brown rice, making it closer to that of fine white rice. However, this application is limited by the natural variation in brown rice. Further, UHP treatment can significantly inhibit the hydrolysis and oxidative rancidity of the lipids in brown rice during storage, which can, in turn, substantially improve the quality of brown rice during storage. In addition, UHP treatment has a significant effect on improving the mechanical strength and dimensional stability of wood; this effect can be utilized for the accelerated strengthening of plantation wood, thereby increasing the range of wood applications. Similarly, UHP treatment can quickly and significantly improve the liquid permeability of wood, which can be used to impregnate various substances, such as coloring agents, flavoring agents, and water-repellent, flame-retardant, insectrepellent, and antibacterial substances, into wood uniformly and at high speed, thus enhancing or improving various properties of the wood. Finally, the author presents his vision of the future trends in the nontraditional applications of UHP technology. In summary, this book aims to broaden the ideas for research on the application of UHP technology in the processing of agricultural products and to provide specific ideas for future research and applications in this field based on the author’s current research. Hangzhou, China

Yong Yu

Contents

1 Chinese Liquor and Related Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Overview of Chinese Liquor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Origin of Chinese Liquor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 General Situation of the Chinese Liquor Industry . . . . . . . . . 1.1.3 Chinese Liquor Aging Technology and Existing Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Status of Research on Aging Technology . . . . . . . . . . . . . . . . . . . . . . 1.2.1 High Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Electric Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Other Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Ultra-High-Pressure Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Overview of Ultra-High-Pressure Treatment . . . . . . . . . . . . . 1.3.2 Research on UHP in the Field of Wine Aging . . . . . . . . . . . . 2 Introduction to Brown Rice and Related Substances . . . . . . . . . . . . . . . 2.1 Introduction to Brown Rice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Nutritional Value of Brown Rice . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Physicochemical Properties of Brown Rice . . . . . . . . . . . . . . 2.1.3 Problems with Brown Rice . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 Opportunities and Challenges in the Development of Brown Rice Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Application of UHP Technology in Agricultural Products . . . . . . . . 2.2.1 Introduction to UHP Technology . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Application of UHP in Processed Food Crops . . . . . . . . . . . . 2.3 UHP Technology for Brown Rice Modification . . . . . . . . . . . . . . . . . 2.3.1 Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Physicochemical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Other Modification Techniques for Brown Rice . . . . . . . . . . . . . . . . . 2.4.1 Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 2 4 5 5 6 7 8 9 10 10 11 15 15 15 19 23 25 26 26 28 33 33 36 42 42

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2.4.2 Germination and Pregelatinization . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Freeze–Thaw Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Fast-Growing Forest and Related Introduction . . . . . . . . . . . . . . . . . . . . 3.1 Fast-Growing Forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Introduction to Fast-Growing Forest . . . . . . . . . . . . . . . . . . . . 3.1.2 The Main Challenges of Fast-Growing Forest Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Research Progress in Wood Modification Technology . . . . . . . . . . . . 3.2.1 Transverse Densification Modification Technology . . . . . . . . 3.2.2 Impregnation Modification Technology . . . . . . . . . . . . . . . . . 3.2.3 Heat Treatment Modification Technology . . . . . . . . . . . . . . . . 3.2.4 Composite Modification Technologies . . . . . . . . . . . . . . . . . . 3.3 UHP Modification of Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Densification Modification of Wood . . . . . . . . . . . . . . . . . . . . 3.3.2 Impregnation Modification of Wood . . . . . . . . . . . . . . . . . . . .

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4 Effect of UHP Processing on the Main Volatile Components and Aging Characteristics of Chinese Liquor . . . . . . . . . . . . . . . . . . . . . 67 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.1.1 Chinese Liquor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.1.2 Processing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.1.3 Liquor Age Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2 Changes in the Main Volatile Compounds of Chinese Liquor . . . . . 70 4.2.1 Chinese Liquor Samples and Chemicals . . . . . . . . . . . . . . . . . 70 4.2.2 Identification and Quantification Analysis . . . . . . . . . . . . . . . 71 4.2.3 Changes in Main Volatile Compounds During Natural Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4.2.4 Impact of Ultra-High-Pressure Treatments on Main Volatile Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4.2.5 Impact of Storage on Main Volatile Compounds . . . . . . . . . . 78 4.2.6 Principal Component Analysis Based on Seven Groups . . . . 80 4.3 Effect of the Ultra-High-Pressure Treatment on the Aging Characteristics of Chinese Liquor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4.3.1 Wine Samples and UHP Treatments . . . . . . . . . . . . . . . . . . . . 81 4.3.2 Electronic Nose Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4.3.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4.3.4 Gas Chromatography Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 89 4.3.5 Sensory Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 4.4 Quality Assessment of Chinese Liquor with Different Ages and Prediction Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Liquor Samples and Chemicals . . . . . . . . . . . . . . . . . . . . . . . . 93 4.4.2 Volatile Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.4.3 Electronic Nose Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.4.4 Prediction of Liquor Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

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4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 5 Improving Taste, Cooking Properties, and Digestibility of Brown Rice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Changes in Quality Characteristics of Brown Rice After UHP Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Cooking Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Moisture Sorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Rheological Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Color and Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Effect of UHP Processing on the Rancidity of Brown Rice During Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Fat Acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Conjugated Dienes Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Thiobarbituric Acid Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Physicochemical Properties of Brown Rice Treated Using Three Different Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Processing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Appearance Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3 Cooking Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Texture Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.5 Rheological Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.6 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.7 Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.8 Phytochemical Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.9 Antioxidant Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Effect of UHP Performance on the Properties of Rice Bran Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Preparation of Rice Bran Protein . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 UHP Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.3 Functional Properties of Rice Bran Protein . . . . . . . . . . . . . . . 5.5.4 Surface Hydrophobicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.5 Protein Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Ultra-High-Pressure Densification of Wood . . . . . . . . . . . . . . . . . . . . 6.2.1 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Moisture Sorption Propertie . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Dimensional Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 105 106 106 114 121 127 131 131 133 136 137 138 139 140 145 150 154 159 166 170 174 175 175 176 185 186 189 191 193 193 195 198 217 229 245

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6.3 Ultra-High-Pressure Dyeing of Wood . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Performance of Dyeing Treatments . . . . . . . . . . . . . . . . . . . . . 6.3.2 Fractal Color of Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3 UV Accelerated Aging Test . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.4 Dye Uptake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Color Distribution of Interior Wood . . . . . . . . . . . . . . . . . . . . . 6.3.6 EDX and SEM Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

254 255 257 259 260 262 263 264 265

7 Conclusion and Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

Chapter 1

Chinese Liquor and Related Introduction

Abstract Chinese liquor is a unique distilled spirit produced in China and aging is a necessary part of the production process of liquor. In this chapter, the origin of Chinese liquor, an overview of the industry, and aging processes and problems are first introduced, followed by the current status of research on high temperature, light and microwave technologies for aging liquor, and finally the research on high pressure treatment in the field of wine aging. Keywords Chinese liquor · Origin · Industry overview · Aging · High pressure treatment

1.1 Overview of Chinese Liquor Chinese liquor is a unique distilled spirit produced in China; it is considered one of the world’s six major distilled spirits together with whiskey, brandy, vodka, gin, and rum. Chinese liquor is produced from starch-rich grains, with Daqu, Xiaoqu or bran koji, and distiller’s yeast used as saccharification starters. It is produced through a series of processes such as cooking, saccharification, fermentation, and distillation. Chinese liquor is usually colorless or slightly yellow and transparent, with a pure and fragrant smell, a sweet and refreshing taste and feel in the mouth, and a compound aroma dominated by esters.

1.1.1 Origin of Chinese Liquor China was the birthplace of wine culture and one of the first countries in the world to master winemaking technology. Chinese wine culture has a long history, spanning thousands of years of Chinese civilization. The currently known archaeological record of wine can be traced back to the Jiahu site, which was excavated in the early

© Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_1

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1 Chinese Liquor and Related Introduction

1960s; the tartaric acid residues contained in pottery found at the site were subsequently identified as the earliest wine residues in the world. Studies have shown that 9,000 years ago, the Jiahu people had developed a winemaking process using rice, honey, and hawthorn as raw materials. This discovery was a platform for 4000 years of human social civilization and wine culture. In general, early wines were divided into two types: colored wine made directly from fruit and grains, such as fruit wine or rice wine, and distilled wine, namely Chinese liquor. The earliest written record of ancient wine was attributed to Yi Di and Du Kang. It is said in Lei Shu Zhuan Yao: “Yi Di created rice wine, Du Kang created sorghum wine.” Yi Di and Du Kang were from the Xia Dynasty; most of the wines at that time were made from fruits, flowers, and trees. Lu Zhanfan’s Yue Xi Ou Ji records that in the deep mountains of Pingle and other prefectures, there are many apes, and they are good at harvesting flowers and making wine. When the woodcutter enters the mountain and finds its nest, dozens of liters of wine can be produced. In the Shang Dynasty, following the development of agriculture, agricultural products became abundant, and although wine brewed using grains as raw materials gradually began to appear, the wine in that period was still mainly brewed with fruit and rice. Later, as human society developed further, the brewing process also improved further. The original processes of cooking, fermenting, and pressing gradually changed to cooking, fermenting, and distillation, which greatly improved the process of alcohol purification. Since then, distilled wine has officially entered the history of Chinese wine culture.

1.1.2 General Situation of the Chinese Liquor Industry The production process of liquor continues to be developed to the present day. Although the production process differs for liquor of different brands and flavors, it includes three main steps: material selection, fermentation, and distillation. In terms of raw material selection, ranging from grain sorghum, corn, and barley to sweet potato and cassava to sugar-containing raw materials such as sugar cane and sugar beet, all can be used for the brewing of liquor. Before fermentation, the raw materials usually need to undergo two processes: pulverization and cooking. The starch in the raw materials can be pregelatinized, making it susceptible to the full action of the subsequent amylase; moreover, the microorganisms in the raw materials can be killed. The fermentation processes can be classified into solid-state fermentation, semisolid-state fermentation, and liquid-state fermentation. The advantages of solidstate fermentation are that it is conducive to the inoculation of natural microorganisms, the enzyme activity is high, and the brewed liquor has a strong aroma; the disadvantages are that the operation process is complex, the yield is low, and the output is relatively unstable. The semisolid method uses solid-state culture of microorganisms and then adds water to liquid fermentation, which has certain advantages in terms of operating procedures and costs; however, the quality of liquor is usually not as good as that of liquor obtained via solid-state fermentation. In liquid-state fermentation,

1.1 Overview of Chinese Liquor

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the materials are in a liquid state from the early stage of microbial culture and saccharification to the later stage of fermentation. The operation is simple, the fermentation time is greatly shortened, and a considerable yield is achieved. The disadvantage is that the quality of liquor is relatively poor. After fermentation, the mature fermented grains obtained are called fragrant grains. Turbid impurities in the fragrant grains can be removed by distillation, and the alcohol, water, and volatile aroma components are evaporated in the form of gas, after which the liquor can be obtained by cooling. Regarding the composition of liquor, its main components are alcohol, water, and trace ingredients, of which alcohol and water account for ~98%, and the remaining 2% of trace ingredients determine its flavor, quality, and taste. These ingredients mainly include alcohols, esters, acids, aldehydes, ketones, aromatic compounds, nitrogen-containing compounds, and furan compounds. These trace components are usually referred to as the “volatile” or “aroma” components of liquor. Owing to differences in raw materials and production processes, the composition of the microcomponents of brewed liquor differ considerably; this results in the development of different flavors of liquor. Although the taste and aroma of liquor are inherent, there was no systematic sensory evaluation system for liquor in China for many years. It was not until the 1950s and 1960s that the liquor industry proposed the sensory evaluation standards of “scent, mellow, sweet, and clean” and “color, aroma, taste, and style” one after another. At the 3rd National Wine Evaluation Conference, Chinese researchers proposed and established for the first time four flavor types for liquor: delicate fragrance, strong fragrance, sauce-flavor fragrance, and rice-flavor fragrance. Later, after meticulous research by liquor production enterprises and related research institutes, liquor was further refined into 12 types of aromas, including delicate fragrance, strong fragrance, sauce-flavor fragrance, rice-flavor fragrance, double fragrance, medicinal fragrance, phoenix fragrance, special fragrance, sesame fragrance, laobaigan fragrance, fragrant fragrance, and fermented bean flavor. From the initial production in small workshops and small factories to the current mechanized production, today’s brewing industry encompasses many enterprises, represented by Maotai, Wuliangye, Luzhou Laojiao, etc., with the development of major production areas, as represented by Sichuan, Guizhou, and Shanxi. The wine industry has become a pillar of the food and drink industry, with wine consumption being well-established as a habit in daily life and culture. Studies have shown that some volatile components in liquor, such as esters and organic acids, can prevent cardiovascular disease to a certain extent, and moderate drinking can help accelerate blood circulation and promote human metabolism. According to data from the National Bureau of Statistics, Chinese liquor production has tripled from 3.78 million kiloliters in 2002 to 11.98 million kiloliters in 2017 while the sales revenue from the liquor industry increased from 49.6 billion yuan in 2002 to 565.4 billion yuan in 2017, an 11-fold increase. Simultaneously, varieties of liquor and output volume have increased. The brewing industry has gradually become an important source of national fiscal revenue and occupies a pivotal position in the national economy.

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1 Chinese Liquor and Related Introduction

1.1.3 Chinese Liquor Aging Technology and Existing Problems The liquor industry continues to develop to this day; although many technologies and processes are relatively mature and complete, there are still some shortcomings. One of which is the traditional aging process, which needs to be improved urgently. After fermentation and distillation, liquor is usually spicy, irritating, and tasteless. At this stage, the bitterness, astringency, spiciness, and strong mouthfeel constitute the primary taste of the new wine. The new wine usually needs to be stored for a certain period before the stimulation of the wine flavor disappears, the taste of liquor softens, and a long aftertaste develops. In the winemaking industry, the storage process to eliminate the pungent taste and increase the softness of the new wine is usually called aging. There have been some different theories about the mechanism of liquor aging. The “association theory” believes that the main reason for the irritating taste of liquor is the free ethanol in the liquor, and both ethanol and water are strong polar molecules. During the storage of liquor, the two exhibit strong association ability, thus forming a new association group, which greatly reduces the number of free ethanol molecules, thereby eliminating the irritating taste of new liquor. The “esterification theory” proposes that during the storage of liquor, many alcohols are first oxidized to aldehydes and then further oxidized to carboxylic acids. Finally, carboxylic acids and alcohols undergo esterification to form aromatic esters, thereby improving the quality and taste of liquor. Previous studies found that during the storage of Gujing tribute wine, the total ester content gradually increased and the total acid content gradually decreased; the esterification reaction increased the content of esters while consuming the organic acid. Researchers who support the “oxidation theory,” based on the increase in the content of acetaldehyde and acetal substances during the storage of new wine, propose that during storage, the gradual oxidation of ethanol to form aldehyde substances leads to the disappearance of the pungent taste of liquor. In contrast, the “volatile theory” is more intuitive, which proposes that during the storage of liquor, irritating sulfur-containing and nitrogen-containing substances with low boiling points continue to diffuse and volatilize, and the reduction in these irritating substances makes liquor soft and mellow; however, because sulfur-containing compounds are difficult to detect, this theory has received little support. In addition, after investigating the influence of different containers on the aging process of liquor, some researchers have proposed the “dissolution theory.” They believe that the density, air permeability, and water absorption of the container are important factors affecting the aging process of liquor. Moreover, the type and content of trace metal elements, such as copper and iron, in the raw materials used to make wine-making utensils may also affect the aging of liquor to varying degrees. In the traditional aging method, the new wine obtained by distillation is divided into clay pots, sealed, and placed in a cool, dark place. The aging period for highquality liquor varies from several months to several years. Lu-type wine is stored for at least 3–6 months, and usually more than 1 year. The storage period is approximately

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1 year for Fen-type wine and usually over 3 years for Mao-type wine. When using traditional aging process conditions, to expand production, manufacturers have to increase the number of storage containers and expand the area of the factory, which not only seriously affects the capital turnover but also greatly extends the production cycle of liquor. During storage, a volume loss of 3%–6% occurs every year. Therefore, there is an urgent need for the liquor industry to identify “rapid aging” technology to replace the traditional technology, thereby shortening the aging cycle and the cost of liquor production.

1.2 Status of Research on Aging Technology Although the mechanism of the natural aging process of liquor remains unclear, under the guidance of some theories and the theories mentioned above, researchers in the liquor industry have explored a series of liquor aging technologies to shorten the liquor aging cycle. Among them, physical catalysis is mainly used, alongside a small number of chemical and biological catalysis technologies.

1.2.1 High Temperature Based on the “volatile theory,” high temperature is more conducive to the volatilization of irritating sulfur-containing compounds and ammonia compounds with low boiling points, which should theoretically accelerate the aging process of liquor. Some researchers used two groups of controlled experiments to compare the effect of high temperature on the aging of liquor, and the experiment was performed in summer. In one group, white wine was aged under traditional conditions (low temperature and in the dark), and the temperature was approximately 18 °C. The other group of white wine was aged in the open air at temperatures ranging from 20–35 °C to more than 40 °C. The aging time for both groups was 90 days. After storage, the researchers conducted a sensory evaluation of the liquor and found that the naturally aged liquor had a new taste and a strong smell of distiller’s grains, whereas the liquor aged at high temperature in the open air had no “new wine” smell and had a slight smell of distiller’s grains, but the aftertaste was clean, sweet, and mellow; liquor aged for 3 months using this method was equivalent to liquor aged for more than half a year naturally. It was hypothesized that under the action of high temperature, the low-boiling-point raw wine-flavor substances in the wine volatilized faster. In addition, the higher temperature increased the collision opportunities between different molecules, which accelerated the progress of various oxidation, reduction, and association reactions, allowing the flavor substances in the liquor to rapidly reach a dynamic balance, thereby achieving the purpose of rapid aging. Other researchers conducted two experiments to explore the effect of temperature on wine aging. The test was divided into two groups of storage conditions: one at 55 °C

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and the other at room temperature (25 °C); the storage time was 1–2 months. The two experiments produced consistent results: in a short period, high-temperature storage was more conducive to aging than normal-temperature storage. In terms of changes in esters, the conversion rate of alcohol acid esters stored at high temperature was much faster than that of wine stored at room temperature; the effect of high-temperature storage for 1–2 months was equivalent to the effect of storage at room temperature for approximately 2 years. Under high-temperature storage conditions, the storage effect was not notably different between 30 and 60 days. After high-temperature storage, the liquor was kept for 7 months. There was no reversion phenomenon, and the sensory evaluation results showed that the taste of liquor was still close to, or even better than that of, liquor aged for 2 years at room temperature. However, the disadvantages of high-temperature storage were also obvious, with the loss of liquor body being more severe than that observed with normal-temperature storage.

1.2.2 Light Infrared equipment was used to treat yellow wine for a week, and the following changes were found: yellow wine before treatment was light yellow; after accelerated aging, the color deepened and turned brown. The wine before treatment had a faint mellow aroma, accompanied by some exotic aromas; the mellow aroma was more intense after aging and the exotic aroma disappeared. Before treatment, the taste of the wine was irritating, with a bitter taste; after aging, the taste was mellow, sweet, and sour. The overall style of the rice wine before and after aging was clear; after aging, it had a typical sweet yellow wine style. Infrared aging can be used to process a large batch of rice wine at a single time and shorten the aging period by approximately 1 year, and there was no phenomenon of taste rejuvenation in the subsequent evaluations. However, it should be noted that during the infrared treatment, the wine temperature reached 50 °C–60 °C; thus, it was difficult to ascertain whether the infrared treatment itself had an effect on the aging or whether the temperature change had an effect. Other researchers found that the content of acetic acid in wine after infrared aging essentially did not change; however, the content of ethyl acetate and ethyl caproate, which were the main aroma compounds, increased, exceeding the content of aroma compounds in the finished wine after natural aging, and this trend was more evident with an increase in the alcohol content. In addition, the sensory evaluation showed that the spicy and pungent taste of the processed wine disappeared, and the taste became rich and mellow, reaching a quality level equivalent to that of wine aged naturally for 1–3 years. The researchers believed that the frequency of energy required for organic molecules to generate fundamental vibration and rotational energy level transitions was in the mid-infrared wavelength range, which constituted the theoretical basis for the infrared aging of liquor. Some researchers believed that the spicy and pungent taste of newly brewed liquor was mainly attributable to aldehydes, particularly acetaldehyde and acetal. After laser treatment, the content of acids, esters, and higher alcohols in liquor increased

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significantly, especially that of ethyl butyrate and ethyl caproate, which play a crucial role in the aroma of the wine. The national liquor sensory assessor’s evaluation of the liquor after laser aging revealed that the treated liquor smelled good and had a strong cellar aroma. The taste was pure, the liquor was harmonious, and the aftertaste was fragrant. The aging effect exceeded that of natural aging of white wine for half a year. The researchers obtained similar results from the laser treatment of 110 liquor samples. The results of chromatographic analysis showed that after laser treatment, the content of the two main esters of high-quality Daqu wine, namely ethyl caproate and ethyl butyrate, increased, and the total ester content also displayed an increasing trend, with different methanol and acetaldehyde contents. Overall, the content of esters that play a role in the aroma of liquor increased and that of the harmful components represented by methanol and acetaldehyde were significantly reduced. Provincial liquor sensory assessors conducted a taste and evaluation of the liquor after laser aging, and the results of three rounds of evaluation showed that the aged liquor was better than the untreated liquor. The test personnel repeated the evaluation of the liquor samples after half a year, and the results were consistent with those previously obtained, with no obvious retrogradation found. Other researchers also observed that after laser treatment, the content of total esters in the liquor increased slightly, the content of alcohol, total aldehydes, and methanol decreased, and the liquor taste was soft, which was equivalent to the quality of liquor aged for approximately half a year. They believed that laser irradiation could promote the breaking of chemical bonds in organic molecules in liquor, such that macromolecular groups were broken into small molecules that existed alone, which was conducive to the mutual contact and reaction between different components, further promoting the oxidation and esterification of liquor. Ultraviolet light has high energy, and the Moutai distillery used ultraviolet light to directly irradiate liquor for accelerating aging. The results showed that the effect of short-term irradiation was better under low-temperature conditions, and the wine had an excessively oxidized odor when the irradiation time exceeded 20 min. This also showed that ultraviolet irradiation had the effect of accelerating the oxidation of volatile components in liquor.

1.2.3 Microwave Some researchers, using liquors provided by Deshan Daqu distillery and Changsha distillery as the research object, used homemade microwave equipment to microwave liquors of different quality, analyzed the influence of different processing conditions on the components of the liquors, and determined the optimal processing parameters; the mechanism of microwave aging was also analyzed and discussed. The study found that in the treated liquor, the alcohol and total acid contents were slightly decreased, total ester content was slightly increased, and other contents were not changed significantly. A possible reason speculated was that microwave treatment accelerated the esterification of ethanol and acid, which resulted in a decrease in

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the content of alcohol and acids and an increase in that of esters. Among the flavor components of liquor, the content and composition of esters are the key factors for the development of different flavors. Therefore, a change in esters is an important reason for the change in liquor flavor. The sensory evaluation results showed that after microwave treatment, the spicy taste of the Chinese liquor disappeared and the liquor tasted soft, equivalent to the Chinese liquor naturally aged for 3–4 months. These researchers believed that water and ethanol molecules in liquor were highly polar and could easily form associative molecular groups. Therefore, the probability of intermolecular collision increased, and esterification was further accelerated. Other researchers studied the aging effect of microwave treatment using yellow wine as the test object. They found that the optimal process conditions for microwave treatment were microwave energy of 87.5 J/s, treatment time of 3 min, and one cycle. The obtained yellow wine had a reducing sugar content of 24.07 g/L, a total acid content of 5.12 g/L, a total ester content of 3.89 g/L, and a pH of 4.18. It was found that the contents of main flavor substances, such as ethyl acetate, isoamyl acetate, and nbutanol, had increased and were generally comparable with those of substances found in yellow wine that had been naturally aged for 3 years. The testers did not perform a systematic sensory evaluation of the treated liquor, nor did they perform further storage to confirm whether there was a regeneration phenomenon. The researchers stated that the natural aging of yellow wine was a very complex process, including a series of esterification reactions, oxidation reactions, and intermolecular polymerization. Hundreds of molecules participated in the reaction, and the mechanism and energy requirements for each reaction were also different. Although the levels of some yellow wine indicators were comparable to those produced by wine naturally aged for several years using a single technology, it was difficult to improve the overall quality. Therefore, to apply a single technology in the actual production of wine aging, more comprehensive exploration and research on various technologies are necessary based on a better understanding of the mechanism of natural aging.

1.2.4 Electric Field At present, research on the effects of electric field on the aging of alcohol is only preliminary. Formerly, researchers believed that an electric field could promote the volatilization of some low-boiling-point compounds and gases in wine that affect the taste, which would eliminate the irritating taste of new wine. Additionally, an electric field could arrange the polar molecules in a certain direction, and simultaneously, the water and ethanol molecules in the wine can penetrate each other to form a large group of associated molecules. Consequently, there were fewer free ethanol molecules, which reduced the pungency of the wine. A study found that the content of acrolein, isoamyl alcohol, and fusel oil in the liquor after electric field treatment decreased, with the acrolein and fusel oil content being decreased by >20%, whereas the content of ethyl caproate, ethyl acetate, and total esters increased. Acrolein has a spicy taste, isoamyl alcohol has an astringent taste, and fusel oil has a pungent taste.

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After processing, the reduction in the content of these three substances improved the pungent taste of liquor. Ethyl hexanoate and ethyl acetate are the main aroma components of liquor, and the increase in these two components also plays an important role in improving the taste of liquor. High-voltage pulsed electric fields have been widely used in the field of food processing in recent years; they are usually used for sterilization or with passivation enzymes to maintain the maximum level of freshness of food. Some studies have also applied high-voltage pulsed electric fields in the field of wine aging. Some researchers used a high-voltage pulsed electric field to accelerate the aging of liquor. The research results showed that the energy provided by the pulsed electric field could accelerate the oxidation and esterification reactions in liquor, reduce the alcohol and odorous substances in the liquor, and simultaneously increase the acid and ester substances. The method is simple and easy to operate and can accelerate the aging of liquor in a few seconds to the same quality as that achieved through 6 years of aging. Other researchers performed a detailed and in-depth study on high-voltage pulsed electric fields to accelerate the aging of liquor. The research showed that the aging-enhancing effect increased significantly with the increase in the pulse number of the high-voltage pulsed electric field and that the content of total acid, total ester, and total aldehyde increased with the increase in the field strength; however, after the electric field strength exceeded 30 kV/cm, the content of total acid, total ester, and total aldehyde decreased with the increase in the field strength. An orthogonal test revealed that the most influential factor for the aging effect was the number of pulses, followed by the field strength, whereas the concentration of the auxiliary agent did not have a significant effect. Finally, the optimal processing parameters were determined to be 50 pulses, field strength of 25 kV/cm, and auxiliary agent concentration of 500 ppm. The total acid, total ester, and total aldehyde content in liquor after high-voltage pulse electric field treatment was between that of 1-year-old and 6-year-old liquor; the total ester content was higher than that of 6-year-old liquor and the fusel oil and methanol content decreased to be comparable to that of liquor that was naturally aged for 6–10 years. After the high-voltage pulsed electric field treatment, the liquor did not regenerate in the subsequent storage process.

1.2.5 Other Technologies Ozone is an unstable gas that is easily decomposed into dioxygen and one oxygen atom when it encounters water; the oxygen of a single atom has strong oxidizing properties. Some former researchers used an ozone generator to generate ozone through a high-frequency voltage for treating newly brewed liquor. The results showed that the total ester content of the liquor decreased by approximately 10% after treatment. The content of ethyl palmitate, ethyl oleate, and ethyl linoleate, the main substances causing turbidity in liquor, had decreased significantly, which could explain the clarity of the wine. The content of ethyl acetate, the main aroma substance of fragrant liquor, reduced, and that of ethyl lactate slightly increased, which may be related to

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the decrease in the new wine flavor of liquor. The increase in the content of phenethyl alcohol, diethyl succinate, dibutyric acid, ethoxyisopentane, and ethyl formate may be related to the increase in the aged flavor of liquor. Meanwhile, the content of the harmful substance methanol slightly decreased. Metal ions are used as catalysts in many reactions, including esterification. Some researchers fired Chenxi soil, Mongolia soil, and other materials rich in metal ions into wine aging materials in a kiln and immersed them in wine to accelerate oxidation and esterification reactions, achieving an aging effect equivalent to 1 year or even several years of storage in a relatively short period. Other researchers used containers with metal catalyst layers, such as nano-silicon carbide, aluminum oxide, and titanium oxide, on the inner wall for wine storage and found that the aging efficiency could be greatly improved. Biological ripening, first used for the production of brandy and whiskey, is characterized by selectivity and specificity, and domestic researchers have not performed much research in this field. Some former researchers extracted zymase and other enzymes from plants to obtain a biological ripening agent called YS-II. After treatment with YS-II for 15–30 days, the irritancy of the liquor was significantly reduced, its taste became soft, and there was no regeneration phenomenon; its quality was comparable to that of half-year-old liquor.

1.3 Ultra-High-Pressure Treatment 1.3.1 Overview of Ultra-High-Pressure Treatment Ultra-high-pressure (UHP) treatment refers to a treatment method in which the material is packaged and placed in an UHP chamber; then, the chamber is sealed using water or other fluids as the pressure transmission medium and processed for an appropriate time under a static pressure of 100–1000 MPa. UHP treatment can cause the destruction or formation of noncovalent bonds (e.g., hydrogen, ionic, and hydrophobic bonds), thereby inactivating, denaturing, and gelatinizing natural macromolecules such as enzymes, proteins, and starches and killing bacteria and other microorganisms, and is used for food sterilization, preservation, and processing. However, UHP does not destroy small molecules such as vitamins and pigments; therefore, it can maximize the original flavor and nutritional value of food during sterilization. In recent years, the advancements in related research have enabled the use of UHP treatment in the sterilization, preservation, and modification of food. UHP technology has also been studied in wine aging.

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1.3.2 Research on UHP in the Field of Wine Aging UHP treatment has little effect on the types of volatile components in rice wine but has a relatively obvious effect on the content of volatile components. After 15 min of treatment at 400 MPa, the alcohols in yellow wine, such as isobutanol, active amyl alcohol, isoamyl alcohol, 2,3-butanediol, and phenethyl alcohol, increased significantly by >twofold compared with the untreated samples. Isobutanol and 2,3-butanediol are the characteristic aroma substances of yellow wine; they are also the precursors of diethyl succinate. Active amyl alcohol and isoamyl alcohol are the main high-grade yellow wine alcohols with mellow and fruity aromas, giving the wine a mellow and soft taste. Isoamyl alcohol can reduce the bitterness of leucine to a certain extent; phenyl alcohol has a rose-like scent. One of the reasons for the increase in the content of these alcohols may be that UHP treatment released the alcohols originally in the form of free glycosides. Similarly, UHP treatment has no effect on the types of free amino acids in rice wine but has a more obvious effect on their content. Some researchers used an automatic amino acid analyzer to analyze the changes in amino acid types and content after UHP treatment and found that after treatment at 400 MPa for 15 min, the types of free amino acids did not change, but the total amount increased slightly. The content of umami and sour amino acids was essentially unchanged, but that of bitter amino acids, astringent amino acids, and tyrosine increased by 14%, 4%, and 22%, respectively. The study also concluded that the increase in the content of bitter and astringent amino acids after UHP treatment was the main reason for the change in the taste of yellow wine. The researchers believed that UHP aging of yellow wine increased its bitterness and astringency, whereas the overall taste of rice wine was more harmonious and comfortable, and the purpose of aging was achieved. This is inconsistent with the common belief that the bitterness and astringency of alcohol gradually disappear during the aging process and that the aroma gradually becomes strong. The author believes that this statement should be questioned. In terms of physicochemical indicators, former researchers found that UHP treatment of rice wine did not change its appearance and acid content significantly but increased its electrical conductivity, total solid content, and total ester content; decreased its fusel oil content significantly; and increased its total aldehyde content significantly. The researchers believed that UHP treatment promoted the oxidation and esterification of various components in yellow wine, resulting in the oxidation of alcohols to aldehydes and acids and the subsequent esterification of acids and alcohols to form esters. Each liter of fresh red wine contains approximately 0.5 g of nitrogenous substances, of which >90% are free amino acids. The main types of amino acids in red wine are proline, serine, leucine, and glutamic acid. Amino acids are important substances that contribute to the flavor of red wine, and the taste and flavor of red wine with different amino acid ratios differ considerably. Studies have shown that in the natural aging process of red wine, the content of sweet, sour, and umami amino acids increases with the increase in time (over years), whereas the content of bitter and astringent amino acids decreases. The results are consistent with the phenomenon that the taste of red wine becomes soft and palatable during the aging process. Previous

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studies found that after UHP treatment at 300 MPa for 90 min, the total concentration of free amino acids increased from 808 to 880 mg/L, but when the pressure exceeded 500 MPa, the total amount of free amino acids did not change much. After treatment of red wine at 300 MPa, sweet amino acids (lysine, alanine, serine, proline, and threonine), bitter amino acids (arginine, isoleucine, leucine, methionine, histidine, and phenylalanine), and sour amino acids (glutamic and aspartic acids) were found in a ratio of 14.5:3.3:1, whereas the ratio in untreated red wine was 15.8:3.9:1. An increase in the proportion of sweet and sour amino acids and a decrease in that of bitter amino acids are important reasons for the improved taste of red wine. The researchers also compared the changes in volatile components of red wine before and after UHP treatment. The volatile components found in high concentrations in red wine included isoamyl alcohol, diethyl succinate, phenethyl alcohol, ethyl 4hydroxybutyrate, butyrolactone, ethyl 3-hydroxybutyrate, 2-hydroxypropionic acid, and ethyl esters; these components accounted for approximately 86% of all components. The content of isoamyl alcohol in red wine decreased and that of phenylethyl alcohol increased after UHP treatment. Studies showed that isoamyl alcohol, as a higher alcohol, was beneficial to the flavor of liquor when its content was low. Excessive isoamyl alcohol content will make it easier to get a headache after drinking, and the taste is also more irritating. Conversely, an increase in phenylethyl alcohol content conferred the taste of rose to red wine, which can significantly improve its flavor. In addition, the increase in the content of esters showed that UHP treatment accelerated the esterification reaction. As esters are the main aroma substances of red wine, an increase in their content was undoubtedly beneficial to the improvement of red wine flavor. The content of other volatile components, such as organic acids, aromatic hydrocarbons, and ketones, did not change significantly. In addition to the relatively small impact of UHP on these types of substances, it was also possible that the content of these substances was inherently low. Small changes caused by processing are difficult to detect. Some researchers also explored the effect of UHP treatment on the quality of red wine. Besides the above changes in volatile components, amino acids, and sensory quality, under the treatment conditions of pressure below 600 MPa and time less than 1 h, the physical, chemical, and sensory quality of red wine did not change significantly. Post-storage experiments also showed that although the immediate differences in physicochemical properties between UHP-treated (UHPT) and untreated red wine may be small, these differences will become obvious after storage. Thus, the storage process after treatment is beneficial to further strengthen the effect of UHP treatment, a finding that has not been mentioned in previous studies. Studies on the effect of UHP treatment on the aging of liquor mainly focus on the volatile components, physicochemical properties, and sensory quality of liquor; however, the conclusions are slightly different. Researchers found that the sensory quality of liquor improved to varying degrees after different pressure treatments, as evidenced by the disappearance of new wine taste and the enhancement of mellow aroma. However, the results of volatile component analysis showed that the changes in esters, acids, and alcohols were not obvious. Other researchers evaluated the aging effect of UHP treatment on liquor from the perspective of physicochemical properties. They found that new wines treated with different pressures had different levels of

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conductivity, redox potential, surface tension, and total acid content. The trend of the change was consistent with that of the natural aging process of liquor, with a better treatment effect found with higher pressure. A previous study found that the content of ethyl lactate and acetal in liquor after UHP treatment was equivalent to that in 2-year-old liquor and that the content of isobutanol, isoamyl alcohol, ethyl acetate, acetaldehyde, and butyric acid was equivalent to that in 3-year-old liquor. The content of ethyl ester and ethyl caproate was essentially the same as in 1-year-old liquor, and the trends of changes in the content of total acid, total ester, and total aldehyde were also consistent with those achieved with the natural aging of liquor. Some researchers found that after UHP treatment, the ethanol content in liquor decreased by 5%, fusel oil content decreased by 42%, and acetal content remained basically unchanged, whereas the trends in total acid and total ester content were consistent with those of naturally aged liquor. The results of sensory evaluation showed that the taste of the treated liquor was remarkably more mellow and softer than that of the untreated liquor. Through the use of various indicators, the researchers believed that the quality of the liquor following UHP treatment was essentially the same as that of 1-year-old naturally aged liquor.

Chapter 2

Introduction to Brown Rice and Related Substances

Abstract Brown rice has a richer and more complete nutritional profile than white rice and is a healthier and more food-efficient alternative to white rice. In this chapter, the nutritional value and physicochemical properties of brown rice as well as the opportunities and challenges for the development of the brown rice industry are first introduced, followed by the application of high pressure technology in the modification of brown rice, and finally other brown rice modification technologies such as milling, germination and pregelatinization and freeze–thaw treatment. Keywords Brown rice · Modification · High pressure treatment · Germination and pregelatinization · Freeze–thaw treatment

2.1 Introduction to Brown Rice 2.1.1 Nutritional Value of Brown Rice Currently, more than three billion people worldwide consume rice as a staple food. According to the US Department of Agriculture, the world produced 497 million tons of rice and sold 484 million tons in 2019. China produced 148 million tons and sold 143 million tons, making it the largest rice-producing country. In recent years, the global rice sales to production ratio has continued to increase, reaching 99% in 2019; if the growth trend continues, sales will likely exceed production in the next 5–10 years. White rice is usually made by milling brown rice to remove the outermost layer of bran (along with the germ) and obtain the endosperm. The goal is to obtain a soft taste and bright white color; because the bran layer is yellow and rough and hard in texture, it will prevent the ingress of water during soaking and cooking, making the inside of brown rice difficult to cook. However, milling also leads to a loss of several nutrients, as approximately 64% of the nutrients in brown rice are concentrated in the bran. Rice bran contains fat, protein, water-soluble active polysaccharides, dietary fiber, γ-aminobutyric acid (GABA), oryzanol, minerals, phytic acid, inositol, and various vitamins. Therefore, the selection of brown rice instead of white rice as a © Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_2

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staple food in the daily diet will not only improve the utilization rate of brown rice and relieve the pressure on grain supply but also provide more nutrients. In addition, brown rice has antidiabetic, anticholesterol, heart-protective, and antioxidant properties. However, because the bran layer is rich in fiber and has a tight texture that prevents the rapid infiltration of water, most consumers find it difficult to accept brown rice because of its poor cooking, taste, and digestibility. Foods prepared using rough rice, such as brown rice bread, has a poor taste, is easy to revive, and has other problems, which limits its promotion. Natural rice flour cannot meet the requirements of industrial application owing to its weak mechanical stability, poor storage properties, and other processing characteristics. Even in today’s increasingly healthy diet, it is still common to grind brown rice further to obtain white rice. During the conversion of brown rice to white rice, 10% of the weight is lost, but 64% of the nutrients are also lost, and the yield of rice is reduced by 15%. In recent years, China’s rice output has exceeded 200 million tons, but the yield of brown rice accounts for only one-thousandth of this, which is equivalent to the loss of the rice output of Hunan Province every year due to milling, a major waste of resources.

2.1.1.1

Starch

Starch is the most important component of brown rice. In brown rice with a moisture content of 14%, the starch content is ~72%, which can provide the material basis (glucose) for the daily energy metabolism of the human body. Starch is composed of amylose and amylopectin. Amylose is a long, helical linear molecule composed of dehydrated glucosidins linked by α-1,4-glycosidic bonds. In addition to the α1,4-glycosidic bond in amylopectin, every 20–30 dehydrated glucosidins have a branching structure formed by the α-1,6-glycosidic bond. The ratio of amylose to amylopectin varies among different varieties of brown rice, but amylopectin is usually dominant. Studies have shown that the fine structure of amylose and amylopectin affects the texture of rice; the smaller the molecular size of amylose and the higher the proportion of long chains, the stiffer the rice. Starch can be classified into rapid digestible starch (RDS), slow digestible starch (SDS), and resistant starch (RS) according to its digestibility. RDS leads to a rapid increase in blood glucose and insulin levels, whereas SDS leads to a more moderate increase. RS has strong digestive properties, helps control weight and blood sugar, and reduces the risk of diabetes, cancer, heart disease, and cardiovascular disease. Therefore, RS is the most attractive starch research object. Compared with traditional white rice, brown rice contains significantly more RS. On the one hand, rice bran is rich in dietary fiber, which has a strong ability to resist digestion. On the other hand, under the same cooking conditions, brown rice is more difficult to gelatinize thoroughly than white rice, and ungelatinized starch is more difficult to digest than gelatinized starch. The content of RS differs among brown rice varieties. The factors affecting the RS content of brown rice can be classified into two categories: internal, which

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includes amylose/amylopectin ratio, starch chain length, starch particle size, and starch crystal structure, and external, which includes storage conditions, germinating state, and processing.

2.1.1.2

Protein

Rice protein (including brown rice and white rice proteins) is a type of high-quality food protein; its digestibility and raw price are higher than those of the proteins of other grains, such as wheat, corn, and barley. The amino acid composition of rice protein is relatively complete, consisting of various amino acids required for human growth, and the composition ratio of various amino acids conforms to the ideal ratio recommended by WHO (World Health Organization)/FAO (Food and Agriculture Organization of the United Nations); in particular, the lysine content is higher than that in other cereal plants. The protein content of brown rice is higher than that of white rice because the protein content of the bran, which is removed during the processing of white rice, is higher than the protein content of the endosperm, which is retained. Lysine is usually the first limiting amino acid in rice protein, and lysine content in rice bran protein is also higher than that in white rice protein. Moreover, the protein efficiency ratio, net protein retention ratio, and net protein utilization ratio of rice bran protein are 2.39, 3.77, and 70.7, respectively, whereas the corresponding values for white rice protein were 1.96, 3.26, and 61.4, respectively. The true digestibility, raw price, and corrected amino acid score for rice bran protein were 94.8%, 72.6%, and 0.90, respectively, which were higher than those for white rice protein (90.8, 66.7, and 0.63). Therefore, the nutritional value of brown rice protein (endosperm protein + rice bran protein) is higher than that of white rice protein (endosperm protein). Based on solubility, rice proteins can be classified into albumin (water-soluble protein), globulin (salt-soluble protein), and gliadin and gluten (alkali-soluble proteins). Because albumin has sufficient net charge and does not have extensive disulfide cross-linking or aggregation, it is soluble in water. However, globulin contamination occurs when albumin is extracted with water, as the minerals present in the rice grains dissolve in water, increasing the solubility of salt-soluble globulin. The content of albumin and globulin in brown rice is low; these are physiologically active proteins and directly participate in the metabolism of brown rice. Gluten and gliadin are storage proteins. Gluten comprises the largest proportion of brown rice protein; it is insoluble in water and salt solutions and soluble in acidic and alkaline conditions. Gliadin is soluble in alcohol solutions, but basically insoluble in water. Therefore, brown rice protein generally has poor solubility in water, which limits its value in practical application. The digestibility of these four proteins follows the order albumin > globulin > gluten > gliadin. Rice protein possesses the advantages of hypoallergenicity, compared with many plant proteins containing antinutritional factors, such as the widespread use of soybean and peanut proteins that contain trypsin inhibitors and blood coagulation

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factors and pineapple that contains bromelain; these substances lead to the occurrence of edible allergic reactions, limiting the application of these vegetable protein in actual production. Therefore, rice protein is a safe source of protein for young children and the elderly.

2.1.1.3

Fat

Brown rice has a fat content lower than its starch and protein content, at approximately 1.6–2.8%; however, the fat content of white rice is even lower, less than a quarter of that of brown rice. This is because the fat of brown rice is mainly distributed in the bran layer (approximately 15–20%). Therefore, studies on the nutritional value of white rice usually do not involve fat. While brown rice is also not high in fat (as opposed to starch), it contributes significantly to processing and nutritional properties. Similar to the essential amino acids, there are also essential fatty acids in brown rice; these are usually necessary to maintain the physiological functions of the body but cannot be synthesized by the body itself and can only be obtained from food. One type is the omega-3 polyunsaturated fatty acids, which are derived from α-linolenic acid (the parent), while the other type is the omega-6 unsaturated fatty acids derived from linoleic acid as the parent. Omega-3 fatty acids are thought to reduce inflammation, blood pressure, and triglyceride levels and improve cardiovascular health. Omega-6 fatty acids help reduce cholesterol levels and regulate hormone action, but they also promote inflammation; therefore, it is important to balance the intake of these two types of polyunsaturated fatty acids. The optimal ratio of omega-6 to omega-3 fatty acids should be approximately 1:1 to 4:1. However, the ratio of omega-6 to omega-3 fatty acids in brown rice is approximately 20:1, which is far from ideal. Because brown rice fat is mainly distributed in the rice bran layer, the most common way to use brown rice fat is to extract rice bran oil, a by-product of white rice processing, as an edible oil. The main components of rice bran oil are palmitic, oleic, and linoleic acids, and the ratio of saturated, monounsaturated, and polyunsaturated fatty acids is approximately 1:2.1:1.8, which is close to the ratio recommended by WHO. However, in the process of rice bran milling and rice bran oil extraction, lipase in rice bran is also released into rice bran oil, meaning that fatty acid degradation could easily occur in rice bran oil.

2.1.1.4

Others

Levels of many nutrients are also higher in brown rice than in white rice. The vitamins rich in brown rice are mainly distributed in the rice bran layer. GABA is a nonprotein amino acid, which calms nerves, improves brain vitality, lowers blood pressure, reduces blood ammonia levels, and promotes alcohol metabolism. The GABA content in brown rice is approximately 4.6 times higher than that in white rice, and brown rice accumulates more GABA after germination. Oryzanol, a special

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ingredient in brown rice, is a natural melanin inhibitor. It can also reduce cholesterol levels and treat and prevent heart disease, nerve disorders, and skin aging. The content of oryzanol in brown rice is approximately 15 times higher than that in white rice; this is because oryzanol is concentrated in the bran layer. The content of mineral elements is markedly reduced after brown rice is milled into white rice. In addition, the total phenol and total flavonoid content also decreased by 86%–90% and 82%– 95%, respectively. Phenols and flavonoids have good antioxidant activity, exhibiting good antiaging effects in the human body.

2.1.2 Physicochemical Properties of Brown Rice Because brown rice contains approximately 80% starch, its physicochemical properties depend mainly on the starch content. Studies on the physicochemical characteristics of brown rice provide objective evaluation methods for its cooking, edible quality, and processing characteristics as well as a basis for innovating and optimizing the modification technologies of brown rice.

2.1.2.1

Gelatinization Properties

The application of brown rice is inseparable from the gelatinization of starch grains. Gelatinization is the phase transition of starch particles from an ordered to a disordered state. Gelatinization can cause hydration, swelling, and amylose leaching of starch particles. It is usually divided into three stages: reversible water absorption stage (amorphous zone hydration), irreversible water absorption stage (crystal zone hydration), and particle disintegration stage (crystallization melting). Therefore, the water absorption of brown rice, interaction between water and starch, swelling of starch particles, and energy required for gelatinization are closely related to its gelatinization characteristics, which affects its cooking quality. There are many methods to detect gelatinization characteristics; one method is the simulation of the cooking process. The swelling behavior of starch particles at different heating temperatures describes the gelatinization process of starch particles in stages, reflects their volume changes under hydration, and their interaction with water. The capacity of starch grains to expand can reflect their strength after hydration; the greater the expansion capacity, the lower the strength. In addition, as the gelatinization process is accompanied by changes in system viscosity, a rapid viscosity analyzer can comprehensively reflect gelatinization characteristics by providing information on the gelatinization temperature, changes in system viscosity. This has become the standard method used by the American Association of Cereal Chemists. The decay and recovery values obtained from the detection can represent the hot-paste and cold-paste stabilities of rice, respectively, which determines the application value of rice and rice flour in the food industry to a large extent and can be used to predict the quality of the final product (such as rice and rice noodles).

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Rheological Properties

The study of rheological properties is also essential for the development and utilization of starch-based materials. In the production and use of brown rice products, the processing characteristics of raw materials during transportation, stirring, and mixing or the taste of the product when eaten and chewed are essentially an expression of the rheological properties of rice. However, natural rice paste usually behaves as a shear-thinning pseudoplastic fluid, which has the disadvantages of poor shear resistance and low strength, which markedly limits its development and application. A hot spot in the research field of starch-based materials is their adaptation to industrial production and processing by modification. According to the different ways in which force is applied, rheological characteristics can be classified into static and dynamic testing methods. Static rheology tests the steady-state shear performance, which mainly reflects the flow capacity of the system, and can be used to describe the consistency and flow characteristics of the fluid by fitting a model (such as a power-law model). Dynamic rheology tests oscillatory shear properties, which mainly reflect the viscoelasticity of the system. Changes in stress and viscosity before and after shear reflect the stability of the structure. Therefore, rheological properties are closely related to machining properties. In addition, the detection of texture properties can help describe the rheological properties perceived by consumers. For example, texture profile analysis can achieve an objective evaluation of the taste of products by simulating chewing in the mouth, which is a universal test method for texture in the food industry. Therefore, the characterization of rheological properties can not only determine the application scope of materials and explain the texture changes in the production process but also evaluate the taste of products and predict their stability. This is extremely important for the development and promotion of brown rice products.

2.1.2.3

Digestibility

Nutrient-rich brown rice has potential as a raw material for the production of food for infants and the elderly. However, owing to the existence of the rice bran layer, the indigestible characteristics of coarse rice-based food limit the expansion of its target consumer group. As a starch-based material, the digestion rate and digestibility of starch contained in brown rice not only directly affect the utilization of its nutrients but also directly affect postprandial blood glucose homeostasis. Therefore, the study of the digestive characteristics of brown rice would provide important reference for the nutritional evaluation of products and the selection of consumer groups. Brown rice-based foods stay in the mouth for a short time. Protein is mainly removed within 30 min in the stomach and then digested in the small intestine for 2 h. Under the action of amylase and glycosylase, it is finally hydrolyzed into glucose, which can be absorbed by the human body. At present, in vitro digestion technology is usually used to simulate the digestive process in the human gastrointestinal tract to achieve the detection of digestibility. Many researchers have studied the kinetics of

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the starch digestion process and obtained relevant information, such as the enzymatic hydrolysis rate constant, to provide a more accurate description of change in glucose content during the enzymatic hydrolysis process. In addition, based on the digestion process of starch in the small intestine, starch can be divided into different types from the perspective of nutrition: (1) RDS, which refers to starch that can be enzymologically digested within 20 min, (2) SDS, which is starch that can be enzymologically digested within 20–120 min, and (3) RS, which is starch that is not enzymatically digested within 120 min. This method is widely used to evaluate the digestibility of starch-based foods.

2.1.2.4

Water Migration Characteristics

As moisture is a basic component of food, the content, distribution, and migration of water have important effects on the physicochemical properties, processing properties, quality, and stability of food. Therefore, the study and control of water dynamics has gradually become a research focus in the field of food processing and storage. Based on its fluidity, water in food can be divided into bound water, hysteresis water (not easy to flow water), and free water. Bound water includes that most closely bound with the nonaqueous component of the combined water and is found in the nonaqueous component of the hydrophilic group around the single layer of water and multilayer water. Free water is water that flows freely in the intercellular space. The fluidity of hysteresis water is between that of bound water and free water; that is, water trapped by microscopic and submicroscopic structures and membranes in tissues. In essence, the three divisions reflect only the degree of combination between water and nonwater substances, which can be transformed into each other and thereby help achieve dynamic balance. Low-field nuclear magnetic resonance (LF-NMR) can be used to study the migration characteristics of water molecules in food by detecting the relaxation behavior of protons. Owing to its rapid, nondestructive, and accurate characteristics, it has attracted considerable attention in the field of food science in recent years. LFNMR relaxation time (T2) can reflect the mobility of water molecules in brown rice products; that is, the degree of binding to nonwater components such as starch.

2.1.2.5

Rejuvenation Characteristics

The processing and storage of brown rice is inevitably accompanied by the occurrence of rejuvenation. Rejuvenation refers to the recrystallization process in which the amylose and amylopectin separated during gelatinization re-gather owing to the slow movement of molecules when the temperature decreases, and then they form hydrogen bonds with the swollen starch particles and return to the ordered structure. Among them, the rapid rate of short-term regeneration is mainly caused by the orderly rearrangement and crystallization of amylose chains, whereas the subsequent longterm regeneration is caused by the recrystallization of branched-chain molecules.

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Resurrection leads to a deterioration in the quality of brown rice, such as a decrease in the water holding capacity, the hardening of taste, and a reduction in digestibility, which not only limits the development and application of materials but also results in substantial wastage of resources. Sometimes, rejuvenation properties can also be used to enhance the texture of foods, such as gelatin-type foods (cold skin, etc.) in a preparation by using short-term rejuvenation properties to obtain better toughness. However, long-term regeneration also results in a series of problems, such as short shelf-life and the abuse of additives, which remarkably limit the industrial production of gel-type foods. Therefore, it is necessary to study the rejuvenation characteristics of brown rice products in development and on the market. There are various methods to detect the rejuvenation characteristics. Among these methods, the changes in the textural properties during storage reflect the degree of rejuvenation perceived by consumers from the perspective of taste. In addition, water not only promotes the migration of starch chains in the free state but also participates in the recrystallization of starch molecules in the bound state. Therefore, the distribution and migration of water significantly affect the regeneration properties of brown rice. Using NMR to study the water migration associated with regeneration from a molecular perspective is helpful to explore the regeneration mechanism and can guide the production and storage of brown rice.

2.1.2.6

Main Influencing Factors

The physicochemical properties of brown rice are not only closely related to starch composition (direct branching ratio), concentration, particle structure, and size but are also affected by nonstarch components (such as protein, lipid, and fiber). Therefore, starch, rice flour, and brown rice flour have different physicochemical properties. However, most of the existing research has focused on starch, with little attention paid to the physicochemical properties of rice flour, particularly brown rice flour, which has greatly contributed to the lack of variety of brown rice products and limited their research, development, and promotion. Nonstarch components such as proteins and lipids in brown rice can inhibit the gelatinization of starch particles in brown rice. Lipids can form complex structures with amylose, which prevents the hydration of starch, thus limiting water absorption and expansion during gelatinization. Proteins can inhibit starch gelatinization by interacting with starch to form complex network structures or by competing for available water. Moreover, nonstarch components can inhibit the progression of regeneration. The proteins and lipids contained in brown rice can form complexes with starch, causing steric hindrance to the migration of starch molecules and hindering the formation of starch networks, thus delaying the regeneration of the system. The reduction in the particle (segment) migration rate caused by the amylopectin–lipid complex affects the order of amylopectin and inhibits the regeneration.

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2.1.3 Problems with Brown Rice 2.1.3.1

Taste

Although brown rice is more nutritious than the target substitute, white rice, it has some disadvantages. The first problem is that brown rice is not as tasty or as soft as white rice. The bran layer, the outermost layer of brown rice, is the main reason for the hard taste of brown rice. The rice bran layer itself is stiff in texture because it contains 7–11% crude fiber, whereas the endosperm contains only 0.2–0.5% crude fiber. However, the bran layer also prevents water from entering brown rice. Even moderate milling (e.g., a milling level of 6.5%, which does not fully grind brown rice to white rice) can increase the water absorption of brown rice by nearly four times. Usually, the higher the water content of brown rice, the softer the cooked rice. In addition, the core process of rice cooking is starch gelatinization; starch expands by absorbing water when heated; hence, water affects starch gelatinization. Consequently, brown rice is more difficult to gelatinize and has a stiffer texture than white rice when subjected to the same cooking conditions. Therefore, the main methods used to improve the texture of brown rice are to destroy the structure of the rice bran layer, increase the water absorption of brown rice, and strengthen the degree of gelatinization while cooking brown rice. The most intuitive way to evaluate the taste of rice is to conduct artificial tasting and sensory evaluation, as the texture of rice can be experienced directly through mouthfeel. However, sensory assessment often requires professional training for volunteers, which is time-consuming and laborious. Moreover, because differences between individuals cannot be completely eliminated, variations usually occur in the final results. Currently, a texture analyzer is most often used to determine the texture parameters of rice; hardness is usually the main parameter, alongside elasticity, viscosity, adhesion, chewiness, etc. The hardness of brown rice is 1.3~2.1 times that of white rice. The difference in hardness between brown rice texture and white rice texture indicates that the harsher nature of brown rice is a nontrivial aspect, and because texture is the first and most intuitive experience for consumers, it is the most important limitation to be addressed.

2.1.3.2

Digestibility

The second disadvantage of brown rice is that it is less digestible than white rice. Although the proportion and content of all types of nutrients in brown rice are better than those in white rice, the actual nutritional value of brown rice does not reach the theoretical value because it cannot be digested or absorbed very well during digestion. The rice bran layer is hard in texture and does not easily disintegrate during digestion; it acts as a barrier to inhibit the diffusion of digestive juices into rice grains, thus limiting acid hydrolysis, structural degradation, and solid leaching. In addition, the antinutritional factors contained in brown rice also hinder the digestion

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and absorption of nutrients. Phytic acid is a typical antinutritional factor, mainly distributed in the bran layer and germ of rice. The phytic acid content in brown rice is approximately 1.8–2.5 times higher than that in white rice. Phytic acid can chelate metal ions and reduce the bioavailability of metal elements. It can also form phytic acid–protein complexes with proteins that are insoluble and unusable under normal physiological conditions, reducing the availability of proteins. Indeed, phytic acid can also directly combine with digestive enzymes, inhibit the activity of the enzymes, and impair the digestion and decomposition of brown rice. Although brown rice is less digestible than white rice, white rice is defined as a high glycemic index (GI) food; thus, the poor starch digestibility of brown rice is not entirely a disadvantage from the perspective of glycemic response. Replacing white rice with brown rice leads to better glycemic control and reduced obesity, mainly because brown rice is less starchy than white rice, and the body ends up digesting and absorbing less glucose for the same amount consumed. However, as starch is the main component of brown rice, the degree of starch digestion and hydrolysis also affect the size of brown rice grains after digestion, impacting digestion and the release of other nutrients. Digestibility is mainly studied from two main perspectives: in vivo and in vitro. In vivo digestion experiments (including human and animal experiments) can provide the most intuitive representation of the digestion and absorption of nutrients, such as through analysis of the body’s blood glucose levels and use of fecal residues to indirectly calculate the absorption and utilization rate of nutrients. They can also show the final health effects of consuming brown rice/white rice for a certain period. However, the differences between individuals inevitably lead to experimental errors, and in vivo experiments also encompass ethical and moral concerns. Currently, in vitro digestion experiments are widely used to analyze digestibility, and there are many welldeveloped simulated digestion methods. Previous studies on the digestibility of brown/white rice have primarily focused on starch, although some have also focused on protein, as starch and protein are the two nutrients with the highest content, which is the main factor influencing the choice between brown rice or white rice to obtain nutrition, especially for white rice. The amount of fat in brown rice is second only to that of starch and protein; however, few studies have investigated the digestibility of brown rice fat. Brown rice is more often used as a supplement, despite it containing other nutrients; indeed, only a few studies have investigated the minerals and phenols released during the digestion of brown rice.

2.1.3.3

Storage

As there are many lipids in the bran layer of brown rice, fatty acid spoilage can occur readily during long-term storage, reducing shelf-life. The rancidity of fat is mainly classified as hydrolytic rancidity and oxidative rancidity. Hydrolytic rancidity, in which triglycerides are hydrolyzed to produce free fatty acids, is usually assessed by the fatty acid value (the mass of KOH required to neutralize free fatty acids in 100 g of sample). Oxidative rancidity refers to the formation of short-chain volatile aldehydes

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and ketones by oxidation reactions of fat and free fatty acids (especially unsaturated fatty acids), which can lead to the production of sour odor and toxic substances. Oxidative rancidity is usually analyzed in terms of peroxide value, conjugated diene value, or malondialdehyde content. The degree of oxidative rancidity is mainly ascertained by the human sense of whether the smell is abnormal (sour taste, rance taste, etc.). The limited shelf-life of brown rice partially restricts consumers’ purchasing trends. White rice can be stored for longer periods and does not require any special packaging and storage conditions. Low-temperature storage is an effective method for preventing fatty acid rot in brown rice. It has been reported that brown rice stored at a low temperature (15 °C) and germinated brown rice exhibited lower levels of lipid hydrolysis and oxidation compared with brown rice stored at higher temperatures (25 and 35 °C). In addition, the fatty acid values and lipoxygenase activity of brown rice stored at 10 °C were lower than those of brown rice stored at 25 °C. Although cryopreserved storage is effective in controlling fatty acid spoilage, it is often expensive. In addition to low temperature, regulating the oxygen level in the storage environment is also a key requirement of inhibiting fatty acid rot in brown rice. The addition of deoxidizer can improve the accumulation of free fatty acids and delay their oxidation during storage of brown rice. If this technique can be combined with the technical means of inhibiting the hydrolytic activity of fat, the advantages can be fully explored. Furthermore, brown rice is more vulnerable to insects than white rice, which is one of the limiting factors for its shortened shelf-life. The insects that are most harmful to brown rice include the Sitophilus oryzae (Linnaeus), the Oryzaephilus surinamensis Linne, the Corcyra cephalonica (Stainton).

2.1.4 Opportunities and Challenges in the Development of Brown Rice Industry China is the world leader in rice production in terms of both output and planted area. In 2011, per capita rice consumption in China was 68.2 kg, of which brown rice accounted for only 0.9%, far below the level in developed countries. At that time, researchers began to realize that the original criteria for evaluating the quality of brown rice could not be used to measure nutrition; therefore, they performed further studies. In recent years, with the continuous improvement of economic development and living standards, the growth of grain consumption has slowed and the consumption of animal food and oil has increased significantly. Human health is being severely put to the test. To cope with this problem, consumers need to enhance their health awareness, increase their intake of whole-grain food, and adopt a reasonable dietary pattern. Researchers need to move beyond the existing technology for breeding, processing. According to the 13th Five-Year Plan for the grain and oil processing

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industry, the “Green and Healthy Grain Ration Project” will be implemented to vigorously develop whole grains and food, strengthen market cultivation, and increase the supply of new, green, high-quality, nutritious, and healthy products such as brown rice, whole wheat flour, coarse grains, potato, and their products. Human health should be a strategic priority to accelerate the development of a “healthy China,” with a focus on the health industry.

2.2 Application of UHP Technology in Agricultural Products 2.2.1 Introduction to UHP Technology For foods, UHP technologies refer to those used to seal the food material into an elastic container or pressure device system (often with water or other fluid as the pressure transfer medium) under high static pressure, generally under a period of time (from a few seconds to a few hours) to ask, to achieve sterilization and material modification and to change the effect of some physicochemical properties of food. As a type of nonthermal processing technology, UHP processing is a purely physical process, with instantaneous compression, uniform effect, safe operation, and low energy consumption; more importantly, it ensures food safety while reducing the degree of food processing as much as possible, thus preserving the original flavor of food. UHP can damage the chemical structure of foods with noncovalent bonds, such as hydrogen bonds, but has no effect on covalent bonds. Thus, UHP can destroy hydrogen bonding in macromolecular material, resulting in the modification or degeneration of macromolecular structures (such as starch and protein) and vitamins, minerals, aroma components, and pigments into small molecules with no effect on covalent bonds. Therefore, compared with traditional processing methods, UHP technology has many advantages. UHP can destroy the cell wall and cell membrane in rice, promote the swelling and gelatinization of starch particles, and improve the structure of rice grains, thereby improving the taste and quality of rice. In addition, starch gelatinized by UHP differs from that gelatinized by heating. Gelatinized starch particles are broken and the starch is dissolved. At low temperatures, starch molecules (especially amylose) quickly regenerate, causing them to reorder and crystallize. The gelatinization caused by UHP actually occurs inside the starch particles, and because UHP does not destroy starch particles, the regeneration occurs within the starch particles. On comparing gelatinized rice starch subjected to UHP and heat treatment, the gelatinization rate under UHP was found to be slower than that under heat treatment, but this phenomenon was not observed with glutinous rice. In recent years, with the continuous exposure to food safety problems, people have paid increasing attention to food safety and quality issues, and the demand for “green” and healthy food has also increased.

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The research and development of new food processing technology is a general trend in the development of international food science and technology. If UHPprocessed brown rice products can be realized, it will have a subversive effect on the diet of the Chinese population and will improve the health of the nation. In recent years, nonthermal processing technology has become one of the hot spots in food processing research and application. Traditional thermal processing can reduce the nutritional value of food. Unlike thermal processing, UHP processing can retain the original nutrition and characteristics of the food to the maximum level, and the technology is considered environmentally friendly. Moreover, it can avoid the defects associated with processing technologies such as microwaves, box lighting, and electric spray fields and has the advantages of saving resources and reducing pollution. UHP processing is a nonthermal processing technology that has been the most well studied and industrialized to the greatest extent. In the food industry, UHP treatment refers to the use of UHP (100–1000 MPa) for sterilization, material modification, acceleration or deceleration of physicochemical reactions of prepackaged food materials in a unit pressure vessel at room temperature or low temperature. The properties of food under UHP follow Pascal’s law. According to this law, the pressure domains are applied uniformly across the entire food material, which means that the effect of UHP treatment is independent of the size, shape, and volume of the material. UHP treatment also follows Le Chatelier’s principle, and a clever pressure treatment moves the equilibrium of the reaction toward the state of obtaining a smaller volume of reactant. The increase in pressure is a purely physical process. When food materials are compressed in liquid medium, the noncovalent chemical bonds such as oxygen, ionic, and hydrophobic bonds that form the three-dimensional structure of polymer material change, resulting in the denaturation of proteins and starch, loss of enzyme activity, and killing of bacteria and other microorganisms. In UHP processing, pressure has no effect on the covalent bonds of low-molecularweight substances such as vitamins and flavoring agents. In particular, small molecular vitamins, amino acids, fruit acid, fructose, aroma substances, and antimutagenic active ingredients in fruits and vegetables are less damaged, and the original flavor of food can be retained after UHP treatment. Starch is an important carbohydrate synthesized by photosynthesis in higher plants. It is widely distributed in the roots, stems, fruits, and seeds of plants and is an abundant natural resource. With the development of the starch industry, an increased quality of starch is required, as starch has many limitations such as insolubility in cold water, rapid aging, poor film-forming properties, and poor shear resistance. UHP technology provides a new possibility for the physical modification of starch. Research into the physical modification of starch using UHP technology commenced at the end of the last century. In the past 20 years, different processing conditions have been proposed for UHP gelatinized starch molecules and the molecular physics of modified starch has been studied, including different stress levels, holding times, starch concentration, temperatures, and sources of starch; these have been used to explore the mechanism of starch gelatinization induced by stress and to understand the effect of UHP on the physicochemical characteristics of starch granules. Many

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reports have shown that UHP processing affects the physicochemical properties of starch polymers. As a nonthermal food processing technology, the greatest advantage of UHP is that it ensures food safety, reduces the degree of food processing as much as possible, and preserves the original flavor of food. At present, UHP technology is widely used in the field of food processing. This includes mature commercial foods such as fruit juice and jam; however, UHPT brown rice products are rarely found in the market. In recent years, the application of UHP technology in the processing of grain products has received increasing attention. Studies on UHP technology have mainly focused on grain starch and protein modification, with less research on grain texture; in addition, UHP technology has been used in grain quality research relating to flavor, nutritional value, and physicochemical characteristics. Therefore, relevant basic research is needed; if it can improve the taste of brown rice, this will help to alleviate difficulties in Chinese food scarcity.

2.2.2 Application of UHP in Processed Food Crops 2.2.2.1

Modification of Crop Starch

Starch, as an important part of cereal crops, is an important biopolymer in the food industry. Starch and its subsequent processing products are widely used in food, textile, paper, medicine, glue, casting, petroleum, and other industries. However, unmodified starch is easily affected by temperature, shear force, and aging; therefore it has not been widely used in the food industry. Methods of starch modification include physical, chemical, enzymatic, and compound modifications. When starch grains are heated in a sufficient amount of water, they absorb water and expand, eventually getting ruptured. However, UHPT starch grains can maintain their integrity, but their surface becomes coarse in early years. UHP treatment was found to reduce the gelatinization temperature of potato, wheat, and bean starch, which promoted further research on the effects of UHP on starch gelatinization. Researchers have studied the influence of different pressures, holding times, and temperatures on the gelatinization temperature and some other physicochemical properties of starch. Some researchers also found that the degree of aging of ordinary rice starch under UHP treatment was smaller than that under heat treatment; however, there was no significant difference in the effect of UHP on glutinous rice starch, indicating that the effect of UHP on starch aging was closely related to starch composition and source. Different types of starch grains have different sensitivity to UHP, which is related to the type of starch crystal. Starches have three crystal structure types: A, B, and C. Type A is mainly derived from cereal starch, such as corn and wheat starches, and is sensitive to pressure. Type B is derived from tuber starch, such as potato, banana, and taro starches, and is relatively resistant to pressure. Type C is a combination of the A and B crystal types and is found in including banana, lotus root, and most bean starches. This explains the sensitivity of different starches to UHP treatment from the perspective of crystal structure. The

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gelatinization pressure of potato starch with type B crystal structure was higher than that of wheat starch with type A crystal structure, and the crystal type of wheat starch with type A crystal structure and cassava starch with type C crystal structure changed to type B crystal structure after UHP treatment. Starch ages differently after UHP and heat treatments. Owing to the limited expansion of starch grains under UHP, almost no amylose is dissolved; thus, the starch grains age following UHP treatment. The aging rate of starch gelatinized by UHP was slower than that gelatinized by thermal processing. It was found that wheat starch was more sensitive to aging after heat treatment. Through these studies, it was found UHP treatment can be used to control the starch gelatinization and rejuvenation process, such that the physicochemical properties of starch are shaped in a specific direction. UHP starch gelatinization can be used to prepare amorphous or hypocrystalline starch; because this is particularly easy to digest, it can be used in foods for infants and the elderly as well as in patients with digestive dysfunction. Moreover, highly crystalline starch, such as microcrystalline starch, is slowly degraded in the large intestine because of its strong antidigestion properties; therefore, it can be used in slimming products. Moreover, because of the slow-release digestive properties of highly crystalline starch, which results in the slow release of glucose to maintain blood sugar levels after meals, it is a good choice for patients with diabetes.

2.2.2.2

Modification of Crop Protein

Protein modification methods include chemical, physical, and enzymatic methods. Chemical methods mainly include acid–base salt modification, acylation modification, deamidation modification, glycosylation modification, phosphorylation modification, alkylation modification, lipophilic modification, etc. Physical methods include mechanical treatment, extrusion, freezing, and other traditional methods as well as new technologies such as UHP, microwave, and ultrasonic treatments. Enzymatic modification includes enzymatic hydrolysis and protein-like reactions. Many studies have reported the application of UHP technology in protein modification. Proteins consist of polypeptide chains that make up their three-dimensional structure. The quaternary structure of proteins is very sensitive to pressure. Appropriate pressure (300 MPa has irreversible effects on proteins but not on covalent bonds. When the pressure is >700 MPa, the stress may affect the secondary structure of the protein. Protein solubility is an important index of protein quality. Poorly soluble proteins cannot be used efficiently. In a study on soybean protein isolate, sulfhydryl group content and surface hydrophobicity of the soybean protein significantly increased after UHP treatment. The subunit composition of soybean protein was clearly changed, with a significant increase in the content of 7S and 11S proteins. In addition, larger protein particles were found to be depolymerized into smaller particles under UHP, which enhanced the dispersibility of the solution. Similarly, in a study

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on peanut protein isolate, it was found that the solubility of peanut protein increased with an increase in pressure and pressure holding time. The gelation of proteins is an important index for solid protein food raw materials. It was found that UHP treatment could destroy the structure of protein colloids, leading to protein agglutination and thus changing the gelation characteristics of the protein. Some disulfide bonds in protein molecules were broken and the content of sulfhydryl groups was increased through UHP treatment, which improved the gel properties of the protein. Foaming is another important indicator of the suitability of proteins as food raw materials; after strong agitation, protein membranes are mixed with the surrounding air, leading to the formation of foam, and the protein surface tension makes foam spherical. In general, UHP technology can change the functional characteristics of proteins by changing the spatial conformation of proteins, changing the composition of groups on the surface of proteins, and unchain or dissociate proteins. However, protein structure is complex and UHP techniques are new to our country; thus, systematic studies and further improvement are required to meet different requirements for protein foods.

2.2.2.3

Effects of UHP on Allergens in Crops

Many ingredients in foods can cause allergies, and protein is a major allergen. The three-dimensional structure of allergen proteins is composed of polypeptide chains folded into each other. UHP affects noncovalent bonds and disrupts the tertiary and quaternary structures of proteins. Although further studies are needed on the effects of UHP on allergens, based on the modification effect of UHP on proteins, we believe that UHP can change allergen potency by changing its allergic epitopes. Milk, seafood, and some other foods are commonly associated with allergy, but many food crops also contain many proteins that cause allergies. For example, globulin in rice has high allergenicity and can easily cause allergic dermatitis, allergic skin osmosis, and other diseases in the human body. When a major cereal crop, such as rice, fails, the impact and damage are immense. Therefore, research and development of hypoallergenic cereal crops is critical, and the market potential is substantial. UHP treatment decreased the allergen reactivity of rice and damaged endosperm cells. Owing to the wall-breaking effect of UHP on cells, the allergen in cells are released and dissolved into the UHP medium, which promotes the elimination of allergens to a certain extent; however, the size of the elimination effect also depends on the solubility of the allergen in the medium. The catalytic action of an enzyme mainly arises from its active center. Studies have found that UHP affects the catalytic activity of enzyme by altering its tertiary structure. Because the tertiary form of the protein is the basis of the enzyme active center, when the tertiary structure collapses due to UHP, the amino acid composition of the enzyme active center changes or the enzyme loses its active center, causing its catalytic activity to change. However, when the enzyme activity began to increase at a low pressure, it was considered that the pressure produced the condensation of the

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enzyme activity center. Enzymes and substrates in intact tissues are often isolated, and low pressure can disrupt this isolation, allowing the enzymes to come into contact with the substrate, thus accelerating the efficiency of the enzymatic reaction. Many studies have reported the effects of UHP treatment on enzymes in animal products, such as proteolysis in egg white, endogenous enzyme activity in sea cucumber, and protein decomposition in grape juice, milk albumin, and soy protein. In plant products, UHP is often used to control the color and flavor of fruit and vegetable products, such as through the inactivation effect of UHP peroxidase, an inhibition effect of polyphenol oxidase in fruits and vegetables, and an effect on pectin methyl enzyme. Overall, it has been shown that the effects of UHP on food allergens vary. If UHP is to be applied to food to eliminate allergens, additional research is required. Although there is no effective method to completely eliminate allergens in all types of food, the combination of UHP technology and some other treatment methods can better meet this requirement. In addition, the effect of UHP on enzymes varies. Appropriate pressure treatment can improve enzyme activity or inactivate enzymes. However, further systematic research is needed to allow it to be applied to actual production.

2.2.2.4

Application of UHP Extraction in Crop Products

UHP extraction technology refers to the use of hydrostatic pressure on the extraction object at room temperature, maintaining the pressure such that the pressure inside and outside the cells of the raw material reach equilibrium (the active ingredients reach dissolution equilibrium) and then quickly depressurize, resulting in a sudden increase in the difference between the osmotic pressure inside and outside the cell; the active ingredients are transferred inside the cell through the cell membrane (the structure of the cell membrane changes under UHP) and then transferred to the extraction solution outside the cell, thereby achieving the purpose of extracting the target ingredients. At present, several studies on UHP extraction technology in China and and other countries have been conducted in this century and many achievements have been made, which has also clarified the advantages of UHP extraction technology. The extraction rate of rice protein was significantly improved at a pressure of 200–400 MPa, but it decreased slightly at a pressure of >400 MPa. It was found that the yield of soybean lecithin was higher under conditions of low pressure and short pressure holding time and UHP and long pressure holding time. Therefore, 360 MPa was considered the critical cutoff point between low pressure and UHP. Under lowpressure conditions, the extraction rate decreased with an increase in the pressure holding time; however, under UHP conditions, the extraction rate first increased and then decreased. UHP technology has many unique advantages in the extraction of active ingredients, such as high extraction rate, shorter time, lower energy consumption, and high product activity. UHP extraction is performed in a closed environment without solvent volatilization, which does not cause pollution to the environment and meets

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the requirements of green environmental protection. However, to realize the industrialization of UHP technology, the selection and residue of extractants still need further study. In addition, although UHP technology does not affect the structure of small biological molecules, it can affect the three-dimensional structure of starch, protein, and other biological macromolecules. However, compared with other techniques, UHP extraction has a less destructive effect on these macromolecules, and this mechanism needs to be further explored.

2.2.2.5

Influence of UHP on Sterilization and Bud Elimination of Crops

It is estimated that the annual loss of grain mildew alone accounts for >3% of the world’s total production, and some molds can also produce toxins, such as Aspergillus, which seriously affect the health of food consumers. Currently, microbial inhibition methods for grain storage mainly include fumigation, mildew control, and low-temperature storage. However, there are few reports on sterilization before grain storage using UHP. After UHP treatment, ATP or some ultraviolet absorbable substances in the cell leak out, while UHP treatment makes substances such as propyl iodide that are not absorbed by the cell enter the cell. Currently, there are few reports examining the control of mold using UHP; however, it is clear that the mold nutrients are sensitive to UHP, whereas that of Cystoisospora is relatively resistant to pressure. We studied the effect of UHP on the killing of Aspergillus oryzae on the surface of rice grains and found that at a pressure of >300 MPa, the survival of A. oryzae began to decline significantly, and this decline was more obvious with an increase in the pressure holding time. Pre-soaking rice before UHP treatment can also effectively improve the sterilization effect of UHP, and the longer the presoaking time, the more obvious the improvement. In addition, the authors found an interesting phenomenon: the survival rate of A. oryzae increased after a relatively low-pressure treatment of 100 MPa, suggesting that low-pressure treatment could induce the germination of mold spores. Regarding this point, similar studies have found that UHP treatment under 200 MPa can induce the germination of bacterial spores. Furthermore, UHP processing can effectively realize the dormancy of some rice and completely remove the need to germinate rice at a low pressure (100 MPa). Therefore, UHP can not only sterilize rice but also effectively prevent the germination of edible rice during storage, reducing the need for the control of environmental conditions for the antigermination of rice during storage. Although UHP can sterilize grains and kill buds in some bulk products such as rice, there is still a gap to be bridged for realizing its industrial application, as UHP technology is relatively dependent on equipment and the equipment processing capacity is small, resulting in a high processing cost. This is a difficult problem for all the above studies in the process of industrialization. However, the processing volume of UHP equipment is continuing to increase, and the technology develops with each passing day. We believe that the development of UHP technology will continue to expand.

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2.3 UHP Technology for Brown Rice Modification 2.3.1 Quality 2.3.1.1

Color

The color of food is an important indicator of its quality. UHP can improve the color of grains. Previous studies have shown that the brightness of nonwaxy and waxy rice increased with pressures of 300 and 400 MPa, respectively. UHP treatment of Panjin rice increased its whiteness, whereas that of Wuchang rice did not significantly change its brightness (L*), red/green degree (a*), and yellow/blue degree (b*). Some researchers studied the effect of UHP on the color of soy sauce. In addition, some studies have shown that UHP has no obvious effect on the color of grain. For pressures of 200 MPa and 300 MPa, the L* value of soy sauce was not changed, whereas the pressure is over 300 MPa, the L* value of soy sauce decreases, which is compared with that of 90 °C. Grain color is usually determined by some pigment molecules present in the rice bran layer. These pigment molecules do not only affect color but also many physiological functions. For example, zeaxanthin in corn has antioxidant and visual protection effects, and black pigment in black rice has antioxidant properties and can restore fatigue, reduce blood lipids, and protect the vascular endothelium. Therefore, it is essential to pay attention to the effects of processing methods on food color.

2.3.1.2

Texture

Texture, a key food quality indicator, is greatly impacted by different processing methods. UHP processing improves food taste while maintaining its original nutritional composition. UHP can be applied uniformly to the food; the application is not limited by the shape of the food. The texture of food is related to the structures of starch, fat, protein, and polysaccharides. UHP can alter the structures of these macromolecules, changing the texture of food. UHP treatment has been shown to reduce the hardness and increase the elasticity and cohesiveness of cooked rice and grains. The ratio of viscosity to hardness in rice was found to increase after treatment at 400 MPa for 10 min. Additionally, UHP treatment of brown rice has been reported to decrease its adhesive property in a pressure-dependent manner, although the effect of pressure holding time remains unclear. Moreover, previous studies showed that pressure holding time does not significantly affect elasticity of rice, but treatment at 500 MPa significantly reduced it; a similar effect has been reported for chewiness. Treatment of Thai fragrant rice at 300 and 400 MPa for 2 and 4 min, respectively, significantly improved its texture and sensory quality. UHP treatment has also been found to increase wheat dough hardness and adhesion. In addition, some studies have shown that UHP treatment

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can be used to make rice cooking more convenient by reducing its cooking time to 3–5 min. In addition, the wall-cracking effect of UHP technology has been studied for improving the quality of aged rice. Generally, the endosperm cell wall and starch film of new rice can be easily destroyed during the cooking process, causing the starch in the amyloplasts inside the endosperm cells to become fully paste-like, and this starch flows out and covers the surface of the rice grain, making it soft and sticky. However, after a long storage period, the water in the cells is lost and the starch film becomes tightly bound to the cell wall, which makes it more difficult to paste the starch and reduces the starch outflow; consequently, the rice is hard but not sticky and has a bad taste. UHP treatment caused partial destruction of the cells, such that part of the structure of the starch plasma membrane in the cell wall was destroyed, the swelling and gelatinization of starch particles were promoted, and the quality of aged rice was improved. Japan has been researching and developing high pressure metrics and products since the late 1980s, a much longer/shorter time than other countries. UHP was found to effectively improve the water absorption of rice grains and shorten the soaking time. The viscosity of rice cooked after the UHP treatment was enhanced. The UHPT nutritional convenience lunchbox (uncooked), produced by a Japanese company, only needs to be heated in a microwave oven for 2–3 min before eating, retaining the maximum nutrients while also providing a soft, sticky, and elastic texture with good taste. At present, the influence of UHP technology on the texture of food has focused mainly on the study of a single component, such as starch or protein modification, with few studies on the overall sensory quality of food. In addition, although food quality and structure can be measured objectively using basic mechanical testing instruments, there is a lack of testing processes that mimic the food in the mouth and chewing. Therefore, there is no comprehensive evaluation of food quality and sensory properties and personal preferences can lead to evaluation errors. Therefore, it is crucial for food processing enterprises to establish a comprehensive and systematic evaluation system for food texture.

2.3.1.3

Rice Aroma

The aroma components of rice are quite complex. Almost 300 types of aroma substances have been found in rice, including aldehydes, ketones, alcohols, esters, fatty acids, alkanes, alkenes, and heterocycles. More than 40 aroma components have been detected in brown rice and 55 aroma components have been detected in fresh cooked white rice. The aroma composition of rice is not only related to the type of rice, grinding degree, storage conditions, etc., but also to the production and processing methods. Only few studies have been conducted on the improvement of rice flavor using UHP technology. Seventy-seven volatile substances were identified in japonica rice and jasmine fragrant rice after UHP treatment. Different pressure levels and different

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rice varieties affected the volatile component profiles, notably alcohol–aldehydes and ketones; after 200 and 400 MPa pressure treatment, the content of alcohols, ketones, esters, and other aroma substances increased in japonica rice and jasmine fragrant rice, whereas that of heterocyclics, pyrohydrocarbons, aromatics, and other substances decreased. Acetaldehyde content is an important indicator of unpleasant odor in rice. UHP can reduce the content of acetaldehyde, which may be related to enzyme activity, and influence its UHP oxidation. Other studies have shown that UHP treatment can reduce the content of acetaldehyde, nonyl aldehyde, and octyl aldehyde. This may be related to the high voltage inhibition of lipoxygenase and hydrogen peroxide cracking synthase activity. The flavor of rice was significantly improved after UHP treatment. The aroma threshold value of 2-acetylpyrrolene is low (only 0.1 μg/kg), which can easily be lost during storage. Therefore, different processing methods have an important influence on the retention of flavor. UHP can destroy the structure of starch, thereby releasing the aroma component 2-acetylpyrrolene combined with starch and increasing its content. The mechanism through which UHP affects the variation in aroma species content has not been clarified. Some researchers speculate that UHP affects the activities of enzymes related to the synthesis or decomposition of aroma substances, subsequently affecting the generation or decomposition of aroma substances. Other researchers believe that UHP hydrolysis changes the structure and chemical composition of food, thus affecting the composition of aroma components. However, there is currently no systematic mechanism to fully explain how UHP treatment affects aroma composition; this warrants further research.

2.3.1.4

Starch Digestibility

Compared with other starch foods, rice has a relatively high GI. Many factors affect the digestion rate of starch in the digestive tract, such as amylopectin proportion, starch particle structure and size, starch crystal structure, crystallinity, starch chain polymerization degree, and processing process. Many researchers have studied the effect of UHP treatment on the physicochemical properties of starch grains, such as wheat, corn, barley, potatoes, chickpeas, puerarin, mung bean, sorghum, lentils, beans, and peanuts. UHP treatment has been found to reduce starch granule swelling, gelatinization temperature, and gelatinization enthalpy as well as change the crystal type of starch and structure and sensitivity of starch hydrolase. At the same pressure, the starch structure was damaged more in a two-step UHP treatment group compared with a one-step UHP treatment group. Moreover, the degree of starch gelatinization was higher, the surface protrusions disappeared, and the content of RS was reduced. Type A starch is more sensitive to UHP, type B starch is more resistant to pressure, and the characteristics of type C starch lie between those of type A and type B. In addition to the type of starch, the time and pressure of UHP treatment affect the digestibility of starch. Based on the rate of hydrolysis, starch can be divided into three types: RDS, SDS, and RS. RDS can cause blood glucose to increase rapidly after a meal, while SDS causes a slow increase in blood glucose levels during digestion after

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a meal, which is beneficial for achieving stable blood glucose levels and preventing diabetes, cardiovascular disease, and obesity. RS can reduce serum cholesterol levels, inhibiting the formation of gallstones and preventing cancer. Therefore, SDS and RS are important for human health. Some researchers have used microwave, citric acid modification, repeated regeneration, branched chain amylase hydrolysis, and other methods to improve the content of SDS. Compared with heat-induced gelatinization, pressure-induced gelatinization has lower swelling power and starch subjected to this method cannot be easily hydrolyzed by starch hydrolase. Studies have shown that UHP can inhibit wheat starch retrogradation. UHP could maintain the integrity of nonwaxy rice starch particles and damage the structure of waxy starch. Moreover, UHP could increase the SDS and RS content of wheat, corn, and glutinous rice starches. In addition, combining UHP technology with other technologies provides the possibility of controlling the gelatinization of rice products to obtain specific products desired by consumers. For example, easily digestible amorphous or low-crystalline starch can be consumed by infants, the elderly, and patients with digestive system dysfunction. Moreover, the antidigestive properties of high-crystalline starch indicate that it can only be slowly degraded in the large intestine; hence, it can be used to formulate weight loss products. Owing to its slow digestibility and release of energy, high-crystalline starch is conducive to maintaining the stability of blood glucose levels after meals, which is the best choice for patients with diabetes.

2.3.2 Physicochemical Properties 2.3.2.1

Starch Gelatinization Characteristics

The pressure gelatinization mechanism comprises two steps: first, the amorphous region is combined with water, which leads to particle expansion; in the second step, the crystallization zone is deformed, and the proximity between the crystallization zone and water improves, which eventually leads to the complete destruction of the particle structure. (1) Factors affecting the gelatinization of ultra-high-pressure-treated starch The degree of starch gelatinization under UHP treatment varies with processing conditions, such as pressure treatment level, pressure holding time, treatment temperature, and starch solubility. Meanwhile, the internal structure and composition of starch particles also affect the gelatinization process under UHP. Pressure and temperature are important factors that determine the gelatinization degree and gelatinization rate of starch during a UHP gelatinization process. At a given temperature, even for a temperature range of −20 to 0 °C, an increase in treatment pressure will increase the degree of gelatinization of the starch aqueous suspension. This may be because the increased pressure forces water molecules to penetrate the crystal structure of the starch, thereby promoting starch gelatinization.

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Although UHP treatment of starch granules is mostly conducted in the temperature range of 20–30 °C, at a given pressure level, an increase in temperature can also promote the gelatinization of starch granules in aqueous starch suspensions. This phenomenon may be due to the weakening of hydrogen bonds and/or the doublehelix structure of amylopectin molecules in aqueous starch suspensions at higher temperatures, making it easier for water molecules to enter the starch microcrystalline structure even at lower pressures (500 MPa), the temperature effect becomes a secondary factor in the UHP gelatinization of starch. The time for which the pressure (pressure holding time) is maintained is another main factor affecting the gelatinization of starch under UHP. Under certain pressures and temperatures, the gelatinization degree of starch granules increases with an increase in the pressure holding time. Below 300 MPa, UHP gelatinization of starch tends to slow down as the pressure holding time increases. However, at pressures exceeding 350 MPa, complete UHP gelatinization of starch takes almost 1 h, and the degree of starch gelatinization may remain unchanged or only slightly increase with an increase in the pressure holding time. In addition, the gelatinization of cereal starch grains tended to be complete within 10 min at pressures over 500 MPa. However, the X-ray diffraction pattern, gelatinization degree, and gelatinization viscosity did not change significantly when starch particles were treated at 690 MPa for 5 min and 1 h. The gelatinization enthalpy was similar when starch particles were pressurized for a long time (1–66 h) at a given pressure level (700, 800, and 1000 MPa). Because of the effect of UHP on starch granules, their structure is primarily composed of alternating crystalline and amorphous regions based on the order of crystal arrangement. In X-ray diffraction analysis, the crystal region is characterized by sharp peaks and the amorphous region is characterized by dispersion. X-ray diffraction analysis showed that starch molecules could be divided into three types: A, B, and C. A-type starches, mainly cereal starches, have strong diffraction peaks at 2θ values of 15°, 17°, 18°, and 23°. B-type starch, which is the abundant carbohydrate in tubers and amylose, has a strong diffraction peak at a 2θ value of 17° and weak diffraction peaks at 2θ values of 20°, 22° and 24°. C-type starch is a mixture of A- and B-type starch patterns and is mainly found in legumes. A-type starch is the most resistant to pressure. When the pressure exceeds 200 MPa, the crystal structure is destroyed, and B-type structure appears. When the pressure reaches 650 MPa, the crystal structure of A-type starch is completely destroyed. B-type starches are more resistant to stress than A or C-type starches. At pressures of 200–450 MPa, the crystallinity is slightly strengthened; beyond this pressure range, the B-type starch crystal structure is gradually destroyed. When the pressure reaches 750 MPa, the B-type starch crystal structure is completely destroyed. The obvious difference in pressure resistance between A-type and B-type starch particles can be explained as follows: in the double-helix structure of amylopectin, the helical column of water molecules in the regular B-type crystal structure may have lower compressibility than the water molecules with randomly distributed A-type crystal structure. In addition, the difference in the pressure tolerance between A-type and B-type starch grains may be due to the difference in the branched structure of amylopectin between these

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two starch grains. It has been speculated that compared with B-type, A-type starch granules have a more branched structure of the amylopectin dispersion, such that its crystal structure is more flexible. Therefore, the A-type crystal structure is more sensitive to UHP treatment than the B-type crystal structure. It was reported that some A-type starches (such as rice, corn, and wheat starches) were transformed into B-type starches after UHP treatment, whereas B-type starches (such as potato starch) retained their original crystal shape. In a study of lotus seed starch, the X-ray diffraction peaks of natural lotus seed starch appeared at 2θ values of 14.86°, 16.96°, 17.75°, and 22.82°, indicating that the natural lotus seed contained C-type starch. When starch granules were subjected to treatment with 500 MPa for 30 min, the X-ray diffraction pattern did not change significantly. However, the peak intensity at 17.75° disappeared and the crystallinity decreased at 14.86° and 22.82° after the application of higher pressure. The X-ray diffraction results showed that the lotus seed starch structure changed from the C-type to the B-type, and the internal crystal structure was significantly destroyed by UHP treatment. The content of water also affected the crystallinity of starch under UHP treatment. The crystal structure of corn starch was studied using an X-ray diffractometer. It was found that with an increase in pressure and water content, the peak value of X-ray diffraction pattern decreased, the peak shape of characteristic peak of corn starch disappeared gradually, the crystallinity decreased, and the gelatinization degree increased. Some researchers found that there was no significant difference in the X-ray diffraction patterns of UHPT1 (740–880 MPa, 5 min–2 h), UHPT2 (960–1100 MPa, 24 h), and UHPT3 (1500 MPa, 24 h) after three UHP treatments with a water content of 70%) decreased the gelatinization temperature of starch.

2.3.2.2

Rheological Characteristics of Starch

The rheological properties of food depend on its chemical composition, molecular structure, molecular interactions, and dispersion system. The rheological properties of starch are closely related to the quality parameters of starch-based foods, including hardness, viscosity, and chewability. Rheological properties therefore have crucial effects on processes such as transport, stirring, mixing, and energy consumption. These properties are sometimes used as a measure of product quality (a measure of change in total solids or molecular size). Rheological data are important for the calculation of processes involved in fluid flow (e.g., pump sizing, extraction, filtration, extrusion, and purification), the description of heat transfer, and the design, evaluation, and modeling of continuous treatments such as pasteurization, evaporation, drying, and asepsis. UHP treatment can significantly affect the structure and rheological properties of starch, which affects the application of starch in food systems. The gelatinizing viscosity of the (relatively natural) starch treated with UHP was reduced, probably because UHP treatment limits the expansion of starch particles. In contrast, the gelatinization temperature of waxy corn starch decreased and the peak viscosity increased. The formation of complex structures between amylose and the lipids of amylopectin and amylose can stabilize the structure of starch granules. These findings may be partially attributed to the lack of amylose and lipids. In addition, the strength of the starch gel obtained by UHP gelatinization was weaker than that

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of hot gelatinization gel, and the dissolution of starch particles was reduced during UHP gelatinization. Some researchers studied the changes in the initial apparent viscosity of normal rice and glutinous rice starches treated under different UHP conditions. Their results showed that the initial apparent viscosity of both rice starches increased with an increase in pressure. The viscosity of starch was also increased by prolonging the holding time and increasing the treatment temperature. The rheological properties of starch paste also varied with water content for a constant pressure and treatment time. When barley starch at a concentration of 10% was subjected to UHP treatment (400–500 MPa), the viscosity of the paste increased less than that of 25% barley starch. However, with an increase in pressure, the G, and G,, of wheat starch paste tended to increase, but pressure had a larger influence on G, , leading to a decrease in starch paste tanδ (G represents the energy storage modulus, G,, represents the energy dissipation modulus, and tanδ is the loss factor). Some researchers compared UHP processing and the influence of hot work on the rheological properties of the rice starch paste; their results showed that after pressure treatment, the consistency coefficient of rice starch and the energy storage modulus were significantly increased, higher than those of rice starch subjected to hot work treatment. These results show that the combination of UHP processing and hot work can be considered as a new technology for starch products, which will improve the texture properties and stability of powdered products.

2.3.2.3

Moisture

NMR is a spectroscopic technique based on the magnetic properties of atomic nuclei. It is a modern instrumental analysis method used for the identification of the structure of organic compounds and the study of chemical kinetics. The basic principle is that the nucleus becomes magnetized in a magnetic field, conferring spin angular momentum; then, when the external energy (radiofrequency field) is the same as the vibrational frequency of the nucleus, the energy absorption of the nucleus occurs as an energy level transition, and the resonance absorption signal is generated. This time interval is the time between two important parameters of NMR, namely, the spin–lattice relaxation time T1 and the spin–spin relaxation time T2. NMR technology in food science research was first used to study the state of water in food. The ‘H nucleus in water is one of the magnetic nuclei often analyzed using NMR. Water in food directly affects the appearance, structure, rheological properties, and sensitivity to microorganisms of the food system. Therefore, methods to accurately measure and control the moisture content of food has become one of the key links in food processing and preservation. The transition time of ‘H is usually of the order 10−12 s, and NMR can measure the water content and dynamic structure at the 10−8 s scale. NMR technology can therefore analyze the water content, water distribution, migration, and other related properties of substances by measuring the longitudinal relaxation time T1 and transverse relaxation time T2 of hydrogen nuclei in a magnetic field.

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As the T1 and T2 parameters are related to the rotation of water molecules, they can be measured to determine the flow and structural characteristics of different types of water molecules (including bound and free water) that are partially immobilized by the substrate. Usually, as the moisture content of food increases, the water mobility increases and the T1 value increases. Macromolecules that exhibit strong adsorption to water have lower T1 values due to the limited mobility of water. In the determination of T2, the signal curves have a clear two-phase difference at higher moisture contents. A fast-decaying component, with a maximum time constant T21, is between 7 and 10 ms, and a slow-decaying component, with a maximum time constant T22, is between 160 and 300 ms; the fast decay component T21 is characteristic of the nucleus and biochemical polymers, that is, the bound water part, indicating that T21 is mainly related to the relaxation time of the protons bound by the macromolecules, and T22 is mainly related to the water protons and –OH exchanged with water. Therefore, the migration of macromolecules can be studied by measuring T1 and T2, and the migration of water molecules themselves and their influence on the migration of macromolecules can be studied. Some researchers have used NMR to determine the proton spin–spin relaxation time (T2) of three different bread formulations during the processing process (dough mixing, fermentation, rising, and baking). The experiments show that the migration behavior of bound phase and free phase is different in the process of bread making. The mobility of the T21 part, namely, the “bound water” part, follows a downward trend, and its content increases slightly in the first three stages, and decreases significantly in the baking stage. The fluidity of the T22 part, namely the “free water” part, follows a downward trend in the first three stages, and rises in the baking stage, and its content follows an upward trend. Some researchers have also used magnetic resonance imaging (MRI) to measure the changes in water content (including ice content) in bread during prefreezing and freezing and found that NMR could be used to distinguish different binding states of water in the frozen state. The results showed that the local ice content was proportionately reduced during freezing. Two types of cheese, made with dough and cooked dough, respectively, were stored at room temperature for 10 days. The water activity of the cooked dough cheese was less than that of the cooked dough cheese measured by NMR. Moreover, the increase in T2 and T1 values during storage indicated that water activity increased and the water holding capacity decreased, which was considered to be the result of protein matrix transformation. Through rheology, NMR, and electron microscopy, it was found that when the dough was compressed into a sheet, the gluten network structure was formed and broken, increasing the mobility of water molecules. After the dough was formed, the network structure was restored, and the mobility of water molecules decreased. Pulsed low-field NMR was used to determine the distribution of water in the dough and the change in kinetic properties during the baking process (dough to bread). The T2 parameter in the baking process curve displays heterogeneity, which can be divided into the change in three components (corresponding to different water level, mild combined water increase, firmly bound water drops, water saturation), with two major shifts observed (gelatinization starch soluble rise early, late pasting swelling absorption, respectively) at 55 °C (corresponding to the starch gelatinization

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temperature) and 85 °C. In a study of bread aging, the texture parameters (hardness and elasticity) of breadcrumbs were correlated with NMR relaxation data, and the results showed a strong correlation. Using MRI technology, the distribution of water can be directly observed from a slice image of the food. Through improvements in this dynamic information, new mathematical models can be developed; this is of great significance for improving the processing conditions of food and improving the quality of food products. Using this method, researchers have performed extensive studies, including the study of soybean seed coat rupture during storage, the study of rice hardness and water mobility changes during cooking and the study of the effect of rice on its shelf life; moreover, MRI was applied to obtain 3D images for the quantitative determination of the distribution of water in food and the study of water mobility of bread during storage. These studies provide valuable information for further research into food characteristics using NMR technology.

2.4 Other Modification Techniques for Brown Rice 2.4.1 Milling In traditional processing, brown rice is usually ground to completely remove the bran layer and produce white rice. Because the bran layer of brown rice is dense and contains α-amylase inhibitors, it is indeed the most direct way to promote the absorption of water and improve the cooking, eating and digestion quality of brown rice. However, the distribution of nutrients in brown rice is very uneven; approximately 64% of the nutrients accumulate in the cortex, aleurone layer and embryo, accounting for 10% of the weight of brown rice. Therefore, deep milling to produce white rice is a process that improves taste at the cost of losing nutrients. The gradual removal of the bran layer is accompanied by continuous changes in the nonstarch components and the integrity of the fibrous pericarp of brown rice, so the physicochemical properties of brown rice can be changed accordingly. To seek a better balance between nutrition and taste, many researchers have studied the effect of different degrees of milling treatment on brown rice, and found that excessive milling cannot create continued improvement in taste. Therefore, the selection of an appropriate DOM (degree of milling) in milling processing and effective control can not only effectively improve the taste of brown rice, but also better retain nutrients, improve production efficiency and reduce energy consumption, which is valuable for further research.

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2.4.2 Germination and Pregelatinization The germination of brown rice is a physiological process in which the pre-soaked brown rice is cultivated for a certain period under specific conditions of temperature, humidity, oxygen and light to make the germ grow to a proper length. During this process, endogenous enzymes such as amylase, protease and phytase are activated to enzymatically decompose the part of starch and crude fiber into smallmolecule sugars, decompose part of protein into amino acids, hydrolyze phosphate residues from phytic acid, and increase the bioavailability of mineral elements. Many researchers have found that in brown rice, the bioactive component content is higher after germination treatment, such as GABA, oryzanol, ferulic acid, and xylan. In addition, the content of amylose and relative crystallinity of brown rice were decreased by germination, and small holes and pits appeared on the surface of starch grains, and the grain size decreased with the increase in germination time. It was found that the cooking time of germinated brown rice was lower, the volume expansion rate and water absorption rate were significantly increased, and the sensory evaluation results showed that germinated brown rice was sweeter, softer and fluffier. Pregelatinization is a hydrothermal treatment technology involving three processes: soaking, steam heat treatment and drying. After pre-cooking, brown rice starch is partially melted into gel state, and the denatured protein and gel starch penetrate into the gaps in the endosperm to tighten the structure closer, increasing the hardness of brown rice, which is conducive to milling and processing, so the yield of white rice can be increased. The pregelatinization process can promote the molecular migration between the inner and outer layers of brown rice: the outer rice bran and the nutrient elements in the embryo migrate into the endosperm, and the water-soluble pigment molecules in the endosperm diffuse out of the rice bran. In addition, some studies have used rice soaked in Fe-EDTA solution combined with steam pregelatinization technology to prepare rice nutritionally fortified with iron, and steam pregelatinization technology to produce rice suitable for patients with diabetes. Researchers have combined germination with pre-gelatinization techniques. The effects of precooking on the milling characteristics, physicochemical properties and texture properties of medium-grain and long-grain brown rice with bud have been studied. Pre-cooking was found to significantly reduce the damage rate, whiteness value and amylose content of brown rice with bud, and increased the content of GABA. It was found that compared with brown rice and white rice, germinated brown rice had higher content of protein, GABA, γ-oryzanol, γ-tocotrienol, ferulic acid, and p-coumaric acid. The biggest problem during the process of brown rice germination is that microorganisms (mainly mold and bacteria) also proliferate rapidly, which requires use of water flushing, water electrolysis, ultraviolet irradiation, sodium hypochlorite immersion, active antibacterial skin treatment methods, etc. Even so, these can only cause temporary inhibition of the growth of microorganisms during germination. During the germination of brown rice by steam pregelatinization treatment, microorganisms

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will be killed by high temperature, solving this issue. However, pre-cooking can also have some negative effects, such as the Maillard reaction between reduced sugars and proteins, which causes brown rice to become more dark and yellow, and brown rice skin can easily to crack after pre-cooking. All these defects will be visually apparent to consumers. In addition, the hardening of the texture of pre-cooked brown rice affects the taste and, inevitably, some reduction in heat-sensitive nutrients occurs.

2.4.3 Freeze–Thaw Treatment Freeze–thaw treatment is a process in which the material is first completely frozen and then thawed, and the formation of ice crystals is used to change the structure and properties of the material. Under normal circumstances, the higher the moisture content, the longer the freezing and thawing times, and the more obvious the effect. Freeze–thawing can improve the porosity of food and largely retain the color, aroma and taste of food to a large extent. It is an advanced nonthermal food processing technology. It was found that the freeze–thaw technology could be used to improve the taste quality of brown rice. In this process, brown rice was soaked at 40 °C for 1 h, dried, frozen at −23 °C for 1 h, and thawed at 40 °C for 30 min. After three freeze–thaw cycles, the sensory score of brown rice reached 69 points (compared with 50 points in the untreated group), the water absorption rate in rice and the iodine blue value in rice soup were also significantly improved. Moreover, studies have examined the effect of a single freeze–thaw cycle on moisture migration in brown rice moisture migration and the influence of the microstructure; LF-NMR results show that the freezing and thawing of brown rice combined water transverse relaxation time and proton density to achieve stable 1/3 time for the untreated group, showing that freezing treatment increased the brown rice water absorption rate, and scanning electron microscopy (SEM) results confirmed increase brown rice starch granule clearance also generated on the surface of rice bran. Some researchers have explored the effect of freeze–thaw cycles on other materials. For example, the effect of freeze–thaw cycles on the mechanical strength of corn grains was studied. The change in the mechanical properties of fresh and air-dried corn grains was negligible after 16 freeze–thaw cycles. The effect of freeze–thaw treatment on the structure and properties of starch gels was also studied. Starch presented an orderly network structure after freeze–thaw treatment. As the number of cycles increased, the network void became larger and the water evolution rate of starch gels decreased. Other researchers reported that freeze–thaw technology could improve the yield of nail burn during anaerobic digestion of rice straw. The results show that freeze–thaw processing can destroy hydrogen bonds in rice straw, cause dense and hard lignocellulose to become loose, shorten the anaerobic digestion time from 23 to 15 days, and increase the yield of nail burn by >twofold. Freeze–thaw processing also has many limitations and shortcomings. For fresh ingredients such as fruits, vegetables, meat and seafood, freeze–thaw treatment can

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cause many adverse effects, such as tissue softening, browning, increased drip loss, and decreased water-holding capacity and tenderness. During the processes of storage and transportation, temperature fluctuations will affect the texture and sensory quality of such materials, so freeze–thaw cannot be used to process these materials. In addition, freeze–thaw cycles can cause brittle materials to crumble, because, during the transition between freezing and melting, drastic temperature changes produce internal stress in the material; in serious cases, the overall structure will be destroyed.

Chapter 3

Fast-Growing Forest and Related Introduction

Abstract Wood is an important raw material in the fields of construction, home furnishing, handicrafts, etc. However, due to its loose material, low mechanical strength, and many defects, it is necessary to modify wood in practical applications. In this chapter, research advances in various wood modification techniques such as transverse densification modification, impregnation modification, heat treatment modification and composite modification are first presented. In addition, the current status of research on the densification and impregnation of wood by high pressure modification is highlighted. Keywords Wood · Modification · Densification · Impregnation

3.1 Fast-Growing Forest 3.1.1 Introduction to Fast-Growing Forest Alongside the development of the national economy and society, the idea of harmonious coexistence between man and nature has become deeply rooted in people’s hearts. Adhering to the concept that “clear waters and lush mountains are gold and silver mountains” and promoting green development are important guarantees for further implementing strategies for sustainable development. Improving the quality and stability of ecosystems has been included in the National Development Plan of the 14th Five-Year Plan, which clearly states that comprehensive control of desertification, rocky desertification and soil erosion will be promoted in a scientific way; largescale afforestation initiatives will be launched; and a forest growth system will be introduced. Based on a policy of completely stopping commercial logging of natural forests, accelerating the rapid development of fast-growing forest timber industry and improving the efficient utilization of fast-growing forest timber resources are important measures to alleviate the contradiction between supply and demand in domestic timber market.

© Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_3

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In China, the average annual output of fast-growing forest wood is approximately 70 million cubic meters, and the planted area ranks first in the world. Fast-growing forest has a wide range of timber planting, covering the whole country, with a wide variety of trees (including poplar, fir, and paulownia) that can be rapidly grown in more than a decade or even a few years. Fast-growing poplar is one of the main fast-growing forest varieties in China, and the artificial planting area of the country is 7,572,300 ha, accounting for 18.93% of the country’s total plantation area. It has various advantages, including short growth cycle, large yield and high survival rate, and is widely used in papermaking, decoration, furniture and other industries.

3.1.2 The Main Challenges of Fast-Growing Forest Development Compared with high-quality wood from natural forests, wood from fast-growing forests has defects including loose material, low density and poor mechanical properties. These defects render the fast-growing forest wood unsuitable for use in construction and other industries that require good material properties, which greatly limits the development and utilization of fast-growing forest wood resources at the present stage, and reduces its commercial value. Therefore, scientific modification of fast-growing forest wood to improve its quality and applicability is an emerging trend in industry, as well as an important means of efficient development and utilization of fast-growing forest wood resources.

3.2 Research Progress in Wood Modification Technology High-quality timber is a scarce resource; it cannot meet the demand of a continuously expanding market. The efficient and value-added use of rapid growth forest resource is an important measure to resolve the contradiction between supply of and demand for Chinese timber. Wood modification technology refers to the material optimization of low-quality, soft fast-growing forest by physical or chemical means, aiming to improve its specific properties and expand the development and utilization of fastgrowing forest resources. At present, research into wood modification technology at home and abroad is mainly focused on heat treatment, impregnation modification and densification of transverse grain.

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3.2.1 Transverse Densification Modification Technology Wood is mainly composed of cellulose, hemicellulose and lignin. It is a material comprising natural macromolecules, and its porous structure will be squeezed and deformed under certain external forces. Wood compression modification technology can make use of the above characteristics of wood densification strengthening treatment: without destroying the integrity of wood cell wall, increasing the content of wood cell wall and other parenchyma per unit volume improve its density and mechanical properties. At present, most wood compression techniques use transverse grain compression, that is, the pressure is applied perpendicular to the direction of the wood grain. Owing to the anisotropy of wood structure, transverse compression can be divided into radial compression and chordal compression. Radial pressure is applicable to all coniferous and broadleaved forests, whereas chordal compression is only applicable to the bulk timber in broadleaved forests. At present, overall densification technology, surface densification technology and shaping compression technology are the mature wood compression modification technologies.

3.2.1.1

Integral Densification Technology

Wood densification technology first appeared in the 1930s and 1940s. In 1930, a compressed wood commodity named Lignostone was used in Germany. Its preparation process was simple: wood was subjected to radial compression treatment by two metal plates for 2 h under high temperature and UHP (140 °C, 250 kg/cm2 ). In the 1940s, researchers from Kyoto University in Japan also prepared compressed wood from beech and comb wood by means of high temperature and pressure. In the early stage, hot pressing modification technology is simple: the hot pressing plate is usually used to compress and finalize the wood under the condition of high temperature and pressure. However, the physical and mechanical properties of the compressed wood are show relatively little improvement, with defects such as poor dimensional stability and high recovery. From the study of the composition and properties of wood, it was shown that hemicellulose, lignin and cellulose and other macromolecules will change from glass state to high elastic state at a certain temperature, which enhances the plasticity of wood. In addition, water, urea, liquid ammonia and other chemicals can penetrate into the amorphous zone of cellulose, hemicellulose and lignin to play a role in moistening and swelling, increase the free volume space, provide space for molecular movement, help to reduce the glass transition temperature, and enhance the softening and plasticizing effect of wood. Some achievements have been made in whole compression modification technology. Thermomechanical treatment (TM) and thermo-hydromechanical treatment (THM) are key topics of research at present. TM modification was used to explore the influence of pre-pressing moisture content on the compression recovery of Japanese cedar. The results show that under the same treatment conditions, the deformation recovery of compressed wood decreases as increase water content increases, and

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the compression recovery rate was lowest for water-saturated samples. Studies have used THM to densify mountain hair sticks, spruce, and pine. The results show that the softening treatment with steam at 150 ºC for 10 min significantly increases the compression rate and physical and mechanical properties of wood. The degree of compression and deformation of the cell lumen was significantly increased, reaching a closed or semi-closed state, with no obvious crack or fracture in the cell wall. Some researchers developed a viscoelastic compression densification technology (VTC) for thin plates (10 mm and below) based on the modification of hydrothermal mechanical compression. VTC is a water vapor softening treatment and compression operation between the addition of 100 s steam in a rapid-release process; the rapid release of steam will cause instantaneous water loss in the wood and a temperature drop, producing a “mechanosorption effect”, further reducing the glass transition temperature of polymer material and improving the plasticity of wood. In the same steam release treatment, the initial thickness of the sample will affect the moisture and temperature distribution gradients in the thickness direction, resulting in different degrees of wood softening in the compression direction, causing a different density distribution of compressed wood products. Therefore, VTC can control the profile density of products and change their mechanical properties such as hardness and flexural strength. There are three main modification technologies of transverse compression: softening, compression and shaping. Softening treatment can improve the plasticity of wood through hydrothermal effects, reduce the damage and destruction to the cell structure in the process of compression, and reduce internal stresses. The shaping process is the main measure to solve the elastic deformation of compressed wood. It reduces the temperature of compressed wood before releasing the load, so that the materials such as hemicellulose and lignin can be cooled and transformed into glass again, and act as a fixing microfiber.

3.2.1.2

Surface Densification Technology

Surface compaction is a new compression modification technology that only causes a compression layer on the wood surface and does not change the structure of the wood core layer. In the 1990s, researchers first reported that when the dried Cryptomeria japonica var. sinensis was soaked in water, the wood surface absorbed some water, could be softened by microwave heating, and then compressed and finalized. Surface compaction technology mainly uses the difference in wood softening and plasticity under different humidity to compress the surface structure of wood. It has been found that the glass transition temperatures of hemicellulose and lignin were closely related to the water content of wood: for a water content of 0%, the glass transition temperatures of hemicellulose and lignin were 200 °C and 150 °C, respectively. When the water content of wood increased to 20%, the transition temperatures of hemicellulose and lignin both decreased significantly (100 and 80 °C), and when the water content of wood increased to 60%, the transition temperature of hemicellulose decreased to 20 °C, whereas that of lignin was 70 °C. It is worth mentioning that the

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glass transition temperature of cellulose is 200–250 °C, which is unaffected by the water content of wood. At present, surface compaction technology mainly uses a single-side or doubleside compaction process, which can reduce the volume loss of wood compression as much as possible and reduce the production cost while meeting the performance requirements of a product. Factors such as compression rate, hot pressing temperature, initial moisture content, compression time and compression rate are important process parameters for surface compaction, which affects the density distribution and microstructure of wood, subsequently altering its mechanical properties, such as hardness. The results showed that when the compression ratio increases, the peak density and average density of compressed wood increase, and the compaction area becomes wider. When the compression rate decreases, the peak density of compressed wood decreases, its distance from the surface increases, and the compression shape variable increases. When the hot pressing temperature increased, the peak width of wood density characteristic peak increased, and its distance from the surface increased. However, the effect of holding time and initial water content of wood is relatively small, and the extension or increase of these two parameters will increase the wood compression deformation. Some researchers optimized the surface compression process and developed laminated wood compression technology, which was able to form compression layers at any position inside the wood. The preheating time is the key control point of this technology; it affects the local softening of wood by controlling the distribution of water and temperature, and realizes the movement of compressed layer. After soaking, the initial moisture content of the surface of Populus tomentosa was high, but when the wood was preheated, the surface water evaporated quickly or penetrated to the core layer. Concurrently, heat is transmitted from the outside to the inside, raising the temperature inside the wood. Under the action of heat, the area with higher moisture content of wood softened first. As the preheating time was increased, this area moves continuously towards the core layer; that is, the compressed layer moves towards the interior of the wood.

3.2.1.3

Shaping Compression Technique

In the early 1990s, a Japanese scholar first proposed technology to modify shaping and compression, which took advantage of the thermal plasticity of wood to soften, compress and shape logs by heating, and therefore directly process logs into materials with specific shapes. In 2000, domestic researchers also developed their own flexible wood compression molding equipment and mold, which softened logs using hightemperature water vapor, shaped and compressed along the diagonal direction, and prepared flexible wood with a square cross section. The results show that the plastic wood has low moisture absorption, water absorption expansion rate and deformation recovery rate, with good dimensional stability. Compared with the material, the density, surface hardness and wear resistance of the plastic wood are significantly improved. In 2004, some researchers have realized production process of molded rectangular shaping wood (nonsquare section), which is more difficult to make. The

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improved shaping wood mold can exert pressure to the log along the long axis and short axis of the molding, which avoids the ring cracking of rectangular-shaped wood. The average density of molded rectangular material increases with an increase in compression rate, but the density distribution is different in the direction of long axis and short axis. Along the short axis, the density decreases from the surface to the center, whereas along the long axis, there is a complex density distribution, which is higher at the midpoint of the surface layer and near the side, and lower in the other parts. A modification technology for shaping compression not only avoids sawing, cutting and other traditional wood processing procedures, but also improves the yield and utilization rate of small- and medium-grade logs such as young wood and thinning forest, thus improving the material, expanding its application value and scope, which brings high economic value and social benefits.

3.2.2 Impregnation Modification Technology Wood is a natural, porous material. Its internal pores are mainly composed of a macrocapillary system (cell lumen, striated pore and cell end perforation) and a microcapillary system (intercellular space and cell wall space). Impregnation modification technology uses the porous structure of wood, incorporating inorganic or organic compounds into the wood interior, through heating, curing and other means, so that a soluble agent is impregnated into insoluble substances or deposition in the cell cavity, cell wall or other wood spaces; alternatively, it can promote substitution or crosslinking reactions with the main components in the cell wall, such as cellulose and hemicellulose, to change the chemical composition of wood. Impregnating modification technology can be based on organic material or inorganic material.

3.2.2.1

Impregnation Modification of Organic Matter

There are many types of organic matter used for impregnation modification; this is a well-studied topic. Some researchers studied the reaction between acetonitrile and wood, and found that after acetylation treatment, the hydroxyl groups in the wood cell wall become esterified, reducing the equilibrium moisture content and the fiber saturation point of wood, and improves the dimensional stability of wood, but the emphasis was on acetylation. The reaction is a single ligation reaction without polymerization, and the acetylation treatment causes no obvious improvement to the mechanical properties of wood. Some researchers used acetic acid crisp, n-butyric acid crisp and hexanoic acid tincture to chemically modify maple tree. The results show that the acylation treatment can effectively improve the dimensional stability of maple wood, and Fourier transform infrared spectroscopy confirmed that the intensity of the light absorption peak of the treated maple wood is significantly weakened. Previous studies of four esterification methods, including maleic anhydride, lauric acid, stearic acid and oxalic acid/hexadecanol, found that the dimensional stability of

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modified poplar was significantly improved and its surface changed from hydrophilic to hydrophobic, reducing the surface wettability with water. In addition, the effect of thermosetting resins such as phenol resin (PF), melamine formaldehyde resin (MF) and dolphin formaldehyde resin (UF) on softwood have been studied. The results showed that the dimensional stability of coniferous forest was significantly improved, and its flexural strength and flexural modulus of elasticity increased slightly. After impregnation and modification with low-molecular-weight phenolic or porphyrin resins, the dimensional stability and natural durability of Chinese fir and poplar were significantly improved. The mechanical properties of oak are significantly improved by modification with styrene or styrene-glycidyl methacrylate: compared with the original material, the hardness of the modified oak is increased by 33% and 32%, respectively; the flexural strength is increased by 97 and 86%; the flexural elastic modulus increased by 121 and 140%. In addition, styrene-glycidyl methacrylate impregnation treatment of oak can greatly improve its dimensional stability. The predecessors impregnated isopropyl glycidylether in Chinese fir and found that the corrosion resistance and lightfastness were significantly improved after etherification treatment. Some researchers found that palmitoyl chloride esterification improved the lightfastness and dimensional stability of rubber wood. In addition, some researchers combined glutaraldehyde and polyvinyl alcohol to modify pine wood, and the antiexpansion rate and mechanical properties of the treated wood are determined by the ratio of the two impregnating agents. In general, organic impregnation modification can improve the dimensional stability, mechanical properties, light resistance and corrosion resistance of wood in a targeted manner, although the use of organic impregnants (except acetic acid, maleic acid and other acid crisps) in the production will produce substances that are harmful to the environment and human body, destroy the original environmental protection characteristics of wood, and do not meet the requirements for “green” environmental protection and sustainable development. The development and synthesis of a nontoxic and efficient organic impregnant are the current research focus of impregnation technology.

3.2.2.2

Impregnation Modification of Inorganic Materials

Compared with organic impregnation, there are relatively few studies on inorganic impregnation; the research has focused mainly on inorganic salt impregnation, inorganic oxide impregnation, and similar. The two-agent diffusion method is the predominant method for inorganic salt impregnation. The wood is immersed in two pre-prepared soluble inorganic salt solutions, which enter the wood through the alternating diffusion of cations and anions, and react to form insoluble inorganic salt precipitates and fill in the cell cavities and other voids. Some researchers have used water–glass and aluminum sulfate to treat Populus tomentosa successively. The results showed that aluminum ions reacted with silicate ions to form aluminum sulfate precipitates, which filled the wood voids, such as cell cavities and microfibril gaps, and improved the dimensional stability, flame retardancy and hardness of wood. The predecessors used sodium carbonate and calcium chloride solution to impregnate

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poplar and found that the mechanical properties, such as flexural strength, elastic modulus and hardness, were increased in poplar modified by calcium carbonate. After the veneer was impregnated with nano-silica, its antibacterial properties were enhanced. Some researchers have used the full cell method to impregnate Chinese fir with silica sol. The results showed that the coefficient of expansion resistance and moisture resistance of Chinese fir/silica composites were increased by 32.6% and 14.8%, respectively, which improved the dimensional stability of Chinese fir. Inorganic salt impregnation does not chemically react with wood, but only plays a simple physical filling role, whereas inorganic oxide impregnation can interact with wood hydroxyl groups through certain media in addition to simple physical filling effect. In general, inorganic impregnants cause little pollution to the environment and human body, but their modification effect is not as good as that of organic impregnations, and there are fewer types compared with organics, which limits their development further.

3.2.3 Heat Treatment Modification Technology Thermal modification of wood is an environmentally friendly method that does not add any harmful substances. It is performed at a high temperature (160–260 °C) for a designated time under the special environment (low-oxygen or oxygen isolation). The application of heat treatment in wood has a long history. In ancient times, craftsmen carbonized wood posts and the parts in contact with the ground to enhance their durability. Research into modern wood heat treatment modification technology began in Europe in the twentieth century. Some researchers used high temperature treatment to reduce moisture absorption, balance moisture content of wood, and improve its dry shrinkage and moisture expansion. Both dry and wet wood were treated with high temperature. Heat treatment clearly improved the moisture absorption of dry wood, but the moisture absorption of wet wood was not obviously different before and after heat treatment. After decades of research and exploration, Finland, France, Germany and other countries have successfully realized the industrial application of wood heat treatment. The heat treatment process varies in different countries. The differences are mainly reflected in the heat transfer medium, initial moisture content, and heat treatment temperature and time. In Finland, thermo wood process uses water vapor as the heat transfer medium in an environment with low oxygen and high temperature (air content to be controlled within 3.5%, the treatment temperature was 185–215 °C), and can be divided into three stages: traditional drying, heat treatment and cooling. The French rectification process uses ammonia as the heating medium (oxygen content less than 2%) to treat air-dried material (moisture content approximately 12%) at a high temperature (210–240 °C) in special equipment. This technology requires precise temperature control to avoid affecting the material quality of the final product. The German oil heat treatment process uses vegetable oils (e.g., soybean oil, canola oil and linseed oil) as the medium for heat treatment of wood (210–220 °C). The heat transfer efficiency of vegetable oil is high, and the wood can

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be isolated from oxygen contact, but this will affect the smell and color of the treated wood. The Dutch Plato process uses steam as the medium to treat raw material or air-dried material, but requires the treatment temperature to be controlled to 160–190 °C. However, the treatment process is relatively complex, with four main steps of wet heat treatment, drying treatment, steam heat treatment and cooling humidification. As the study of heat modification is relatively recent, an optimized process has not yet been formed. Research has mainly focused on the effects of high temperature treatment on wood properties such as balanced moisture content, dimensional stability, mechanical properties and durability. The steam heat treatment of poplar, spruce and Pinus sylvestris was performed, and the results show that heat treatment clearly reduced the moisture absorption and improved the dimensional stability of wood. Researchers conducted atmospheric pressure and pressurized steam heat treatment on pine and oak, respectively, and found that the pressurized steam heat treatment led to improved moisture absorption and dimensional stability of wood. The changes in wood functional groups before and after heat treatment were studied by Fourier transform infrared spectrometry. The results showed that as the heat treatment temperature and time were increased, the light base absorption peak intensity of the heat-modified materials notably decreased, whereas the monkey base absorption peak intensity displayed a slight decrease. The chemical reactions of cellulose and hemicellulose, the main hygroscopic materials in wood, occur at high temperature (cellulose molecular chain bridging, hemicellulose thermal degradation), which significantly reducing the light base content of wood and improving the dimensional stability of wood. It is found that heat treatment can significantly improve the anti-expansion and shrinkage ratio of poplar, although its density, hardness and flexural strength decrease to different degrees. The optimized heat treatment conditions identified were steam medium (oxygen content less than 2%), temperature less than 185% and heating time of 5 h. The effects of heat treatment on the mechanical properties of wood are complex, but follow these main rules: the mechanical properties of wood decrease gradually with the increase in temperature and time, and some properties, such as bending strength and compressive strength along the grain, were most notably decreased. After a short period of heat treatment at low temperature, the mechanical properties do not clearly change, and even the flexural elastic modulus and surface hardness increase. The equilibrium moisture content of heattreated materials decreases, and the formic acid and phenolic substances produced by hemicellulose during the heat treatment process can prevent or delay the growth of deciduous bacteria and improve the anticorrosion performance of fast-growing forest.

3.2.4 Composite Modification Technologies The above modification technologies have inherent disadvantages: heat treatment modification has adverse effects on the density and mechanical properties of wood; the dimension stability of the product modified by cross-grain compression is poor; it

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can be easily recovered by hydrothermal environment. The impregnation modification process is complicated, and most of the impregnating agents will affect the environmental characteristics of wood, pollute the environment and cause adverse effects on human health. Using original modification technologies as a basis, domestic and foreign researchers have explored and studied the effects of various combinations of modification treatments. Hot pressing modification combined with heat treatment, especially oil bath heat treatment and high-temperature water vapor treatment, can fix the compression deformation within a short time and resolve the defects of poor dimensional stability of compressed wood. The hemicellulose and cellulose amorphous regions of wood will be degraded at high temperature, which decreases the mechanical properties of compressed wood after heat treatment, but they remain much higher than that of untreated material. The combination of hot-pressing treatment with impregnation is also a focus of research into joint modification techniques. Impregnation modification technology can effectively fix the compression deformation, and the compression treatment can improve the permeability of wood, improve the impregnation effect, and reduce the amount of impregnating agent, although its products still have pollution problems. Combined modification utilizes the characteristics of each modification technology to compensate for their deficiencies and to improve the modification effect.

3.3 UHP Modification of Wood 3.3.1 Densification Modification of Wood 3.3.1.1

Ultra-High-Pressure Densification Technology

Ultra-high-pressure densification (UHPD) is a green and efficient wood densification technology. This technology transmits pressure quickly and can quickly reduce the volume of pores in the wood to improve the density and mechanical properties of fast-growing forest wood. In addition, unlike conventional densification modification techniques, UHPD does not require pre-softening treatment and the treatment time is very short. Studies have shown that UHPD can increase the density of fir and poplar by 2–3 times within 5 min. After UHPD treatment, the wood profile density was significantly increased while maintaining the homogeneity of the log, which facilitated further cutting and processing of the densified wood, regardless of the uneven profile density. Some researchers have studied the influence of UHPD treatment on the mechanical properties of tung wood. The treatment parameters of 20, 40, 60, 80 and 100 MPa were used for UHP treatment. The profile density and hardness of UHP-densified paulownia increased by 88%–170% and 84%–173%, respectively. The results showed that the elastic modulus and Brinell hardness of poplar wood increased by 162% and 67%, respectively, after UHP treatment at 150 MPa for 30 s. However, when the pressure was over 150 MPa, the mechanical properties of poplar

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wood would decrease owing to the destruction of its cell wall structure. The Young’s modulus and fracture modulus of Chinese fir increased by 88.3% and 172%, respectively, after the UHPD treatment of 100 MPa. Some researchers have studied the hygroscopic characteristics of UHP-densified copper mold. The results show that after UHPD, the permeability and hygroscopic properties of paulownia wood were reduced owing to the closure of the water transmission channel and the decrease in hydrophilic hydroxyl groups and carbonyl groups, which will be beneficial to the storage of compacted wood. In addition, it was found that the wood size was stable when the UHP-densified Chinese fir was stored in a low relative humidity environment, whereas the wood size changed significantly when the UHP-densified Chinese fir was stored in a high relative humidity environment. In conclusion, UHPD technology, as a new method that can be applied to wood densification research, can significantly improve the mechanical properties such as density, hardness and the elastic modulus of fast-growing forest wood. However, to further improve the environmental adaptability of UHP-densified wood, the problem of dimensional stability needs to be further explored and solved.

3.3.1.2

Improved Technology for Dimensional Stability of Wood

After wood has been strengthened or modified, it is very sensitive to the moisture in the environment. During the process of moisture absorption, the compression deformation of the wood undergoes extensive rebound, and the dimensional stability is poor. This is a result of the existence of a large amount of residual stress inside the densified wood, which makes the compression deformation of the wood exhibit a metastable state. When the wood absorbs moisture, the residual stress is released, causing a rebound of the compression deformation. To solve this problem and improve the dimensional stability of densified wood, post-treatments for densified wood have been studied extensively. At present, there are two ways to improve the dimensional stability of densified wood: (1) reduce the hygroscopicity of the wood and reduce the moisture in the environment from entering the wood; and (2) eliminate the residual stress inside the wood, so that the compression deformation reaches a stable state. A coating treatment of water repellent can effectively prevent water molecules in the environment from entering the interior of the wood, reducing the water absorption rate by providing a water barrier or making the wood hydrophobic, which reduces the swelling and deformation of the wood. Tung oil, a dry oil obtained by pressing the seeds of tung tree nuts, dries quickly when exposed to air, forming a transparent film that blocks contact between the wood surface and water molecules in the environment. It was reported that after the coating treatment of tung oil, the moisture absorption rate of oak wood and Pinus sylvestris is significantly reduced. The coating treatment can improve the swelling resistance of compacted wood. After tung oil coating treatment and epoxy resin coating treatment, the compression recovery rate of densified wood decreased significantly, and the effect of epoxy resin coating treatment was improved. The coating treatment of epoxy resin could significantly improve

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the dimensional stability of wood. This is because the epoxy resin can form a threedimensional cross-linked structure during the film-forming process, which prevents slippage between the molecules after cross-linking and curing, making it difficult for external moisture to penetrate into the epoxy-coated wood. Some researchers used epoxy resin as binder and nanoform silica and fluorosilane to provide roughness and low surface energy, and prepared a type of superhydrophobic coating. The superhydrophobic coating can be constructed by applying it to the wood surface, which greatly reduces the moisture absorption of the wood. However, these hydrophobic treatments only delay the entry of water into the wood. Some researchers have impregnated wood with paraffin oil, and found that its tangential and radial swelling ratios decreased significantly, and the degree of decline increased with an increase in impregnation time. After 4 h of dipping, the radial and tangential swelling of the wood samples decreased by 55% and 60.2%, respectively. In addition, the low-molecular-weight phenolic resin impregnation treatment also improved the dimensional stability of the densified wood, as the phenolic resin impregnation reduced the hygroscopicity of the wood and formed a rigid crosslinked network upon curing. However, impregnation treatment may potentially cause environmental pollution problems while improving the dimensional stability of densified wood. High-temperature heat treatment is widely used as a post-treatment for hot-pressed wood densification to improve the dimensional stability of the densified wood. The principle of high-temperature heat treatment for the improvement of the dimensional stability of wood encompasses two main aspects: (1) High-temperature treatment may change the polar side groups on the molecular structure of cellulose, hemicellulose, lignin and extracts, resulting in reduced moisture absorption by the wood; (2) During high-temperature heat treatment, the crosslinks within the microfibers and between the cell wall polymers are rearranged, so that the residual stress inside the wood is released. At present, the methods of high-temperature heat treatment also tend to diversification. The main differences between methods are reflected in the heat transfer medium, heating temperature and treatment time. Some researchers have used water vapor heat treatment as a post-treatment of heat pressing densification, and explored the effect of water vapor heat treatment time on the dimensional stability of densified wood. The results showed that the compression recovery of surface-densified wood decreased as the steam heat treatment time increased. In addition, combining UHPD and high-temperature heat treatment found that after high-temperature heat treatment, when densified radiata pine was exposed to moisture, almost no swelling of the wood occurred. After steam heat treatment at 160–220 °C, the dry shrinkage rate, wet expansion rate and equilibrium moisture content of wood were reduced, and the expansion resistance rate and dimensional stability were significantly improved, especially when the temperature exceeded 200 °C. However, as the heat treatment temperature increased, the flexural strength, elastic modulus, and surface hardness of wood all tended to first increase and then decrease, so it is necessary to optimize the conditions of high-temperature heat treatment. While the dimensional stability of wood is improved, the loss of mechanical properties of wood should be reduced.

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3.3.2 Impregnation Modification of Wood 3.3.2.1

Wood Permeability

Wood permeability refers to the degree of difficulty of fluid (liquid or gas) moving in and out of wood. It is an important physical property of wood. In the process of wood functional design and processing, wood pretreatment and leaching and the uniform distribution of functional substances in wood are closely related to wood permeability. Therefore, the study of wood permeability crucial to the functional design and processing of wood. . Wood permeability theory The theory of wood permeability is based on the porous structure model of wood. According to the theory of fluid flow in a porous medium, a simplified wood structure model is constructed, and then the fluid flow equation in the wood is established based on the theoretical flow of fluid in a porous medium, allowing the analysis of the fluid flow law in wood. When a fluid passes through a porous medium, if the viscous force dominates, the fluid components follow each other by inter-particle forces, causing the fluid to flow in a streamlined manner. The flow type is viscous flow or linear laminar flow, and the flow is steady-state, which follows Darcy’s law; that is, the volume flow rate and energy loss of fluid flow in porous media are linearly related, negatively related to the length of the medium, and proportional to the stable conductivity. According to Darcy’s law, the permeability of liquid in wood can be expressed as: Q=

K 1 AΔP K 1 A(P2 − P 1 ) = μL μL

(3.1)

Q Lμ Q/A μ= ΔP × L AΔP

(3.2)

or K =

where K is the permeability of liquid in wood, expressed in m3 ·m−1 , Q is the volume flow rate in m3 ·s−1 , L is the length of wood in the direction of liquid flow in m, A is the cross-sectional area of wood in the direction of vertical liquid flow in m2 , ΔP is the pressure difference between the two ends of the wood in Pa, and μ is the fluid viscosity in Pa·s. The viscous flow in a porous medium can also be described by the Poiseuille equation. Assuming that the porous medium consists of a bundle of uniformly parallel straight capillaries, each of which obeys Poiseuille’s law, the viscous flow of liquid through the porous medium can be expressed as follows:

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3 Fast-Growing Forest and Related Introduction

Q=

N πr 4 ΔP 8μL

(3.3)

where N is the number of uniformly parallel straight capillaries and r is the capillary radius in m. Both the Darcy and Poiseuille equations are derived from the Navier–Stokes equation, which assumes that the fluid is in a steady state, the porous medium is in the shape of a parallel straight capillary, and the inertial force in the flow is negligible. The Darcy and Poiseuille equations are both capable of describing fluid flow in porous media and are equivalent. Consequently the following equation can be used: K =

nπr 4 N πr 4 = 8A 8

(3.4)

where n is the number of capillaries per unit cross-sectional area in m−2 . It can be concluded that the permeability of liquid in wood is positively related to the number and diameter of capillary tubes inside the wood. . Factors influencing permeability In general, materials that are permeable to fluids must be porous, but not all porous materials are permeable to fluids. Some studies have shown that wood is a porous material, but the permeability of wood to fluids is relatively low. This is because the ability of wood to transport fluid depends on the pore size and number of ultramicropores on the pit membrane connecting the intercellular channels of wood and the infiltrate. The smaller the pore size and the fewer the number of micropores in the grain pore membrane, the higher the capillary tension at the fluid and gas interface when the fluid flows through the wood; consequently, the greater the obstruction presented by this force against the fluid flow and the harder the fluid penetrates inside the wood. In contrast, the larger the pore size and the larger the number of micropores, the higher the permeability of wood. Therefore, the capillary tension of the micropores prevents the immersion and flow of liquid in the wood, and an external pressure greater than the capillary tension of the micropores is required to achieve the flow of liquid in the wood. The penetration of fluid in wood involves two aspects: longitudinal and transverse dredging. For broadleaf wood, the longitudinal permeation channels of fluid in wood are the main conduit molecules. Many duct molecules are connected longitudinally in broadleaf wood, forming tubular structures of variable length. There are perforated structures at both ends of the duct molecules. The fluid passes through the perforation between catheters when flowing longitudinally and through the grain hole when flowing horizontally between catheters. For coniferous wood, the longitudinal permeation channels of fluid in wood are mainly tracheid in series with each other. The tracheid has no perforated structure at either end, and the intertracheid fluid flow must pass through the perforated holes in tracheid. Therefore, for coniferous wood, the number of cell spacers of the same length in the longitudinal direction is greater than that of broadleaf wood, and these cell spacers hinder the flow of liquid in the

3.3 UHP Modification of Wood

61

interior of coniferous wood, resulting in the longitudinal permeability of coniferous wood being generally lower than that of broadleaf wood. On transverse dredge, broadleaf material and conifer material are basically consistent. Rimmed pores between wood fibers in broadleaf timber and axially between tubular cells in coniferous timber are the main channels for the chordal penetration of liquids in the wood. Ray tracheids and ray parenchyma cells are the main channels through which fluid flows radially in wood. Therefore, the aperture, number, and degree of opening and closing of the grain holes on the axial tracheid of broadleaf wood fiber and conifer wood determine the liquid dispersion ability in the wood chordal direction. The number of ray tracheids and ray parenchyma cells, the pore size, and number and occlusion of the two ends of ray parenchyma cells determine the radial fluid dispersion ability of wood. Coniferous wood generally has fewer ray parenchyma cells than broad-leaved wood; therefore, the radial permeability of coniferous wood is generally lower than that of broadleaf wood. The pore size and the number of micropores on the porous membrane are direct determinants of the permeability of wood. Therefore, the analysis and calculation of the pore size and the number of micropores on the grain pore membrane provide an important reference for the study of wood liquid permeability. Initially, researchers mainly used the model to calculate the pore size and number of micropores. The Petty model was used to calculate and analyze the micropores in the grain pore membrane of red pine heartwood, and the size was found to be 0.95 µm. Using the theory of fluid dynamics in porous media, the pore size and number of micropores in Larch and Pinus chinensis were calculated by the wood parallel capillary model. The results were in good agreement with the microscopic morphology observed by scanning electron microscopy. With the continuous advancement of testing technology, the technology for measuring the internal pore size of wood has been fully developed. The mercury intrusion method can analyze pores in the range of 1.8–58,000 nm. The diameter of the larger tracheids in wood is greater than 5 µm, and the diameter of the smaller tracheids is ~0.1–5 µm. The pore size of the upper plug edge of the edge of the ribbed pores is generally 0.1–0.7 µm, and the diameter of the ribbed pores is distributed throughout the above ranges. Therefore, the microporous structure in wood can be analyzed by the mercury injection method. In addition, NMR, X-ray scattering, gas adsorption, and other techniques can be used to study the pore size distribution of wood. . Methods for improving wood permeability According to the analysis of the influential factors of wood permeability, to improve the penetration of fluid lumber from the pit, specific technical methods to expand the pits on microporous membrane apertures, increase the number of pores, and reduce capillary tension on the interface of liquid and gas would help improve the speed of liquid flow in the wood and the permeability of liquid into wood. Much in-depth and effective research has examined methods to improve the permeability of wood.

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3 Fast-Growing Forest and Related Introduction

These methods can be divided into three categories of treatment methods: chemical, physical, and biological. (1) Chemical Treatment Methods In chemical treatment, chemical reagents are used to degrade the pit film or to replace the extracted substances in the pit film, such that the pore size on the edge of the pit film is enlarged, to expand the transmission channel of the fluid inside the wood, thereby increasing permeability. Supercritical fluids have the characteristics of high solubility and high diffusion, which can improve the permeability of wood by dissolving extractives that hinder the penetration of fluids inside the wood. Usually, supercritical CO2 fluid is used to improve the permeability of wood. An appropriate amount of entrainer (generally a substance with strong affinity for the extracted components that can be added to the supercritical CO2 fluid to improve the solubility of the supercritical CO2 fluid in the extracted components) can be added, thereby achieving better improvements in permeability. In previous studies, supercritical CO2 fluid was used to treat Eucalyptus urophylla grandis wood, and it was found that supercritical CO2 fluid could dissolve and remove the extract from the wood. Meanwhile, the rapid pressure change in supercritical CO2 fluid could open the closed grain holes, reduce the occlusion rate of grain holes, and thus improve the wood permeability. Using supercritical CO2 fluid as the carrier solvent for anticorrosion treatment of wood composite materials can impregnate the preservative iodide propargyl carbamate into the materials to improve the anticorrosion performance. Adding methanol, ethanol and benzene– ethanol as entrainment agents in supercritical CO2 fluid improved the permeability of Chinese fir and Masson pine. Substituting the extracts with chemical agents can also achieve improvements in permeability. Using alcohol to displace and dry spruce can improve the permeability of spruce. The underlying principle is that, after alcohol is displaced, the ribiscus membrane is still in the middle open position, and the blocking rate of ribiscus is almost the same as the wood state, which is conducive to the permeation and transmission of fluid in wood. It was reported that the permeation channels in wood were dredged by extraction of wood with 0.4% NaOH at 60 °C for 24 h, and the longitudinal, radial and chord penetration rates of acid red dye were 67.21%, 40.45%, and 22.41%, respectively, which were significantly higher than for the untreated wood. Mulberry wood was extracted with 2% NaOH at 65 °C for 2 h. NaOH was found to destroy the intercellular pore membrane and increase the porosity of wood cell wall, thus improving the permeation of liquid into wood, promoting cellulase to penetrate into the wood and decompose cellulose. However, the use of chemicals may negatively affect wood properties. After refractory impregnation treatment with supercritical CO2 fluid, the bending properties of western red cedar and Ingleman spruce were significantly reduced, the modulus of rupture of western red cedar was reduced by 23.1%, and the maximum working load was reduced by 31.0%. The fracture modulus of Engelmann spruce decreased by 10.8–21.6%, and the maximum workload decreased by 7.2–29.0%. In conclusion, chemical treatment has

3.3 UHP Modification of Wood

63

a significant effect on the permeation of liquid into wood, but it must be used carefully to avoid negative effects on mechanical properties and other wood properties. (2) Physical Treatment Methods Physical treatment methods encompass the use of mechanical or laser scoring, microwave, ultrasonic, steam blasting, freezing or other treatments to increase the penetration channels in the wood. Mechanical or laser scoring is used to carve cracks of appropriate depth and width on the surface of wood with laser or mechanical tools, which cut the fiber on the surface of wood into segments, thereby increasing the penetration channel of fluid in wood. This technology can be used in wood anticorrosion treatment. During the process of ultrasonic treatment of wood, the porous structure inside the wood will produce cavitation, and when the cavitation bubbles burst, it will have a blasting effect on the fiber components of the wood. The blasting effect and the mechanical action of ultrasonic waves impact on the wood fiber structure simultaneously, destroying the fiber structure and improving the permeation of liquid into wood, which can be used in the process of wood flame retardant, auxiliary dyeing and so on. Freezing treatment means that the free water in the wood cell cavity freezes under low temperature conditions, causing volume expansion and squeezing the wood cell wall, thereby destroying the pit membrane and improving the permeation of liquid into wood. When the freezing treatment time was extended, the cracks on the parenchyma cell wall and the wood fiber cell wall continued to increased, and the damage to the pit membrane between the ducts increased. After a 48-h freezing treatment of Eucalyptus grandis wood, it was found that many ray cells were destroyed, and obvious cracks appeared on the pit membrane with bordered pits. The destruction of the internal structure of the wood expanded the flow path of fluid in the wood, thereby increasing the permeability. Steam explosion refers to the softening of wood under high temperature steam conditions, which instantaneously reduces the pressure. At this time, the pressure difference between the inside and outside of wood cells is formed, and the impact of this pressure difference impact destroys the wood parenchyma and weakens the position of the grain pore membrane, expanding the fluid transmission channel and improving the permeability. The wet heartwood of fir was treated by steam blasting, and the microstructure of the treated material was analyzed. Five changes were noted in the occluded pore: the occluded pore reset; the offset of the pore plug and the distortion of the pore membrane; the rupture of the pore plug and the detachment from the pore orifice; the tear of the pore edge and the cell wall; and the detachment of the pore edge from the cell wall. The changes in these occluded pores expand the permeation channels of the fluid in the wood, thus improving the permeability of the wood, which is helpful for the drying and functional impregnation modification of the wood. After steam blasting treatment of wood, the occluded pore membrane is reset and the pore membrane is broken, which causes some damage to wood cell walls, which significantly improves permeability. However, one disadvantage of this treatment is that the treatment effect is not uniform.

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3 Fast-Growing Forest and Related Introduction

The mechanism by which microwave treatment improves wood permeability is the same as that of steam blasting. Both treatments use the pressure difference generated by high-temperature steam to impact and destroy the microstructures, such as ray parenchyma and grain pore membrane, of treated wood, which expands the transmission channel of fluid in wood and improves the permeation of liquid into wood. Wood moisture content, microwave power, microwave treatment time, and other parameters alter the treatment effect. (3) Biological Treatment Methods Biological treatment methods use enzymes, bacteria, fungi and other microorganisms to erode the grain pore membrane or parenchyma of wood, which expands the infiltration channels for liquid into wood and improves permeability. Previous studies on the changes in wood microstructure after enzymatic treatment showed that both the pit plug and plug edge were degraded and destroyed to a certain extent. Such degradation and destruction could increase the pore size and the number of micropores on the ribbed pore membrane, thus improving the permeability of wood. It was found that the permeability of treated sapwood increased by 29 times, whereas the permeability of heartwood increased only 1.52 times. This is because bacteria such as Bacillus brevis can degrade the marginal porous membrane of sapwood after wood erosion, but there is no significant degradation effect on heartwood. Fungal treatment comprises the inoculation fungal spores on the surface of the wood to be treated. Fungi move inward through the lumen of wood cells, and mycelia can penetrate the intercellular striatum membrane. Simultaneously, fungi secrete enzymes that can also degrade the ribbed pore membrane, increase the pore size and number of micropores on the edge of the ribbed pore plug, and even destroy the ribbed pore membrane to form a cavity structure; all these effects improve the permeation of liquid into wood. Common fungi include wood-rotting fungi, mold and discoloration fungi. After the erosion of wood, mold seriously damages the grain pore membrane, and can even degrade the cell wall of treated wood, which has a negative impact on the mechanical properties of wood. Studies on Chinese fir treated with Trichoderma erosion for 1–9 weeks showed that the sapwood permeability was significantly improved, whereas the heartwood permeability was almost unchanged. In a study of the permeability characteristics of Norway spruce treated with Trichoderma, the permeability of the treated sapwood was improved. Further research found that the mechanical strength of the treated wood did not decrease significantly. A study of the microstructure of the treated wood showed that only the pit membrane of the sapwood was damaged, and the cell wall of the sapwood and the heartwood were essentially undamaged. Therefore, this method is only suitable for improving the permeability of sapwood. Fungal erosion of fir has been reported to degrade cell wall material, but fungal hyphae block fluid channels and instead reduce permeability.

3.3 UHP Modification of Wood

3.3.2.2

65

Effect of UHP on Material Permeability

As an emerging processing technology, UHP is mainly used in the food industry to achieve the effects of improving food quality and prolonging food shelf life. Researchers at home and abroad have applied UHP treatment to processing to improve the permeability of food and materials, and have made important research progress. UHP can destroy the integrity of cells and the three-dimensional structure of biological macromolecules, resulting in cell fragmentation, the destruction of noncovalent bonds (e.g., ionic bonds, hydrogen bonds, and hydrophobic bonds), the denaturation of macromolecules such as starch and proteins, the discharge of intercellular gas and penetration enhancement. Previous studies have shown that UHP treatment can shorten the curing time and speed up the curing process of chicken breast. The salt diffusion rate reached a maximum when curing at 150 MPa, and curing pressure above 200 MPa caused damage to muscle cells and muscle fibers. The treatment of fresh-cut potatoes with 200–400 MPa damaged the cell tissue structure, which manifested in the destruction of the cell wall, the blurring of the intercellular space, the loose arrangement of the cells, and the increase in the relative electrolyte exudation rate. For pressures above 400 MPa, the relative electrolyte exudation rates no longer changes with the increase in pressure. UHP treatment leads to the deformation and rupture of fruit and vegetable cell structure, as well as the exudation of cell fluid, which leads to the decrease in Ca2+ content in fresh-cut potatoes treated with UHP. Further research found that this is because UHP treatment can improve cell permeability, thereby accelerating ion penetration. The rice husk, pericarp, seed coat, etc., will hinder the water absorption by rice grains, and UHP soaking can expand the starch grain spacing of rice grains, open channels for water molecules, improve the water absorption rate of rice grains, and shorten the cooking time. UHP treatment can have a significant influence on the pores and internal structure of the material, changing the fluid permeability of the material. The research into lactic acid-based nano-silver composites treated with UHP shows that UHP treatment has no effect on the crystal structure of the composite, does not change the peak and intensity of the infrared spectrum, and does not affect the glass transition point, cold crystallization temperature, melting point temperature, etc. However, UHP can improve the crystallinity of the composite films. With an increase in pressure, the water vapor transmission rate and oxygen transmission rate of the UHPT polylactic acid-based nano-silver composites increased, and the barrier properties decreased. The effect of different pressure treatments on the CO2 permeability of the composite membrane is quite different, and the highest CO2 permeability of the composite membrane was achieved with 400 MPa pressure treatment. Researchers have studied the seepage characteristics of rocks under high water pressure, and found that under high water pressure, the internal pores of the rock are squeezed and collapse, and deformation and cracks appear in the rock, improving the permeation of liquid into the rock.

66

3.3.2.3

3 Fast-Growing Forest and Related Introduction

UHP Dyeing

UHP dyeing is a new type of wood dyeing technology that enables dye molecules to rapidly enter the wood under the action of UHP, without the need for high temperature, and produces uniform and deep dyeing. In addition, compared with traditional hot-dip-dyeing, UHP dip-dyeing greatly reduces the processing time, taking approximately 5 min to complete the dyeing of wood blocks of 100 mm × 50 mm × 30 mm in size. Some research shows that the effect of UHP dip-dyeing increases with the increase in processing pressure and holding time, and both the dyeing depth and dyeing uniformity are better than with traditional hot-dip dyeing.

Chapter 4

Effect of UHP Processing on the Main Volatile Components and Aging Characteristics of Chinese Liquor

Abstract This chapter focuses on changes of volatile compounds in Chinese liquor after high pressure treatment and a combination of high pressure treatment and following storage comparing with natural aging process. With gas chromatography and electronic nose, the aging effect of high pressure treatment on Chinese liquor is evaluated with multiple models including principle components analysis, cluster analysis, linear discriminant analysis, and partial least square regression. Keywords Chinese liquor · Aging · High pressure treatment · Volatile compounds · Electronic nose · Liquor age prediction

4.1 Introduction 4.1.1 Chinese Liquor Chinese liquor is a popular alcoholic beverage that plays an essential role in national economic development. As listed in Wikipedia, the earliest evidence of alcohol in China is traced to jars from Jiahu, dating to ~7,000 BC. This early rice mead was produced as a fermented product from rice, honey, and fruit. Chinese liquor is an alcoholic beverage made as a distillate of traditional grain ferments. Owing to its mellow taste and full aroma, this liquor has been one of the most popular alcoholic beverages for centuries, with a consumption of more than 4 million kL each year worldwide, creating an annual revenue of over 500 billion Chinese Yuan. The top brands of Chinese liquor include Maotai, Wuliangye, Xifeng, Shuanggou Daqu, Yanghe Daqu, Gujing Tribute, Jinnanchun, Luzhou Laojiao Tequ, Fen, and Dong. The typical manufacturing process for the liquor involves primary fermentation, distillation, aging, and, if required, blending. Though minor differences may exist in the manufacturing techniques of various liquors depending on the manufacturing area and flavor, three essential processes are generally involved: fermentation, distillation, and aging. The primary grain used for manufacturing Chinese liquor is sorghum or a mixture of sorghum containing wheat, rice, and corn. These raw materials are milled, © Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_4

67

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

cooked, and mixed with Daqu powder (starter culture). After several months of fermentation, the liquor is distilled using steam. Upon distillation, the liquor (young, raw, or fresh) often has an undesirable harsh taste. Hence, the freshly distilled liquor is usually aged for several years to refine the flavor and develop the bouquet aroma. Therefore, the economic value of Chinese liquor is highly associated with its age since aging plays an indispensable role in producing high-quality liquors. During the natural aging of Chinese liquor, many chemical reactions such as oxidation, esterification, and hydrolysis can occur, accompanied by the permeation and migration of some small and polar volatile compounds. Recently, dozens of constituents have been extensively investigated and recognized as volatile compounds of distilled alcoholic beverages. Regarding aging, previous research showed that acetal showed the highest and most positive relationship with the ages of Malvasia and Bual wines. In contrast, volatile phenol showed the lowest correlation. Specifically, 3-furfural, pantolactone, trans-dioxane, and 2,2-diethoxyethanol are suggested as potential aging markers of Malvasia and Bual wines. It was reported that 13 compounds in Chinese liquor, including acids, alcohols, esters, aldehydes, and furans, decreased significantly during the first year of aging and were maintained at the same levels for the next three years. Besides, ethyl lactate was the most stable volatile compound during natural aging. Because the economic value of Chinese liquor is highly associated with its age, some dishonest producers counterfeit their younger products as several years aged liquors to get a higher selling price. Thus, the discrimination of liquor age and related predictive analysis is urgently needed to safeguard the market order and protect consumer rights.

4.1.2 Processing Technology As liquor aging is usually a lengthy process, alternate technologies are often sought to shorten the required aging time. Several methods based on physical or chemical modifications have recently been reported in the literature for liquor aging. Previous studies investigated the effect of pulsed-electric-field treatment on the composition of phenolic compounds in brandy during aging in oak barrels. Their results demonstrate that phenolic compounds, such as tannins, total phenols, and volatile phenols, significantly increased after the pulsed-electric-field treatment, as commonly observed during aging. Thus, electric field treatment could be used to accelerate aging. Some former scholars used 20-kHz ultrasonic waves to accelerate the aging of different wines and then related changes in pH, alcohol content, gas chromatography (GC) measurements, and sensory evaluation scores to the aging process. They observed that the ultrasonic treatment helped to age the rice wine more rapidly and with similar quality, than standard aging; however, they also noticed that the treatment did not accelerate the aging of maize wine with comparable quality. The aging of Brandy de Jerez on a laboratory scale was accelerated through extraction by employing oak chips and ultrasound treatment. After 30 days, samples obtained using the developed

4.1 Introduction

69

method were reported to possess analytical and sensorial characteristics similar to those aged traditionally for an average time of 6–18 months. During the past decade, the application of UHP treatment for food processing and preservation has increased rapidly. As a nonthermal technology, UHP processing is currently used worldwide for various food processing applications to achieve higher quality products that were earlier impossible to produce. This approach offers several advantages compared to conventional thermal techniques. In addition, applications of UHP treatment in wine and Chinese liquor have been reported. In comparison, changes in wine treated under lower UHP conditions (400–500 MPa for 5 min) were perceptible only after several months of storage (had no immediate effect). Furthermore, UHP significantly influenced the chromatic characteristics and the phenolic composition of the wine after treatment at 650 MPa for 0.25, 0.5, 1, and 2 h. Moreover, UHP treatment for 2 h at 650 MPa was reported to significantly reduce the intensities of the sour and fruity odor of the wine. Another study reported that UHP treatment modified the α-helical and β-sheet structures of wine proteins. UHP treatment has been proven to be beneficial for accelerating cheese ripening. However, our previous study observed immediate changes in Chinese liquor even after lower conditions of UHP treatments. The results showed that UHP treatments at 300 and 400 MPa significantly decreased the alcohol content and total acid content of sauce-flavor Chinese liquor; meanwhile, an increase in the total ester content and total solid content were also observed. Moreover, in sensory evaluation tests, pressurized liquors achieved better performance than the control group (without UHP treatment). These findings agree with those observed in the case of natural aging process. However, to the best of our knowledge, the effect of UHP on the aging of liquors, especially Chinese liquor, has not been studied yet, and the effects of UHP treatment and the following storage process on the main volatile compounds of Chinese liquor are still largely unknown.

4.1.3 Liquor Age Prediction The current strategies used to identify liquor age can be divided into two categories: sensorial evaluation and instrumental analysis. Well-trained analysts usually conduct sensory evaluations to grade Chinese liquor according to its color, aroma, taste, and overall quality. The evaluation result lacks objectivity and reproducibility as it can be easily affected by external environmental conditions and the internal physical and mental status of the analysts. For the instrumental analysis, GC or a combination of GC and mass spectrometry (MS), high-performance liquid chromatography (HPLC), atomic absorption spectroscopy (AAS), and spectrograph technique have been widely applied to analyze the flavor characteristics of Chinese liquor based on their volatile compounds. However, a sophisticated pretreatment, long experimental period, and skilled operating technician are usually needed to acquire accurate results,

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

which is challenging. Above all, it is important to find an ideal method to characterize the flavor features of Chinese liquor precisely and conveniently to determine the liquor age. During the last decades, more attention has been focused on developing the electronic nose, a nondestructive and rapid method for detecting the aroma characteristics of food matrices. The core component of the electronic nose system is the sensor array; each sensor generates a specific response to the corresponding aroma substance in the headspace of food samples to simulate the human nose. Then, a data-collection unit usually acquires response values of the sensor array; then, discrimination and prediction analysis can be realized using multivariate statistical techniques. Apart from low cost and time-saving features, the electronic nose has also been proven to perform well in volatile compounds analysis of various foods, including fruit juices, alcoholic beverages, and green tea.

4.2 Changes in the Main Volatile Compounds of Chinese Liquor GC with a direct injection method was used to identify and quantify the main volatile compounds in Chinese liquor. After UHP treatment, a more specific short-term storage test (2–6 months) was included to investigate the stability and impact of UHP treatment. The main objective of this research was to lay a theoretical foundation for the effect of UHP treatment and short-term storage strategy on changes in the main volatile compounds of Chinese liquor and to explore the possibility of UHP being used as a new technology for accelerating the aging of Chinese liquor from the perspective of volatile compounds.

4.2.1 Chinese Liquor Samples and Chemicals Chinese liquor “Junchang” (light flavor) was provided by a local liquor factory in Sichuan province and stored at room temperature before use. Young liquors without storage and 1-year-aged liquors were used. Young liquors (15 samples) were randomly separated into two subgroups—the control group containing young liquor (5 samples) and the 1-year-aged liquor (5 samples)—and the experimental group (10 samples) were subjected to UHP treatments for subsequent use. A mixed standard solution (Donglilong Information Technology Co. Ltd., Lanzhou, China), unique for Chinese liquor GC analysis, was used for the identification and quantitation analysis. As shown in Table 4.1, this mixed standard solution contained 50 compounds, covering almost all the main volatile compounds of Chinese liquor. To ensure the accuracy of quantitation results, amyl acetate was used as an internal

4.2 Changes in the Main Volatile Compounds of Chinese Liquor

71

Table 4.1 Composition of mixed standard solution (concentration: mg/L). Reprinted from Ref. [1], used with permission Number Compound

Concentration Number Compound

Concentration

1

Acetaldehyde

606.0

26

Ethyl lactate

916.7

2

Propanal

367.2

27

Ethyl oenanthate

363.8

3

Acetone

440.9

28

Acetic acid

1548.2

4

Ethyl formate

285.7

29

Trimethyl pyrazine

662.8

5

Methanol

318.0

30

n-Heptanol

412.5

6

Ethyl acetate

325.0

31

Ethyl caprylate

546.2

7

Acetal

646.0

32

Furfural

540.9

8

Ethanol

Solvent

33

Iso-amyl caproate

385.7

9

Ethyl propionate

298.6

34

Propionic acid

388.4

10

Diacetyl

356.9

35

Isobutyric acid

140.3

11

Ethyl isobutyrate

326.1

36

Butanoic acid

120.4

12

2-Pentanone

401.1

37

Fuefuryl alcohol

456.0

13

2-Butanol

358.4

38

Isovaleric acid

136.4

14

1-Propanol

426.3

39

Ethyl caprate

450.0

15

Ethyl butyrate

127.3

40

Diethyl succinate

672.1

16

Butyl acetate

336.2

41

Valeric acid

395.8

17

2-Hexanone

357.7

42

Ethyl phenylacetate

653.8

18

Isobutanol

1092.3

43

Hexanoic acid

412.2

19

n-Butanol

386.9

44

Ethyl lactate

536.8

20

Ethyl valerate

365.9

45

Phenylethanol

210.1

21

Isoamylol

1038.2

46

Heptylic acid

502.9

22

n-Amyl alcohol

395.8

47

Octanoic acid

447.3

23

Ethyl hexanoate

298.0

48

Ethyl myristate

496.8

24

Hexyl acetate

376.2

49

n-Nonoic acid

512.2

25

2-Hydropxyheptane 365.4

50

Ethyl palmitate

460.2

standard substance. All groups were analyzed before and after storage (2 and 6 months).

4.2.2 Identification and Quantification Analysis . Measurement Identification and quantitative analysis of volatile compounds in Chinese liquor were performed via GC. Five samples from the young group, five samples from the 1year group, five samples from the 400 MPa-15 min group (immediately after UHP

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treatment), and five samples from the 400 MPa-30 min group (immediately after UHP treatment) were first analyzed through GC to characterize the volatile composition of all groups. To evaluate the effect of short-term storage on the volatile composition of UHP-treated liquors, GC tests were also performed on 10 pressurized samples (5 samples for 400 MPa-15 min and 5 samples for 400 MPa-30 min) after 2 and 6 months of storage. The amount of liquor sample in each pottery jar was ~500 mL, and ~30 mL of the sample was used for each batch of GC test with five replicates for each sample; the rest of the sample was sealed and stored in the jar (Fig. S1). The process from opening to sealing was less than 20 s, which was even shorter than the subsequent filtering procedure. Thus, it had little effect on the GC results and the volatile composition of the remaining sample. GC analysis was performed using an GC system (Agilent 7890A, California, USA) equipped with a flame ionization detector (FID). The column carrier gas was nitrogen at a constant flow rate of 1 mL/min. All samples were analyzed on an LZP950 column (50 m × 0.32 mm; 1.0-μm film thickness). First, liquor samples were filtered into a 2-mL autosampler vial using a filtering membrane with a pore diameter of 0.45 μm to remove impurities. Then, 1 μL of the filtrate was injected into GC from the autosampler vial, and the split ratio was 1:1. The oven temperature was held at 65 °C for 8 min, then raised to 200 °C at a rate of 5 °C/min, and held at 200 °C for 50 min; the injector and detector temperatures were 230 °C and 250 °C, respectively. The identification analysis was performed by comparing the retention times of all compounds in the mixed standard solution (Fig. 4.1a) with those in the liquor sample (Fig. 4.1b) based on the theoretical foundation, which states that a certain substance has a specific retention time under the same chromatographic condition. The volatile compounds in Chinese liquor were quantified using calibration curves. In addition, amyl acetate was added to the samples as an internal standard substance, and the final quantification results were corrected using this internal standard (350 mg/L in 1 mL of liquor sample). All liquor samples were analyzed using the direct injection method, replicated five times, and the values were averaged. . Results Typical chromatograms of the mixed standard solution and the liquor samples are shown in Fig. 4.1a, b; all compounds were numbered according to Table 4.1. For liquor samples, 21 compounds were the abundant and dominant constituents in this type of Chinese liquor. Considering chemical groups of acids, alcohols, esters, aldehydes, and furans, the quantification results of 21 compounds are presented in Table 4.2. A radar map was used to visualize the general variation of the main volatile compounds during natural and artificial aging (UHP treatment and shortterm storage). As shown in Fig. 4.2, the concentration of each compound in young liquor was set as 1, and the relative concentration of each compound in the other six groups was calculated by the actual concentration of the compound versus the corresponding concentration in young liquor. Among the 21 compounds, 16 were used in the radar map analysis as the other 5 compounds were only detected in the 1-year group.

4.2 Changes in the Main Volatile Compounds of Chinese Liquor

73

Fig. 4.1 Typical chromatograms of the a mixed standard solution and b liquor sample. All compounds were numbered according to Table 4.1. Red circle: amyl acetate (internal standard). Reprinted from Ref. [1], used with permission

4.2.3 Changes in Main Volatile Compounds During Natural Aging To evaluate the aging effect of UHP treatment, changes in main volatile compounds during the natural aging were first characterized by following short-term storage on Chinese liquor. As shown in Table 4.2, 17 compounds were detected in young liquor, while 21 compounds were found in the 1-year-aged liquor. Results show that natural aging significantly affected all studied chemical groups, especially acids and esters. Acids in Chinese liquor are products of sugar oxidation or alcoholic fermentation during liquor making. Acetic acid was the most abundant compound in this group,

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

Table 4.2 Main volatile compounds in liquor samples with different treatments. All values are expressed as mean (mg/L) ± standard deviation (SD). Different letters indicate significant differences (p < 0.05). ND: Not detected. (S2): Samples stored for 2 months. Reprinted from Ref. [1], used with permission Number Compounds

Young

400-15

400-30

Young (S2)

1

Acetaldehyde

237.81 ± 11.87a

212.50 ± 12.72b

217.75 ± 10.66b

178.89 181.42 ± 6.97c ± 12.49c

2

Methanol

79.79 ± 65.10 ± 70.42 ± 64.37 ± 64.78 ± 64.28 ± 46.75 ± 3.46a 1.89c 2.12b 2.39c 2.50c 2.69c 2.06d

3

Ethyl acetate

1741.48 1418.78 1450.86 1472.49 1524.67 1454.40 694.23 ± ± ± ± ± ± ± 87.52a 85.08b 76.20b 69.85b 88.60b 59.43b 36.17c

4

Acetal

519.72 ± 30.60a

5

2-Butanol

29.97 ± 22.65 ± 23.21 ± 18.30 ± 18.41 ± 18.36 ± 20.54 ± 1.35a 1.04b 1.76b 1.88c 1.96c 2.47c 2.54c

6

1-Propanol

338.84 ± 16.54a

7

390.36 ± 25.19b

378.95 ± 28.88b

402.45 ± 25.46b

400-15 (S2)

416.49 ± 24.61b

400-30 (S2)

One year

176.52 152.99± ± 9.66c 12.67d

401.96 ± 30.19b

275.31 ± 22.76c

291.73 ± 13.38b

296.90 ± 20.40b

248.92 250.50 ± 9.67c ± 15.25c

250.29 ± 16.79c

261.24 ± 20.35c

Ethyl butyrate ND

ND

ND

ND

ND

ND

4.28 ± 0.53

8

Isobutanol

637.67 ± 39.72a

545.46 ± 20.09b

553.07 ± 38.16b

477.81 ± 18.13c

480.14 ± 25.44c

479.77 ± 28.13c

593.72 ± 29.01a

9

n-Butanol

13.84 ± 11.37 ± 11.60 ± 9.84 ± 1.73a 1.26a 1.19a 1.64a

10.13 ± 10.07 ± 12.53 ± 1.57a 1.98a 1.79a

10

Isoamylol

800.53 ± 41.79a

697.90 ± 44.61b

710.83 ± 45.37b

594.50 ± 38.17c

597.56 ± 37.39c

598.68 ± 40.96c

793.02 ± 38.19a

11

Ethyl hexanoate

ND

ND

ND

ND

ND

ND

52.82 ± 4.69

12

Ethyl lactate

446.38 ± 30.40a

423.64 ± 20.71a

451.06 ± 28.87a

302.33 ± 17.52b

306.78 ± 14.99b

309.78 ± 12.05b

465.10 ± 31.16a

13

Ethyl oenanthate

ND

ND

ND

ND

ND

ND

14.43 ± 1.62

14

Acetic acid

1194.00 1095.88 1158.02 592.01 ± ± ± ± 91.82a 86.71a 79.46a 41.40c

599.30 ± 40.80c

605.75 ± 38.26c

701.73 ± 40.19b

15

Furfural

19.17 ± 16.65 ± 18.09 ± 8.82 ± 1.09b 1.88b 1.62b 1.03c

9.21 ± 0.96c

9.36 ± 1.41c

36.05 ± 2.56a (continued)

4.2 Changes in the Main Volatile Compounds of Chinese Liquor

75

Table 4.2 (continued) Number Compounds

Young

400-15

400-30

Young (S2)

400-15 (S2)

400-30 (S2)

One year

16

Propionic acid 7.83 ± 0.57a

7.11 ± 0.69a

7.79 ± 0.77a

4.71 ± 0.62b

2.35 ± 0.55c

2.48 ± 0.41c

7.61 ± 0.41a

17

Isobutyric acid

18

Butanoic acid 20.87 ± 18.20 ± 20.01 ± 9.12 ± 1.56b 1.71b 2.07b 0.79c

19

Isovaleric acid

3.62 ± 0.26a

3.28 ± 0.30a

3.73 ± 0.29a

20

Phenylethanol 5.90 ± 0.44b

4.43 ± 0.56b

21

Ethyl palmitate

ND

14.95 ± 14.17 ± 15.09 ± 13.27 ± 11.46 ± 11.69 ± 9.18 ± 1.92a 2.04a 2.27a 2.41a 2.07a 1.89a 0.60b

ND

7.84 ± 0.86c

8.11 ± 0.91c

29.69 ± 2.01a

2.55 ± 0.21b

1.02 ± 0.16c

1.07 ± 0.15c

2.87 ± 0.18b

5.48 ± 0.69b

ND

ND

ND

11.59 ± 0.68a

ND

ND

ND

ND

13.09 ± 0.95

Fig. 4.2 Radar map based on main volatile compounds. Acetaldehyde (1), methanol (2), ethyl acetate (3), acetal (4), 2-butanol (5), 1-propanol (6), isobutanol (7), n-butanol (8), isoamylol (9), ethyl lactate (10), acetic acid (11), furfural (12), propionic acid (13), isobutyric acid (14), butanoic acid (15), isovaleric acid (16). (S2): Stored for 2 months. Reprinted from Ref. [1], used with permission

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with a concentration of 1,194 mg/L in young liquor, which agreed with previous studies. After 1 year of aging, acetic acid content decreased from 1,194 to 701 mg/L (p < 0.05) owing to the volatilization through the jar and the esterification with alcohols during the natural aging. This decreasing trend could produce high-quality Chinese liquor as acetic acid negatively contributes to the liquor bouquet. In addition, a slight increase (p < 0.05) in butanoic acid content and decreases (p < 0.05) in isobutyric and isovaleric acid contents were observed. Still, no significant change was found in propionic acid content. Acids affect the taste and mouthfeel of liquors, increase color stability, reduce oxidation, and along with ethanol, are primarily responsible for the microbial and physicochemical stability of liquors. In the ester group, ethyl acetate and ethyl lactate were the most prominent representatives, with concentrations of 1,741 and 446 mg/L in young liquor, respectively. A decrease of 60% (p < 0.05) was observed in ethyl acetate content after 1 year of aging, i.e., from 1,741 to 694 mg/L. Ethyl acetate significantly affects the organoleptic characteristics of wines and distilled liquors, depending on its concentration, namely, “fingernail polish remover” and fruity properties at high and low concentrations, respectively. In addition, long-chain esters, such as ethyl butyrate, ethyl hexanoate, ethyl oenanthate, and ethyl palmitate, were detected in the 1-year-aged liquor. During the manufacturing of Chinese liquor, unlike ethyl acetate, long-chain esters are mainly generated due to moderate esterification between alcohols and corresponding acids during aging rather than fermentation and distillation. Thus, they gradually form after 1 year of aging. Moreover, there might be very small amounts of long-chain esters in young liquor, which were much lower than the detection limit and could not be detected in this study. It seemed that ethyl lactate was highly stable during aging as no significant difference was observed between young and aged liquors (at least during the first year of aging). For alcohols, isobutanol, n-butanol, and isoamylol were maintained at the same levels (p > 0.05) while methanol, 2-butanol, and 1-propanol decreased (p < 0.05). Meanwhile, a slight increase was observed in the phenylethanol content (p < 0.05) after 1 year of aging. It is recognized that there will be some decline in alcohol content during the natural aging of liquors, which has been linked to changes in the structure of water and alcohol molecules. Acetaldehyde and acetal are the most toxic metabolite created by alcohol metabolism originating from fermented raw materials. Therefore, a significant decrease was found in acetaldehyde and acetal contents during the first year of aging, reducing the potential toxicity (caused by acetaldehyde and acetal) of Chinese liquor. Besides, the accumulation of furan, as presented in Table 4.2, is usually considered an aging marker as it can be formed by the pyrolysis of carbohydrates, dehydration of sugars through the Maillard reaction, and caramelization, which occurs during fermentation and the aging process.

4.2 Changes in the Main Volatile Compounds of Chinese Liquor

77

4.2.4 Impact of Ultra-High-Pressure Treatments on Main Volatile Compounds . Ultra-High-pressure treatments and storage UHP treatments were performed using laboratory-scale UHP equipment. The system consisted of an UHP unit (UHPF-750, 5 L, Kefa, Baotou, China) equipped with Ktype thermocouples (Omega Engineering, Stamford, CT, USA) and a data logger (34970 A, Agilent Technologies GMBH, Germany) for temperature measurement and a thermostat jacket connected to a water bath (SC-25, Safe, China) for maintaining the processing temperature. The intensifier used for generating the pressure was a batch-type unit that built up the pressure in a stepwise ladder-like process. This study used water as a pressure transmitting medium (PTM), and the pressure vessel was maintained at 25 °C before pressurization. Sample temperature was monitored during the tests and was recorded at 1-s intervals. Normally, sample temperature is expected to increase by 3 °C after every 100-MPa-pressure rise due to adiabatic compression. However, to minimize this adiabatic heating, a low rate of pressurization was maintained (~100 MPa/min) so that the sample temperature easily equilibrated to the set point temperature of 25 °C. The pressure release time was maintained at less than 5 s. Sufficient young liquors were packaged with polyethylene terephthalate (PET) bags and sealed using a plastic-envelop machine. Afterward, young liquors were treated under UHP at 400 MPa-15 min and 400 MPa-30 min. Each condition was evaluated with five replicates (five liquor samples for each condition). After UHP treatments, 10 liquor samples were individually transferred into 10 pottery jars (500 mL for each pottery jar) for subsequent use. All the pottery jars were placed under protection from light at ambient temperature (~25 °C) to imitate the natural aging of Chinese liquor. . Results It can be observed from Table 4.2 that the UHP treatments significantly affected some chemical groups of pressurized liquor, especially for alcohols and aldehydes, when compared with young liquor. As can be seen, acetic, propionic, isobutyric, butanoic, and isovaleric acid contents were maintained at the same level (p > 0.05) after UHP treatments. A similar phenomenon was also observed in the ester group. Though ethyl lactate content was slightly altered after treatment, differences were not statistically significant. Furthermore, ethyl butyrate, ethyl hexanoate, ethyl oenanthate, and ethyl palmitate were not detected in young liquor or pressurized liquors. Only ethyl acetate was significantly affected by UHP treatments, with decreasing rates of 19% and 17% at conditions of 400 MPa-15 min and 400 MPa-30 min, respectively. These results, however, contradicted our previous findings, in which we stated that UHP treatments significantly increased ethyl acetate content in sauce-flavor Chinese liquor. This may have been caused by the differences in the volatile composition of the

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two types of Chinese liquor (sauce flavor and light flavor), and the chemical reaction equilibrium under UHP conditions could be affected differently. However, more studies are needed to better understand the reaction mechanism of compounds in Chinese liquor under UHP conditions to confirm which chemical reaction equilibrium has been affected. Results from Table 4.2 also revealed a significant reduction in the alcohol group except for n-butanol and phenylethanol after UHP treatments; however, differences still existed in comparison with the 1-year-aged liquor. The aldehyde groups—acetaldehyde and acetal contents—significantly decreased (p < 0.05) after UHP treatment, which was consistent with the natural aging process. Furan maintained the same level, and no significant difference was observed between young and pressurized liquors. Though the mechanism of how UHP treatments affect the volatile compounds of Chinese liquor is still unknown, these changes have been explained by the principle of Le Chatelier, which states that an increase in pressure enhances any phenomenon accompanied by a decrease in reaction volume and vice versa. Thus, chemical reaction equilibrium in Chinese liquor can be altered through UHP treatments, resulting in volatile composition changes. Furthermore, the decreasing trends of some compounds might have been caused by the enhanced volatilization and hydration under UHP conditions. In general, changes in alcohols and aldehydes after UHP treatments were consistent with those in natural aging, as can be seen intuitively in Fig. 4.1. However, UHP treatments failed to alter the content of acids and long-chain esters in this research. In addition, no significant difference was found between the two selected pressure conditions (400 MPa-15 min and 400 MPa-30 min). It should be noted that despite some differences in the main volatile compounds of the pressurized liquors compared to young liquor, significant overall gaps still existed between pressurized and 1-year-aged liquors (Fig. 4.1).

4.2.5 Impact of Storage on Main Volatile Compounds . Impacts of 2 months of storage on the main volatile compounds Previous studies elucidated that UHP treatments with a processing time of ~5 min and pressures between 400 and 500 MPa affected the volatile compounds of white and red wine. However, the effect was only perceptible after several months of storage, changing the aroma characteristics of the wine. The results led us to hypothesize that UHP treatment accelerates changes in volatile compounds of Chinese liquor, and the effect can be enhanced by storing. As a control group, young liquor was stored for 2 months under the same condition as pressurized liquors. Results from Table 4.2 show that after 2 months of storage, few differences were observed among the volatile compositions of three storage groups, including young (S2), 400-15 (S2), and 400-30 (S2). Ethyl lactate contents in the three groups were even much lower than that in the 1-year-aged group, indicating an increased trend during the storage process. A slight reduction was observed in the ethyl acetate

4.2 Changes in the Main Volatile Compounds of Chinese Liquor

79

content of young liquor, whereas no significant change was found in the ethyl acetate content of pressurized liquors. Furthermore, the short-term storage still did not affect long-chain esters; these long-chain esters were detected only after 1 year of aging. As for the acid groups, as shown in Table 4.2, four acids decreased (p < 0.05), while isobutyric acid stayed at the same level (p > 0.05) after 2 months of storage. As the dominant compound in this group, a sharp decline from 1,100 to 600 mg/L was observed in acetic acid, which was also lower than that in the 1-year-aged liquor (700 mg/L). Acetic acid, an organic acid with a short carbon chain, is volatile to some extent; the initial decline of acetic acid might have been caused due to volatilization during the storage of the first two months; subsequently, the concentration of acetic acid increased gradually with the decomposition of ethyl acetate after storage, as ethyl acetate decreased dramatically during the first year of aging. Similar anomalous changes were also observed in other compounds; more specifically, concentrations of furfural and almost all alcohols after short-term storage were below the range of the young and 1-year-aged liquor (Fig. 4.1), which was unexpected. Based on these findings, we presumed that the major factors affecting the volatile compounds of Chinese liquor change with the storage time during natural aging. As stated before, volatilization plays an important role in the initial storage, decreasing most compounds in the first two months. As the storage time increases, volatile compounds are mainly affected by a series of complex chemical reactions, which are catalyzed by metallic substances that exist in the pot used to store liquor; consequently, after 2 months of storage, some compounds decreased while others increased during storage till 1 year. That could be the reason for anomalous changes observed in some volatile compounds of Chinese liquor during storage; however, this hypothesis has not been verified by any research so far. Moreover, the decreasing trends observed in acetaldehyde and methanol contents were consistent with the natural aging process (Fig. 4.1). Primarily, short-term storage failed to enhance the differences in volatile compounds between young and pressurized Chinese liquor; on the contrary, the initial impact of UHP treatments disappeared if samples were stored for 2 months after UHP treatments, which was unexpected. Hence, storage played a more important role than UHP treatments in changing the volatile composition of Chinese liquor during this process. These findings were paradoxical with former studies as presented before. In addition, the anomalous changes found in some of the compounds suggested the existence of complex chemical reactions during the storage of Chinese liquor. . Impacts of 6 months of storage on the main volatile compounds The results were unexpected as this study was conducted on the hypothesis that UHP treatment accelerates changes in volatile compounds of Chinese liquor based on previous studies. Hence, the experiments were repeated after 6 months of storage. The results of 6 months of storage were almost identical to that of 2 months of storage, reconfirming the conclusions from the preceding section. Hence, the differences between young liquor and UHPT liquors found immediately after the treatment gradually disappeared within a short to medium term of storage. The variation trends of the main volatile compounds detected in the three groups (stored for 6 months)

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were consistent during the storage process. In conclusion, some compounds were maintained at the same level (acetaldehyde, methanol, 2-butanol, 1-propanol, ethyl butyrate, n-butanol, isovaleric acid, phenylethanol, and ethyl palmitate). In contrast, others were slightly decreased (ethyl acetate, acetal, isobutanol, isoamylol, ethyl lactate, and acetic acid). Moreover, increasing trends in furfural and long-chain esters (ethyl hexanoate and ethyl oenanthate) were observed as compared to 2 months of stored liquors. However, the sophisticated chemical reactions resulting in these changes during the storage process are still unknown.

4.2.6 Principal Component Analysis Based on Seven Groups As presented in Fig. 4.3, a scores plot was used to study the character location of Chinese liquor with different treatments and evaluate overall differences in volatile composition among all groups. The first principal component (PC1) and the second principal component (PC2) were taken as coordinate axes, which explained 82.85% of the total variance of all compounds. In principal component analysis (PCA), the higher the cumulative contribution rate, the more original information will be reflected. Generally, the first and second principal components are regarded as good representatives of original information when the accumulated variance exceeds 80%. Liquor samples were located in three different quadrants, and different groups were well-separated. Young and pressurized liquors were distributed in the first quadrant and located closely with each other, which verified the conclusion as stated before; although some differences were observed in the volatile compounds of the pressurized liquors in comparison with young liquor, the impact of UHP treatments

Fig. 4.3 Scores plot of PCA based on the main volatile compounds. Young liquor (0); 400 MPa-15 min (1); 400 MPa-30 min (2); young liquor stored for 2 months (3); 400 MPa-15 min stored for 2 months (4); 400 MPa-30 min stored for 2 months (5); 1-year-aged liquor (6). Reprinted from Ref. [1], used with permission

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging …

81

was still minimal. Liquor samples with 2 months of storage (young and pressurized liquors) were clustered as one category in the third quadrant, signifying the disappearance of the initial difference in volatile compounds between young and pressurized liquors. Moreover, 1-year-aged liquors were isolated in the fourth quadrant; the relatively long distance, as shown in the coordinate system between the 1-year-aged group and other groups, indicates that overall gaps still existed between artificially aged liquors under selected conditions (UHP treatments and short storage) and naturally aged liquors even after 1 year of aging.

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging Characteristics of Chinese Liquor The objective was to evaluate the effect of UHP treatment on the major volatile flavor components in a specific Chinese liquor (Junchang brand, which is a popular local brand produced by Junchang Liquor Factory in Sichuan province) subjected to 15–30 min treatment at 100–400 MPa. For volatile profile analysis with an electronic nose, four chemical tests, namely, total acid, alcohol, total ester, and total solid content, and GC, were conducted to compare the profiles of treated liquors, untreated liquors, and the liquor stored for 6 years. In addition, a sensory evaluation was performed.

4.3.1 Wine Samples and UHP Treatments Chinese liquor “Junchang” (sauce flavor) was obtained from a local source and stored at room temperature before use. Junchang is a local brand produced by Junchang Liquor Factory, a winery in the Sichuan province of China. This factory was founded more than 10 years ago, with an annual output of ~300 kL in recent years. This brand is commercially not well recognized like the top 10 brands detailed earlier because it is a local brand sold locally. UHP treatments were performed using a laboratory scale UHP equipment, as shown in Fig. 4.4. UHP treatments were performed using the same procedure described in Sect. 2.2.4. Different combinations of UHP treatment involving pressure level and time were used in the study—pressure (100, 200, 300, and 400 MPa) and holding time (15 and 30 min)—and each combination was triplicated.

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

Fig. 4.4 Schematic diagram of the ultra-high-pressure experimental setup. Reprinted from Ref. [2], used with permission

4.3.2 Electronic Nose Analysis . Measurement A FOX model 4000 from AlphaMOS (Toulouse, France) was used as the electronic nose system. The FOX 4000 instrument consisted of 18 metal–oxide–semiconductor (MOS) gas sensors and three temperature-controlled chambers (Table 4.3). A pure air generator (Whatman), attached to a dehydration cartridge filled with phosphoric anhydride, was used for flushing FOX. The multisensory array was interfaced with a computer that collected the sensor signals via an RS-232 port. A supervisor program (Alpha Soft Version 9) was equipped to control the whole system and analyze signals with chemometric methods. A 10-μL liquor sample was diluted to 1,000 μL using deionized double distilled water, placed in a 10-mL headspace bottle, and sealed using a bottle capper. The diluted liquor samples were maintained at normal temperature for 1 h to let the aroma components volatilize sufficiently. Further, 2-mL headspace gas was injected into the electronic nose for detection. Two pattern recognition methods, namely, the

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging … Table 4.3 Metal–oxide–semiconductor sensor name. Reprinted from Ref. [2], used with permission

Chamber Chamber CL

Chamber A

Chamber B

Number

83

Sensor

1

LY2/LG

2

LY2/G

3

LY2/AA

4

LY2/GH

5

LY2/gCTL

6

LY2/gCT

7

T30/1

8

P10/1

9

P10/2

10

P40/1

11

T70/2

12

PA2

13

P30/1

14

P40/2

15

P30/2

16

P40/2

17

T40/1

18

TA2

principal component analysis (PCA) and discriminant function analysis (DFA), were used to analyze the signals gathered by electronic nose sensors. . Results (1) Principal component analysis results As generally defined, PCA is a statistical procedure that uses an orthogonal transformation to convert a set of possibly correlated variables into a set of linearly uncorrelated variables called principal components. The number of principal components is less than or equal to the original variables. This transformation is defined in such a way that the first principal component has the largest possible variance, i.e., it accounts for as much of the variability in the data as possible, and each succeeding component, in turn, has the highest variance possible under the constraint that it is orthogonal to the preceding components. PCA is a type of multivariate statistical method in which data converses and dimensionality reduces the extracted multi-index information of sensors. It linearly classifies the characteristic vector after dimensionality reduction and displays the major two-dimensional map on a PCA analysis map. In the absence or lack of sample information, PCA can quickly scan all data to determine the associated features of the samples and make a conclusion based on the available information.

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The classification results of different groups through PCA analysis are shown in Fig. 4.5. PC1 and PC2 were taken as coordinate axes for the PCA analysis on samples, and it was found that the linear combination of PC1 and PC2 explained an overall variance higher than 93%. The figure also shows that, except for Samples 2 and 5, the different groups of liquor components were well-separated from each other in the selected coordinate system. Furthermore, response values of Samples 7, 8, 9, and 0, which referred to 300 MPa-30 min, 400 MPa-15 min, 400 MPa-30 min treatments, and 6-year-aged liquor, respectively, were negative on PC1, while the other groups were positive. As most of the 18 sensors used in the electronic nose were placed on PC1 (explained by an overall variance of 83.9%), the result implied that Samples 7, 8, 9, and 0 could be clustered as one category. In contrast, Samples 1–6 are clustered as another category. Meanwhile, an apparent difference between Sample 0 (6-year-aged liquor) and Samples 7, 8, and 9 in PC2 can also be observed in Fig. 4.5, indicating that some difference remained between these variables. (2) Discriminant function analysis results DFA is another multivariate technique for describing a mathematical function that distinguishes among predefined groups of samples. DFA has a strong connection to multiple regression and PCA. In addition, DFA is the counterpart to the analysis of variance (ANOVA); in DFA, continuous variables (measurements) are used to predict a categorical variable (group membership), whereas ANOVA uses a categorical variable to explain variation (prediction) in one or more continuous variables. DFA is a classification technique that optimizes the distinguishing ability of variables by recombining sensor data. Mathematical manipulation minimizes the differences between the same data type and broadens the disparities between data from different categories to establish a data recognition model.

Fig. 4.5 PCA map of nine samples: 6-year-aged (0), control (1), 100 MPa-15 min (2), 100 MPa-30 min (3), 200 MPa-15 min (4), 200 MPa-30 min (5), 300 MPa-15 min (6), 300 MPa-30 min (7), 400 MPa-15 min (8), and 400 MPa-30 min (9). Reprinted from Ref. [2], used with permission

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging …

85

Fig. 4.6 DFA map of nine samples: 6-year-aged liquor (0), control (1), 100 MPa-15 min (2), 100 MPa-30 min (3), 200 MPa-15 min (4), 200 MPa-30 min (5), 300 MPa-15 min (6), 300 MPa-30 min (7), 400 MPa-15 min (8), and 400 MPa-30 min (9). Reprinted from Ref. [2], used with permission

The DFA map of the nine samples used in this study is shown in Fig. 4.6. The electronic nose could distinguish the liquor samples based on the different treatment conditions. DF1 and DF2 explained a total variance of 77.9% and 11.7%, respectively. Thus, the first factor allowed effective discrimination of different liquor sample groups. As observed with PCA, Samples 7, 8, 9, and 0, which referred to 300 MPa30 min, 400 MPa-15 min, and 400 MPa-30 min treatments, and 6-year-aged liquor, respectively, were positive on DF1, whereas other groups were negative. As a result, Samples 7, 8, and 9 were clustered as one category while samples 1–6 were clustered as a second category. There was also a difference between the 6-year-aged liquor and Samples 7, 8, and 9 on DF2, similar to the PCA results. The PCA and DFA analysis results demonstrated that the UHP treatments altered the volatile composition of fresh (young) liquors compared to the aged control. Furthermore, the isolation of Samples 7, 8, 9, and 0 on both maps indicate that UHP treatments at 300 and 400 MPa influenced the liquor-aging parameters more than that at 100 and 200 MPa. However, the results from PC2 and DF2 also demonstrated that some differences still existed between pressure-treated samples and the 6-year-aged liquor, indicating further exploration of the treatment effects and some level of aging would be needed following treatment. Pressure-treated samples at 300 and 400 MPa were selected for further chemical analysis.

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4.3.3 Chemical Analysis 4.3.3.1

Total Acid Content

Total acid content was measured according to the standard Chinese liquor analysis methods. The total acid content of control, 300 MPa-15 min, 300 MPa-30 min, 400 MPa-15 min, 400 MPa-30 min, and 6-year-aged samples were 0.854, 0.817, 0.808, 0.794, 0.788, and 0.726 g/L, respectively, which were within the commercial specification range of Chinese liquors (0.6–1.58 g/L) (Fig. 4.7). Statistical analysis demonstrated that UHP treatments lowered the total acid content (p < 0.01). UHP treatment at 400 MPa for 30 min yielded the lowest total acid content of 7.7%, while a decline of 14.1% was observed in the 6-year-aged liquor. The loss of volatile acids may have caused the reduction in acidity due to the esterification between acids and alcohol components during UHP treatments. Hence, this is considered as an indicator of the aging of liquor. It was reported that most acids in TN sweet, TN dry, and Malaysia wines decreased after aging for 1 month. Overall differences between the four pressure-treated samples were not significant when they were considered together; however, differences existed between 300 and 400 MPa-treated samples (either for 15 or 30 min), indicating that pressure level could be a more effective factor than holding time in influencing the acid composition.

Fig. 4.7 Total acid content of liquor samples treated under different conditions. The error bars indicate the standard deviation. Different letters above the bars indicate the significance under p < 0.05. Reprinted from Ref. [2], used with permission

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging …

4.3.3.2

87

Alcohol Content

The alcohol content was measured according to the standard Chinese liquor analysis methods. A slight decline in alcohol content was observed compared to the young liquor without UHP treatment, with the lowest alcohol content obtained following UHP treatment at 400 MPa-30 min (Fig. 4.8). The alcohol content of the aged sample was lower than all UHPT samples, although much closer to the 400 MPa-30 min treated sample. Some gaps still existed between the UHPT and aged samples with respect to the alcohol level. However, no significant decrease was observed between the untreated control and samples treated with pressure (p > 0.05). Traditionally, there will be some decline in the alcohol content during the natural aging process of liquors. Through long-time maturation, the odor peculiar to ethanol in spirits is reduced, and consequently, their tastes are altered to be favorable after aging. These have been linked to the changes in the structure of water and ethanol molecules. Previously, the hydrogen bonding of water-ethanol in aged whiskey was studied by the 1 H NMR chemical shifts of the OH in water and ethanol. It has also been reported that some volatile and nonvolatile compounds could assist the formation of ethanol–water clusters during aging, which reduces ethanol stimulation. Generally, in a wine sample, the aroma compounds represent only ~1 g/L, while the ethanol and water concentrations are ~100 and 900 g/L, respectively.

Fig. 4.8 Alcohol content of liquor samples treated under different conditions. The error bars indicate the standard deviation. Different letters above the bars indicate the significance under p < 0.05. Reprinted from Ref. [2], used with permission

88

4.3.3.3

4 Effect of UHP Processing on the Main Volatile Components and Aging …

Total Ester Content

Total ester content was measured according to the standard Chinese liquor analysis methods. The total ester content of pressure-treated samples was significantly high (Fig. 4.9), which is used to characterize the liquor samples after natural aging. The total ester content of control, 300 MPa-15 min, 300 MPa-30 min, 400 MPa-15 min, and 400 MPa-30 min samples were in the range of 0.95–1.02 g/L. The highest level of total ester was obtained at 400 MPa-15 min with an increase of 5.7%. The total ester content of the 6-year-aged liquor was 2.053 g/L, twice the amount in liquors without aging. Esters could be the most important class of all the aromas constituted in Chinese liquor. Trace component analysis indicates that more than 30 types of esters have been found in some famous brands of Chinese liquors, such as Maotai, Daohuangxiang, and Luzhoulaojiao, and the total ester of the aged liquor could be as high as 2 g/L. Esters are products of esterification between acids and alcohols during fermentation and aging, especially ethyl esters, which could be a reason for the decrease in total acid and alcohol contents during aging. The enhancing effect of pressure on esterification has been explained by the principle of Le Chatelier. As a result, UHP treatment favors the increasing levels of total ester content in Chinese liquor.

Fig. 4.9 Total ester content of liquor samples treated under different conditions. The error bars indicate the standard deviation. Different letters above the bars indicate the significance under p < 0.05. Reprinted from Ref. [2], used with permission

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging …

4.3.3.4

89

Total Solid Content

The total solid content was measured according to the standard Chinese liquor analysis methods. The total residual solid content represents the nonvolatile and volatile substances with high boiling points. A significant increase (p < 0.05) was observed in the total solids content of UHPT samples (Fig. 4.10). Solid content is an undesirable index in commercial liquors; therefore, a lower solid content is desired in high-quality liquors. The interaction between organic acid and metal ions during manufacturing is the leading cause of solids. Previous studies have reported that metal ions, especially calcium and sodium ions, increased rapidly after aging for several years, which appeared to be mainly dissolved from the container. The generation of high-boiling-point esters followed by UHP treatments might be responsible for the increase in solid content. However, further research is required to substantiate this claim.

4.3.4 Gas Chromatography Analysis GC was performed using the same procedure described in Sect. 2.2.2. Aroma components in the liquor samples were identified by comparing the retention times with those of authentic compounds in a mixed standard solution. Aroma components

Fig. 4.10 Total solid content of liquor samples treated under different conditions. The error bars indicate the standard deviation. Different letters above the bars indicate the significance under p < 0.05. Reprinted from Ref. [2], used with permission

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

were quantified based on calibration curves obtained with the mixed standard solution under the same chromatographic conditions as those used for liquor samples. All liquor samples were analyzed using the direct injection method, replicated five times, and the values were averaged. Only the liquor treated at 400 MPa-30 min was selected as this group performed well in the electronic nose and chemical analysis. As presented in Table 4.4, four acids and six esters were identified. Acetic acid was the most abundant aroma component of the acid group, with a concentration of 1,131 mg/L in young liquor, while the total content of the other three acids was below 100 mg/L. Generally, the total acid content in Chinese liquor is expressed as the concentration of acetic acid as more than 90% of the acidity is contributed by it. The concentration of acetic acid was 670 mg/L in the 6-year-aged liquor, with a decrease rate of 41% (p < 0.05) compared with the young liquor. Meanwhile, a reduction of 15% (p < 0.05) was found in the concentration of acetic acid after UHP treatment under selected conditions (400 MPa-30 min). However, after UHP treatment, no significant change was observed in the concentrations of propionic, butanoic, and isopentanoic acids. Ethyl acetate and ethyl lactate were the most prominent representatives in the ester group, with concentrations of 1,656 and 1,014 mg/L in the young liquor, respectively. Concentrations of ethyl acetate in aged and UHPT liquors were 1,904 and 1,753 mg/ L, respectively, which were significantly higher than in young liquor. It appears that ethyl lactate is highly stable during aging and UHP treatments as no significant change was observed in its concentration. UHP treatment also increased (p < 0.05) the ethyl hexanoate concentration from 11.5 to 55.7 mg/L. However, UHP treatment did not affect the concentration of ethyl butyrate (p > 0.05). Ethyl oenanthate and ethyl palmitate were 54.6 and 82.2 mg/L in the 6-year-aged liquor, respectively; Table 4.4 Gas chromatography analysis. Note: All values are mean (mg/L) ± standard deviation (SD). Values followed by different letters are significantly different (p < 0.05). ND: not detected. Reprinted from Ref. [2], used with permission

Components

Young

6-year-aged

UHP

Acetic acid

1131.5 ± 50.6a

670.2 ± 38.9c

969.6 ± 52.7b

Propanoic acid

15.8 ± 0.9a

14.9 ± 1.5a

13.1 ± 0.8a

Butanoic acid

41.4 ± 3.7b

55.7 ± 5.6a

45.5 ± 2.7b

Isopentanoic acid

28.8 ± 1.4a

25.9 ± 2.3a

25.6 ± 2.1a

Ethyl acetate

1656.6 ± 66.2c

1904.3 ± 50.7a 1753.8 ± 49.6b

Ethyl butyrate

8.2 ± 0.7b

Ethyl hexanoate 11.5 ± 1.2c

17.5 ± 1.6a

7.4 ± 0.7b

205.4 ± 10.2a

55.7 ± 3.3b

Ethyl lactate

1014.7 ± 50.3a

1053.2 ± 70.2a 1119.9 ± 66.9a

Ethyl oenanthate

ND

54.6 ± 3.2

ND

Ethyl palmitate

ND

82.2 ± 7.4

ND

4.3 Effect of the Ultra-High-Pressure Treatment on the Aging …

91

however, ethyl oenanthate and ethyl palmitate were not detected in the young and UHPT liquors.

4.3.5 Sensory Analysis A panel of six judges participated in the sensory evaluation. According to the standard Chinese liquor analysis method, all panelists were trained to be familiar with the different quality attributes of typical Chinese liquors. A quantitative descriptive analysis method was used to determine the differences in the sensorial characteristics among the UHPT, untreated, and 6-year-aged Chinese liquors. All test samples were presented to each panelist after storing them at room temperature for 3 days following the UHP treatment. Seven sensorial attributes associated with Chinese liquor, mainly related to appearance, odor, taste, and overall quality, were selected by consensus. The definition of sensorial attributes and anchors used in the quantitative analysis are shown in Table 4.5. The appearance, odor, and taste parameters were individually averaged to determine the overall sensory scores. Then, the combined average was considered to provide an equal weightage to appearance, odor, and taste-related quality parameters. The differences in the means were statistically tested as detailed earlier. Table 4.5 Definitions of sensorial attributes used in the descriptive analysis of Chinese liquor. Reprinted from Ref. [2], used with permission Definition

Anchoring points (full mark = 10)

1. Color

Colorless and transparent

Colorless and transparent (10)–yellowish (0)

2. Clarity Odor

Lack of cloudiness

Clear (10)–dull (0)

3. Pungent

Strong and sharp; acrid

Imperceptible (10)–very intensive (0)

4. Exotic fragrance

Fragrance that does not belong to liquor

Imperceptible (10)–very intensive (0)

5. Bouquet taste

Typical pleasing odor of liquor

Imperceptible (0)–very intensive (10)

6. Harsh

Coarse; spicy

Imperceptible (10)–very intensive (0)

7. Mellow

Full and pleasing flavor

Imperceptible (0)–very intensive (10)

Overall quality

Overall quality of the sample

High quality (10)–inferior quality (0)

Attributes Appearance

Odor

Taste

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

Sensory analysis is the most straightforward technique to evaluate the quality of Chinese liquor because it reflects consumer perceptions. Therefore, liquors were subjected to quantitative descriptive analysis of sensory tests to investigate the influence of UHP treatment on the sensorial characteristics of Chinese liquor. Liquors treated at 400 MPa-30 min were chosen to conduct sensory analysis as this group performed well in the electronic nose and chemical analysis. The testing procedure was based on unpublished data, which has reported that UHP makes the flavor profiles of liquor to be transient and completely disappear 2 days after the treatment. The influence of UHP treatment on Chinese liquor sensorial attributes has been illustrated as a spider plot in Fig. 4.11. Attributes related to liquor appearance (Attributes 1 and 2)—the most dominant sales-related factors—were found to be highly influenced by UHP treatment (p < 0.05), where changes in color and clarity induced by UHP were visible. As for the odor, the intensity of the pungent smell declined (Attribute 3) after UHP treatment (desirable). However, UHP treatment also brought exotic (external) fragrance (Attribute 4) to the liquor, which may have been caused by the packaging film used for subjecting the samples to the UHP treatment (and possibly overcome using more appropriate packaging materials). Meanwhile, the bouquet flavor remained the same (Attribute 5). The average score for UHPT liquor was significantly higher than the one for fresh liquor (p < 0.05) for three parameters, except for the taste (Table 4.6). Gustatory attributes were not statistically significant between the fresh and UHPT liquors. However, a reduction in the intensity of mellowness was observed after the UHP treatment. This could be the exact reason for the formation of the external exotic smell, which could have been caused by the packaging bag. The overall quality of the UHPT Chinese liquor was calculated by the average value of appearance, odor, and taste (Table 4.6), which indicated that the UHPT liquor scored higher than the untreated young liquor (p < 0.05), while no significant difference was found between the UHPT liquor and 6year-aged liquor (p > 0.05), indicating the overall quality of the UHPT sample being much closer to the aged liquor. As expected, the 6-year-aged liquor scored the highest color and flavor profiles (p < 0.05), and UHPT samples had profiles much closer to the six-year-aged liquor.

4.4 Quality Assessment of Chinese Liquor with Different Ages and Prediction Analysis Chinese liquors with different aging were used in this study, and nine dominant volatile compounds were selected and researched. The main objectives of this part were: (1) to investigate the possibility of using electronic nose combined with PCA and linear discriminant analysis (LDA) to discriminate Chinese liquor with different ages and (2) to realize the prediction analysis of liquor age and the contents of main volatile compounds in Chinese liquor based on response values and partial least squares regression (PLSR).

4.4 Quality Assessment of Chinese Liquor with Different Ages …

93

Fig. 4.11 Effect of UHP processing on the sensory characteristics of Chinese liquor. Reprinted from Ref. [2], used with permission

Table 4.6 Sensory analysis results. Note: All values are means± standard deviation (SD). Values followed by different letters are significantly different (p < 0.05). Overall quality was calculated by the average value of appearance, odor, and taste. Reprinted from Ref. [2], used with permission

Attribute

Young

6-year-aged

UHP

Appearance

7.70 ± 0.67b 9.40 ± 0.51a 8.90 ± 0.56a

Odor

7.53 ± 0.63c

Taste

6.70 ± 0.48b 7.80 ± 0.42a 6.40 ± 0.69b

8.73 ± 0.59a 8.13 ± 0.63b

Overall quality 7.31 ± 0.59b 8.64 ± 0.51a 7.81 ± 0.63ab

4.4.1 Liquor Samples and Chemicals Chinese liquor was provided by “Junchang” Liquor Factory, a very popular local brand located in Sichuan Province. Young liquor and liquors aged for 1, 2, 3, 4, and 5 years were used. All liquor samples were stored in sealed potteries at normal temperature before use.

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

4.4.2 Volatile Compounds . Measurement GC was performed using an Agilent 7890A GC unit equipped with a FID. All samples were analyzed on an LZP-950 column (50 m × 0.32 mm; 1.0-μm film thickness), which is a specialty column for Chinese liquor GC analysis. Pure reagents were firstly diluted with ethanol to obtain solution-to-ethanol ratios of 1:0, 1:1, 1:3, 1:7, 1:15, 1:31, 1:63, and 1:99; this series of the diluted solution was used for the protraction of standard curves. Before the injection, all samples were filtered into a 2-mL autosampler vial using a filtering membrane with a pore diameter of 0.45 μm to remove impurities. Then, a measurement volume (1 μL) of the test sample was injected into GC from the autosampler vial with a split ratio of 1:1. The column carrier gas nitrogen was delivered at a constant flow rate of 1 mL/min. The oven temperature was held at 65 °C for 8 min, raised to 200 °C at a rate of 5 °C/min, and held at 200 °C for 50 min; injector and detector temperatures were set at 230 °C and 250 °C, respectively. The identification analysis was made by comparing the retention times of volatile compounds in liquor samples with those of the corresponding reference compounds. The quantification analysis was made using the established calibration curves. To eliminate the error caused by sample injection, amyl acetate was added to all samples as an internal standard substance, and the final quantification results were corrected using this internal standard. All liquor samples were analyzed and calculated in five replicates, and the values were averaged.

4.4.2.1

Quantification of the Volatile Compounds

Although hundreds of volatile compounds have been found in Chinese liquor by previous research, only the dominant compounds were selected in this study, as the main objective of this research was to explore the possibility of predicting volatile compound contents using electronic nose signals. As shown in Table 4.7, two aldehydes, four alcohols, two esters, and one acid were included as the nine dominant compounds, and changes in them during aging were also characterized. Aldehydes are generally formed by oxidizing alcohols in food and usually have the odor of overripe apples. As presented in Table 4.7, concentrations of acetaldehyde and acetal decreased rapidly during the first year of aging with rates of 35% and 47% (p < 0.05), respectively, and no significant change was found in the following three years. However, a slight decrease (p < 0.05) was observed in the fifth year, which could harm human health because a high concentration of acetal can damage the liver. Alcohols are mainly produced by the deamination of amino acids under anaerobic conditions and the decarboxylation of sugars under aerobic conditions during fermentation. Methanol and 1-propanol decreased significantly in the first year and were maintained at the same level (p > 0.05) in the following years, while a reduction of isoamylol and isobutanol contents (p < 0.05) was only observed in

4.4 Quality Assessment of Chinese Liquor with Different Ages …

95

Table 4.7 Concentration of major volatile compounds in different aged Chinese liquors. All values are expressed as mean (mg/L) ± standard deviation (SD). Different letters indicate significant differences (p < 0.05). Reprinted from Ref. [3], used with permission Number Compound

Young

1 year

2 years

3 years

4 years

5 years

1

Acetaldehyde 237.81 ± 11.87a

152.99 ± 148.14 ± 144.73 ± 153.67 ± 118.96 ± 12.67b 6.34b 13.72b 5.76b 4.53c

2

Methanol

79.79 ± 3.46a

3

Ethyl acetate

1741.48 ± 694.23 ± 677.85 ± 692.20 ± 627.80 ± 446.29 ± 87.52a 36.17b 44.60b 164.33b 32.33b 20.55c

4

Acetal

519.72 ± 30.60a

275.31 ± 260.37 ± 267.91 ± 255.42 ± 183.57 ± 22.76b 11.09b 23.17b 18.14b 6.46c

5

1-Propanol

338.84 ± 16.54a

261.24 ± 247.04 ± 267.68 ± 270.81 ± 341.57 ± 20.35b 7.04b 23.10b 13.65b 10.96a

6

Isobutanol

637.67 ± 39.72a

593.72 ± 573.37 ± 551.78 ± 549.03 ± 390.93 ± 48.15a 29.01a 39.16a 35.42a 22.71b

7

Isoamylol

800.53 ± 41.79a

793.02 ± 766.17 ± 740.56 ± 744.52 ± 588.39 ± 38.19a 19.15a 52.14a 44.75a 18.53b

8

Ethyl lactate

446.38 ± 30.40a

465.10 ± 445.73 ± 450.21 ± 454.67 ± 443.85 ± 31.16a 27.98a 19.36a 31.46a 23.94a

9

Acetic acid

1194.00 ± 701.73 ± 688.29 ± 673.71 ± 676.13 ± 570.52 ± 91.82a 40.19b 37.69b 44.96b 38.74b 30.31c

46.75 ± 2.06b

41.09 ± 1.66b

44.62 ± 3.97b

44.68 ± 2.35b

39.86 ± 1.11b

the fifth year. Notably, the control of methanol level is highly important during the manufacturing of Chinese liquor due to its toxicity, though the content is generally much lower than other abundant alcohols. As for esters, the content of ethyl acetate was reduced to 40% within the first two years and then maintained at the same level (p > 0.05) in the next three years; however, a further reduction was found in the fifth year, with a rate of 30%. As an exception, ethyl lactate seems to be the most stable compound as no significant change was observed throughout the studied aging process from the first to the fifth year. This relatively stable content of ethyl lactate can conduce to the mellow sensorial properties of Chinese liquor. The concentration of acetic acid was 1,194 mg/L, contributing more than 90% of the acidity in Chinese liquor. In the first year, a 41% decline from 1,194 to 701 mg/L was found for acetic acid; then, no significant change was observed until the fifth year. Acids in Chinese liquor can impact sensory characteristics, contribute to color stability, and increase the antioxidant power of the products. Above all, eight compounds decreased during aging, except for ethyl acetate. Besides the complicated physicochemical reactions, volatilization and the possible permeation have played an indispensable role during this process.

96

4.4.2.2

4 Effect of UHP Processing on the Main Volatile Components and Aging …

Prediction of Volatile Compound Content

As previously discussed, of all the nine compounds researched, the content of ethyl lactate was maintained in the range of 443–446 mg/L from the beginning to the end of aging, and no significant difference was observed among six liquor groups with different ages. Based on that, the prediction of ethyl lactate content was optional, at least during the first five years of aging; thus, only eight compounds were studied in this part. For all the prediction modeling of eight compounds, 90 samples (15 samples for each group) were randomly selected as the calibration set and other 30 samples (5 samples for each group) were used as the validation set. After the extraction of factors, cross-validation was also performed using the leave-one-out technique to choose the optimized number of factors used for the subsequent modeling. Calibration and validation results are listed in Table 4.8. The total sum of the square consisted of the regression sum of the square and the residual sum of the square, and the regression sum of the square versus the total sum of the square calculated the fitting degree (R2 ) of the PLSR model. The R2 of eight PLSR models in the calibration set was within the range of 0.8602–0.9379, demonstrating a good correlation between the actual and predicted values. As for the validation set, the prediction ability of the 1-propanol PLSR model was deficient, with a low R2 of 0.5439, while the R2 values of the other seven compounds ranged from 0.7053 to 0.7614. As shown in Table 4.8, in the calibration set, R2 values of 1-propanol, isobutanol, and isoamylol were 0.8602, 0.8935, and 0.8775, respectively, while the other five compounds were all ~0.9300. Furthermore, in the validation set, relatively lower R2 values were obtained with these three compounds when compared with the other five compounds. The response values of the studied sensors and modeling technique seemed to have drawbacks in predicting alcohol contents, except for methanol. Beyond that, the established method performed well in the prediction of acetaldehyde, ethyl acetate, acetal, and acetic acid. Table 4.8 Prediction results of volatile compounds based on partial least square regression. R2 : fitting degree; RMSE: root mean square error. The larger the R2 values and the lower the RMSE value, the better the prediction model. Reprinted from Ref. [3], used with permission Compound

Calibration set R2

Validation set RMSE

R2

RMSE

Acetaldehyde

0.9327

9.60

0.7459

Methanol

0.9379

3.43

0.7603

6.74

Ethyl acetate

0.9304

111.79

0.7542

210.13

18.63

Acetal

0.9343

27.04

0.7614

51.54

1-Propanol

0.8602

14.12

0.5439

25.50

Isobutanol

0.8935

25.08

0.7106

41.35

Isoamylol

0.8775

24.81

0.7053

38.49

Acetic acid

0.9354

55.51

0.7574

99.89

4.4 Quality Assessment of Chinese Liquor with Different Ages …

97

4.4.3 Electronic Nose Analysis A PEN2 portable electronic nose (Airsense Analytics, Germany) was used in the experiment. The sensor array of this system was composed of 10 MOS-type sensors with different chemical compositions and thicknesses; each sensor generated a specific response when exposed to corresponding volatile substances, namely, S1 (W1C, aromatic compounds), S2 (W5S, nitrogen oxides), S3 (W3C, ammonia), S4 (W6S, hydrogen), S5 (W5C, alkanes, less polar compounds), S6 (W1S, methane), S7 (W1W, sulfur compounds), S8 (W2S, alcohols), S9 (W2W, organic sulfur compounds) and S10 (W3S, high concentrations of methane). A data acquisition instrument was also equipped for collecting and recording response values. Further, 3 mL of the liquor sample was diluted with deionized water to 300 mL, and then, 5 mL of the diluted solution was transferred into a 500-mL beaker. After that, the beaker was sealed using a preservative film and let stand for 15 min to get the headspace to reach equilibration. The detection time was set as 80 s, which was long enough for all sensors to reach stable response values. Afterward, 100 s of cleaning (nitrogen) was executed to ensure the sensor array was normalized to the initial state. All liquor samples (20 replicates multiplied by six groups) were detected under the same conditions at ambient temperature (25 °C).

4.4.3.1

Selection of Response Values

The response value of sensors was calculated as G/G0, where G0 is defined as the electric conductivity of nitrogen (cleaning gas) and G is defined as the electric conductivity of tested samples. Typical response curves of the sensor array during the detection period of liquor samples are given in Fig. 4.12. The initial response values of 10 sensors were all ~1, which means that the sensor array was well-calibrated before the detection. This process lasted for ~5 s, and then, the response values of almost all sensors diverged from 1 and gradually became stable after 40 s (except for S2). Considering the continuous decrease in S2, the response values of all sensors at 70 s were chosen as the original data. As shown in Table 4.9, low RSD values and significant differences (p < 0.001) were obtained for all sensors among six liquor groups with different ages, indicating that the response values of the sensor array were stable and credible when exposed to liquor samples and all the 10 sensors were closely related with liquor ages. Thus, the response values of all sensors (S1 to S10) were used as the data matrix for the following discrimination and prediction analysis.

4.4.3.2

Classification Based on Principal Component Analysis

As shown in the scores plot (Fig. 4.13), the first two principal components (PC1 and PC2) were taken as coordinate axes, which explained the total variance of 90.36% (82.65% for PC1 and 7.71% for PC2). It showed that liquor samples clustered closely

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4 Effect of UHP Processing on the Main Volatile Components and Aging …

Fig. 4.12 Typical responding curves of liquor samples obtained from the electronic nose. Reprinted from Ref. [3], used with permission

Table 4.9 Results of the RSD test and one-way ANOVA of the response values. Each RSD value was obtained from 20 samples of each group. One-way ANOVA of each sensor was performed with 120 samples (20 replicates multiplied by six groups). Reprinted from Ref. [3], used with permission Sensor Young (%) One (%) Two (%) Three (%) Four (%) Five (%) F value

P value

S1

2.21

1.96

2.76

2.04

2.29

2.08

73.088 0.05) in FS among different pressure treatments or between control and UHP treatments at 200 and 300 MPa. It has been reported that the FS of potato protein treated at 200, 400, and 600 MPa was increased; however, there were no significant differences among the UHP treatments. Another research has shown that the increased FS might be due to the enhanced protein–protein interactions after pressure treatment. Foam stability requires a cohesive, thick film around the air bubble, which was likely enhanced by UHP treatment.

5.5.3.5

Emulsifying Properties

• Measurement of emulsifying activity index and emulsion stability index The emulsifying activity index (EAI) and emulsion stability index (ESI) of rice bran protein were measured as follows: 12 mL of rice bran protein solution was mixed with 4-mL soybean oil, making the oil fraction equal to 0.25. Then, the mixture was homogenized using a high-speed homogenizer (FJ 200-SH, Songben, Shanghai, China) at 10,000 kr/min for 2 min. Further, 50 μL of the freshly prepared emulsion was taken from the bottom of the emulsion and diluted with 5-mL 0.1% SDS. After vertexing the solution to mix it well, the absorbance was measured at 500 nm. The EAI and ESI were calculated using Eqs. (5.19 and 5.20). ( ) E AI m 2 /g =

2×2.303×A0 , 0.25× pr otein weight (g)

E S I (min) =

A0 A0 −A10

× ΔT,

(5.19) (5.20)

where A0 and A10 are the absorbances of the diluted sample at 0- and 10-min individualities and ΔT is 10 min. • Results of emulsifying activity index and emulsion stability index The effect of UHP treatment on EAI and ESI of rice bran protein is shown in Fig. 5.23. UHP treatment significantly improved the EAI of rice bran protein. The EAI of the

5.5 Effect of UHP Performance on the Properties of Rice Bran Proteins

183

untreated control sample was only 17.5%, while after a 100-MPa treatment, it sharply increased to 25.4%. However, a further increase in pressure level did not significantly change the EAI of rice bran protein. Similar results have also been reported for the EAI of soy protein isolates (1%, 3%, and 5%), in which treatment at 200 MPa significantly increased the EAI. Still, no differences were observed with a further increase in pressure. It has been reported that the emulsifying capacity was greatly influenced by surface hydrophobicity. The improved EAI after UHP treatment may be due to the unfolding of rice bran protein and exposure of more hydrophobic groups, leading to higher protein absorption at the oil–water interface, thereby improving the emulsifying activity. Proteins are good emulsifying agents, and stable emulsions can be formed due to the existence of hydrophobic and hydrophilic groups in the protein moiety. EAI represents the ability of a protein to rapidly absorb polar and nonpolar components at the water–oil interface by preventing coalescence. UHP treatment also increased the ESI of rice bran protein. ESI reflects the ability of an emulsion to keep itself emulsified for a quantifiable period. ESI of the control sample was only 18.2 min, while after UHP treatment, it increased several folds, especially from 100 to 400 MPa; however, at 500 MPa, it slightly decreased. The emulsifying capacity and stability of protein depend on the hydrophilic and lipophilic balances, which could be affected by UHP treatment. It is understood that UHP treatment helps to open the structure of rice bran protein, exposing hydrophilic and lipophilic groups. This facilitates interactions between proteins and solvents and prevents the coalescence of oil drops. It has been reported that molecular flexibility

Fig. 5.23 Emulsifying activity index (EAI) and emulsion stability index (ESI) of untreated control and UHPT (100–500 MPa) rice bran proteins. Different letters on top of the columns indicate significant (p < 0.05) differences among samples treated under different conditions. Reprinted from ref. [10], copyright 2017, with permission from Springer Nature

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5 Improving Taste, Cooking Properties, and Digestibility of Brown Rice

is also important for food emulsion stability. The reduction of ESI to 500 MPa might be related to the decrease in molecular flexibility after a higher severity of pressure treatment.

5.5.3.6

Least Gelation Concentration

• Measurement The minimum protein concentration required to form a gel was studied as follows: a series of rice bran protein suspensions from 6 to 18% with an increment of 2% was prepared using distilled water. These were taken in glass test tubes, heated in boiling water for 1 h, followed by rapid cooling under running tap water, and allowed to stand at 4 °C in a refrigerator (BCD-256KFB, Haier, Qingdao, China) for 2 h. The lowest gelation concentration was the lowest concentration at which the sample in the inverted tube did not fall or slip. • Results The effect of UHP treatment on gelling properties of rice bran protein is summarized in Table 5.20. The least gelation concentration (LGC) was used as an index of the gelation capacity of the protein. From a magnitude point of view, the lower the value of LGC, the better its gelation capacity. The control sample did not form a gel until the final concentration of 18% was used. This gelation concentration level agrees with the value reported by another study for native rice bran protein. The LGC value significantly decreased to 12% and 10% following UHP treatments at 100 and 200 MPa, respectively, while a further increase in pressure resulted in no further changes (p > 0.05) in LGC values. Some similar results have also been reported by other researchers. It has been reported that the LGC of UHPT cowpea protein isolate showed better results at 200 and 400 MPa. Moreover, the LGC of rapeseed protein isolate was significantly decreased after UHP treatment. The gelation mechanism and appearance have been reported to be predominantly influenced by hydrophobic interactions. Therefore, the decrease in LGC could be due to the unfolding of rice bran protein and exposure of hydrophobic core groups due to the UHP treatment. The unfolded protein can more easily interact through hydrophobic bonding to increase the strength of the resultant gel network and reduce the amount of protein required to form a gel. Furthermore, gelation is the aggregation of denatured molecules, and UHP treatment partially denatures the rice bran protein, resulting in more aggregation than in the untreated control sample.

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Table 5.20 Gelling properties at different concentrations of untreated control and UHPT (100– 500 MPa) rice bran proteins. Reprinted from ref. [10], copyright 2017, with permission from Springer Nature Sample concentration (%)

Pressure (MPa) Control

100

200

300

400

500

6

−

−

±

±

−

−

8

−

±

±

±

±

±

10

−

±

+

+

+

±

12

−

+

+

+

+

+

14

−

++

++

++

++

+

16

±

++

++

++

++

+

18

+

++

++

++

++

++

(−) liquid, (±) viscous, (+) gel, (+ + ) firm gel

5.5.4 Surface Hydrophobicity • Measurement The surface hydrophobicity index (H 0 ) of rice bran protein was measured as follows: ANS was used as the fluorescence probe. The rice bran protein solution was serially diluted with phosphate buffer (pH 7.0) to obtain different concentrations ranging from 0.025 to 0.2 mg/mL. Further, 20 μL of 8-mM ANS solution was added to 4 mL of the protein solution. The fluorescence intensity was measured using a spectrophotometer (Synergy H1, BioTek, USA) at the excitation wavelength of 370 nm and emission wavelength of 490 nm. A plot of fluorescence intensity vs. protein concentration was prepared, and the H 0 was calculated from the slope of the initial fluorescence intensity vs. protein concentration curve. • Results The effect of UHP treatment on the H 0 of rice bran protein is shown in Fig. 5.24. The surface hydrophobicity property was used to detect the conformational differences between the untreated control and pressure-treated samples. This technique involved the use of ANS as the extrinsic fluorescence probe. UHP treatment resulted in a gradual but significant increase in H 0 when the pressure level increased from 100 to 400 MPa. The highest H 0 was achieved at 400 MPa with a 65% increase. A slight decrease was observed at 500 MPa; however, H 0 was still higher than that of the untreated control sample. These results are consistent with the general notion that UHP treatment unfolds the protein structure, which brings the hydrophobic groups to the surface of the protein molecule. Similar results have been reported by the UHP treatment of soymilk protein, which reported an increase in the relative fluorescence intensity of ANS with increasing pressure from 100 to 400 MPa; the relative fluorescence intensity of ANS decreased at 500 MPa. This was caused due to complete protein denaturation, exposing more hydrophobic regions. They have

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5 Improving Taste, Cooking Properties, and Digestibility of Brown Rice

Fig. 5.24 Surface hydrophobicity (H 0 ) of untreated control and UHPT (100–500 MPa) rice bran proteins. Different letters on top of the columns indicate significant (p < 0.05) differences among samples treated under different conditions. Reprinted from ref. [10], copyright 2017, with permission from Springer Nature

postulated that the decrease in fluorescence intensity at 500 MPa might be related to the refolding of the protein structure and concealing of the hydrophobic residues after high-severity pressure treatment. Another study showed that the surface hydrophobicity of amaranth protein increased with pressures of 200, 400, and 600 MPa owing to the unfolding of protein structure.

5.5.5 Protein Structure 5.5.5.1

Fourier Transform Infrared Spectroscopy

• Measurement Fourier transform infrared spectroscopy (FTIR) of UHPT and untreated control samples was performed using an FTIR spectrometer (AVA TAR 370, Nicolet, WI, USA). The protein samples were mixed with KBr by the ratio of 1:100 and then pressed into pellets. Measurements were recorded in the scanning range of 400 to 4,000 cm−1 during 64 scans with 4-cm−1 resolutions. • Results The FTIR spectra of rice bran proteins with different pressure treatments are shown in Fig. 25a. FTIR is a useful method to study the conformational changes of

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187

secondary protein structure. In the spectra of the unpressurized sample, 11 strong bands were observed, namely, 3,292, 2,922, 2,851, 1,743, 1,652, 1,539, 1,455, 1,395, 1,239, 1,159, and 1,080 cm−1 . Bands in the range of 3,200–3,600 cm−1 and 2,800– 3,000 cm−1 represent the stretching vibration of the –OH and –CH groups, respectively. The bands between 1,600 and 1,700 cm−1 are important for identifying the protein structure. The amide I band (1,600–1,700 cm−1 ) is attributed to the C = O stretching vibration of the peptide bond, and the amide II band (1,500–1,600 cm−1 ) is mainly due to the C–N stretching and N–H bending of amide groups. After UHP treatment, many frequency shifts were observed. The –OH stretching vibration peak of the unpressurized sample was 3,292 cm−1 . While after 100 MPa, it shifted to 3,290 cm−1 . The increased pressure further shifted, reaching 3,286 cm−1 at 400 MPa. The result indicated that the –OH stretching vibration absorption peak red-shifted due to the influence on hydrogen bonds. This is also in accordance with the surface hydrophobicity caused by UHP treatment, unfolding the protein structure and bringing the hydrophobic groups to the surface of the protein molecule, thereby increasing the probability of interacting via hydrogen bonds. The band at 2,922 cm−1 , representing the –CH stretching vibration, shifted to 2,921 cm−1 after UHP treatments. Moreover, the C = O stretching vibration was shifted from 1,743 cm–1 of the unpressurized sample to 1,745 cm−1 of UHPT samples, and the C–N stretching vibration shifted from 1,239 to 1,238 cm−1 after UHP treatments. All these results indicated small changes in the secondary structure of rice bran protein caused by UHP treatment.

5.5.5.2

Intrinsic Fluorescence Spectroscopy

• Measurement Intrinsic fluorescence spectra of UHPT and untreated samples were determined using an F-4500 spectrofluorometer (Hitachi, Tokyo, Japan). Protein solutions (0.2 mg/mL) were prepared in 50-mM phosphate buffer (pH 7.0). Protein solutions were excited at 290 nm (slit width = 5 nm), and emission spectra were recorded from 300 to 400 nm (slit width = 5 nm). • Results The intrinsic fluorescence spectra of rice bran protein at different pressures are shown in Fig. 25b. UHP treatment increased relative fluorescence intensity at 100 and 200 MPa but decreased at 300 and 400 Mpa. As pressure increased, the wavelength of fluorescence emission peak blue-shifted from 340 nm for the untreated control sample to 339 nm for the UHPT rice bran protein at 400 Mpa. The UHP modification of the protein conformation in the vicinity of the tryptophan (Trp) residues likely contributed to the changes in the fluorescence spectra, relative fluorescence intensity, and blue shift. Polar solvent quenches the fluorescence intensity of Trp; therefore, when they are exposed to a more hydrophilic environment, the fluorescence intensity decreases. Thus, the fluorescence quenching indicates the exposure of the protein

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Fig. 5.25 FTIR curves a and intrinsic fluorescence measurements b of untreated control and UHPT (100–400 MPa) rice bran proteins. Reprinted from ref. [10], copyright 2017, with permission from Springer Nature

hydrophobic regions to the solvent. With a continued elevation of treatment pressure to 400 Mpa, the relative fluorescence intensity declined near that of untreated samples. This indicated that Trp was contacted with more polar solvents at 300 and 400 Mpa. Fluorescence spectrum analysis confirmed the changes in the tertiary and quaternary structures of rice bran protein after UHP treatment.

5.6 Summary

189

5.6 Summary Compared with traditional soaking treatment, PUHP significantly shortens the cooking time of brown rice from 34 to 14 min. The hardness of brown rice treated with PUHP reduced remarkably, which is lower than that of soaking and similar to that of white rice. The gumminess and springiness of brown rice dramatically decreased under pressure above 500 MPa. However, the water uptake capacity of brown rice treated by PUHP was not affected, and its moisture content were much lower than that of soaked samples. The analysis of thermal properties revealed that PUHP influenced the enthalpy of brown rice, and brown rice components were denatured. These results and microstructure analysis revealed that the pericarp and aleurone layer of brown rice was damaged by PUHP, which allows water to be easily absorbed by the rice kernel during cooking. PUHP could be a potential pretreatment for improving the cooking properties of brown rice. Soaking and UHPT both change the quality of brown rice. The water absorption ratio and lightness values of brown rice were increased by soaking and UHPT. The hardness and gumminess values of cooked brown rice were reduced, while springiness and cohesiveness were elevated by UHPT. SEM indicated that UHPT improved brown rice texture by disrupting the rice bran layer, which allowed easier water penetration into the rice grain during cooking. Moreover, the two-cycle UHPT resulted in a lighter color and softer texture for cooked brown rice than that for single-cycle UHPT, primarily due to the severe structural disruption of the bran layer. Overall, two-cycle UHPT after soaking could potentially improve the quality of brown rice, taking approximately the same amount of time as the singlecycle UHPT. Furthermore, the quality improvements with the two-cycle UHPT were facilitated at lower pressure levels, providing better commercial processing opportunities. The desorption behavior of brown rice was relatively humidity-dependent at any given temperature. UHP caused an upward change in adsorption and desorption isotherms. EMCs of the control were higher than that for the treated samples. The 300-MPa treated sample had the lowest EMC. A hysteresis phenomenon was found over the investigated ERH, as the EMC for desorption is higher than that for adsorption. The hysteresis area of the 300-MPa test samples was the largest, and the 100-MPa test sample was closest to the control. A quadratic polynomial equation was established to describe the relationship between EMC determined from UHPT samples, the ambient temperature, and ERH. GAB and BET equations were analyzed through nonlinear regression to evaluate the monolayer content of brown rice. UHP treatment significantly lowered the water uptake. SEM results showed destruction on the aleurone layer of brown rice by UHPT. Hydrolytic and oxidative rancidity are the main causes of brown rice quality deterioration during storage. Brown rice was soaked in water and treated with UHP. The effect of UHP treatment on moisture content, fat acidity, conjugated dienes (CD), and 2-thiobarbituric acid (TBA) value during 3 months of storage at room temperature was evaluated and compared with soaked (0.1 MPa) brown rice and untreated brown rice. After storage, the moisture content of UHPT brown rice was significantly higher than that of untreated but lower than that of soaked, apart from

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5 Improving Taste, Cooking Properties, and Digestibility of Brown Rice

the brown rice treated at 400 MPa-0 min, which was the lowest in moisture content. UHP treatment at 400 MPa enhanced the fat acidity immediately after the treatment, while samples treated at 200 MPa-0 min showed a lower level of hydrolytic rancidity during storage. Better stabilities were observed based on CD content, and TBA values were observed in brown rice when UHPP at 200 MPa-10 min was applied. Moisture sorption isotherms (MSI) of presoaked brown rice after UHP treatment (100–500 MPa for 10 min at ~20 °C) were evaluated at 20 °C–40 °C, and five existing models were tested for prediction performance. All MSI models performed well, and the modified Guggenheim–Anderson–de Boer (GAB) model provided the best fit. The mobility and distribution of water in equilibrated brown rice were studied through nuclear magnetic resonance (NMR) analysis. The proton distributions of the samples revealed three or four distinct water populations, which varied with ERH (water activity). UHP treatment resulted in a change in the water distribution and caused the water to be less mobile. SEM confirmed that UHP treatment altered the surface structure of the starch granules. This study helped develop a modified MSI model and better understand the water distribution in UHPT brown rice. Germination and UHP contributed to restricting swelling. Germination treatment induced more severe UHP modifications. Besides, germination reduced viscosity and hardness, while UHP enhanced them. Further, the setback and breakdown pasting property values decreased after germination and UHPT. Changes in starch granule size and flour structure due to germination might be responsible for the different effects of UHP among samples. The texture of FTC-treated BR was the closest to the texture of white rice. Improved water absorption ratio, UHP, FTC-induced modification of the bran layer, and GP-induced partial starch gelatinization were responsible for enhancing the texture of BR. All treatments improved the in vitro digestibility of BR starch in the increasing order of FTC < UHP < GP. FTC treatment also resulted in a minimal glycemic index (GI), while GP treatment resulted in a higher GI. The amylose content was generally lower for untreated BR than for treated BR. Further, the UHP, GP, and FTC treatments showed improved amylose/amylopectin ratios. UHP and GP decreased the gelatinization enthalpy, while FTC increased it. GI positively correlated with amylose content and amylose/amylopectin ratio, while gelatinization enthalpy had a negative correlation. UHP treatment at 100 and 200 MPa significantly improved the solubility and oil absorption capacity, while water absorption and foaming capacities increased further and reached the maximum at 500 MPa. Compared with the untreated control sample, the emulsifying activity and foam stability of treated samples were significantly higher and the least gelation concentration was lower; however, none of them showed any specific trend with pressure level. Emulsion stability and surface hydrophobicity increased with the pressure level until 400 MPa and decreased slightly at 500 MPa. Pearson correlation coefficients showed that surface hydrophobicity positively correlated with water absorption capacity, foaming capacity, emulsifying activity index, and emulsion stability index but negatively correlated with the least gelation concentration. The pressure-treated rice bran protein possessed good functional properties for use as a food ingredient in the formulations.

References

191

References 1. Yu Y, Ge L, Zhu S, Zhan Y, Zhang Q (2015) Effect of presoaking high hydrostatic pressure on the cooking properties of brown rice. J Food Sci Technol 52(12):7904–7913 2. Yu Y, Pan F, Ramaswamy HS, Zhu S, Yu L, Zhang Q (2017) Effect of soaking and single/two cycle high pressure treatment on water absorption, color, morphology and cooked texture of brown rice. J Food Sci Technol 54(6):1655–1664 3. Yu Y, Ge L, Ramaswamy HS, Wang C, Zhan Y, Zhu S (2016) Effect of high-pressure processing on moisture sorption properties of brown rice. Drying Technol 34(7):783–792 4. Zhang Q, Ge L, Ramaswamy HS, Zhu S, Yu Y (2017) Modeling equilibrium moisture content of brown rice as affected by high-pressure processing. Trans ASABE 60(2):551–559 5. Wang H, Hu F, Wang C, Ramaswamy HS, Yu Y, Zhu S, Wu J (2020) Effect of germination and high pressure treatments on brown rice flour rheological, pasting, textural, and structural properties. J Food Process Preserv 44(6):e14474 6. Wang H, Zhu S, Ramaswamy HS, Hu F, Yu Y (2018) Effect of high-pressure processing on rancidity of brown rice during storage. LWT 93:405–411 7. Du Y, Zhu S, Ramaswamy HS, Wang H, Wu J, Yu Y (2019) Comparison of germination– parboiling, freeze–thaw cycle, and high pressure processing on the cooking quality of brown rice. J Food Process Eng 42(5):e13135 8. Wang H, Zhu S, Ramaswamy HS, Du Y, Yu Y, Wu J (2021) Dynamics of texture change and in vitro starch digestibility with high-pressure, freeze-thaw cycle, and germination-parboiling treatments of brown rice. Trans ASABE 64(1):103–115 9. Yu Y, Du Y, Ramaswamy HS, Wang H, Jiang X, Zhu S (2018) Comparison of germinationparboiling, freeze-thaw cycle and high pressure processing on phytochemical content and antioxidant activity in brown rice evaluated after cooking and in vitro digestion. International Journal of Food Engineering, 14(11–12) 10. Zhu SM, Lin SL, Ramaswamy HS, Yu Y, Zhang QT (2017) Enhancement of functional properties of rice bran proteins by high pressure treatment and their correlation with surface hydrophobicity. Food Bioprocess Technol 10(2):317–327

Chapter 6

Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Abstract The scientific modification of fast-growing forest timber to improve the quality and suitability of the timber is the development trend of the fast-growing forest timber industry at this stage and an important means of efficient exploitation of fast-growing forest timber resources. In this chapter, the focus is on assessing the mechanical properties, moisture absorption, dimensional stability and microstructure of wood after high pressure densification. In addition, the effect of high pressure dyeing on the properties of the wood is investigated. Keywords High pressure densification · High pressure dyeing · Dimensional stability · Fractal dimension

6.1 Introduction The strength properties of wood are positively related to its density. As an environmentally friendly renewable material, high-density wood is widely used in daily life (e.g., in construction, furniture, and floor industry). However, the high-density wood supply is far from satisfying the growing demand of the market, primarily because of its long growth period. To solve this problem, fast-growing plantation forests could be used as a fungible resource, as they have not been effectively exploited owing to their poor mechanical properties in relation to their low density. Two main modification methods to improve wood density and mechanical properties have been developed to fully use the fast-growing forest resource. The first modification is an impregnation method that involves filling wood cell cavities with fluid substances. The other modification is to densify wood through compression by reducing its void volumes without adding chemicals. As the impregnation method can damage the ecological characteristics of natural wood, the material contains potentially harmful residues to the environment and human health. Hence, compression technologies are more widely used in wood densification for furniture use. Wood compression technology initially emerged at the beginning of the twentieth century. At the early stage, wood samples were directly compressed to desired © Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_6

193

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

thickness in the radial direction using heated metal plates. The defects caused by compression can be minimized with a better understanding of the composition and natural characteristics of wood, unidirectional static compression with softening pretreatment, and cooling posttreatment. This technique improved the quality of densified wood products. During the past few decades, thermos–hydro–mechanical (THM) compression, thermos–mechanical (TH) compression, viscoelastic thermal compression (VTC), and sandwich compression have been developed to densify wood products. Studies reported that the mechanical properties of spruce, pine, and beech wood were significantly enhanced through THM treatment, particularly the shear strength, with more than a tenfold increase rate. Previous studies have found that the modulus of rupture (MOR) and modulus of elasticity (MOE) of VTC wood with a 132% degree of densification were 103% and 129%, respectively, greater than those of nondensified hybrid poplar wood. However, these hot-pressing densification methods have fatal drawbacks, such as complicated procedures and long treatment times, resulting in low productivity and slow industrialization of these technologies. Recently, several novel wood-compression methods have been studied. Small pieces of spruce (5 MPa) and cherry (19 MPa) were treated under hydrostatic pressure in sealed cylindrical containers. They found that the average density increased by 26% for spruce and 46% for cherry. Moreover, the densified woods became less heterogeneous and less anisotropic than before. Another study investigated the effects of semi-isostatic compression on the properties of Scots pine in a Quintus press that can yield pressures of up to 140 MPa. Their findings concluded that applying pressure in all directions prevented compression defects (e.g., fracture, fragmentation, spreading, and checking). Small fir logs were subjected to UHP treatment (i.e., pure equalization compression) to improve wood properties. Results showed that pressurized woods exhibited better mechanical properties and stability at a moderate relative humidity (RH). Above all, results based on previous studies suggest that purely isostatic wood compression is a less destructive technique than traditional technologies. In conventional wood production, logs are generally sawn into wood boards to speed up drying. To the best of our knowledge, the effects of UHP treatment on the density distribution, mechanical properties, and microstructure of wood boards have not been studied yet. The appearance and surface color of prepared wood samples are important attributes of wood quality and commodity value. Nonuniformity in appearance, especially color, can result in a reduced market value. Artificial dyeing has been employed for rapid-growing plantation products to effectively eliminate color differences, improve the visual quality of wood, and impart the color characteristics of other highly valued wood species. Dyeing is widely used for various types of wood products, such as furniture, decorations, and unique designs, in addition to the final finish that might still be applied. The heating–impregnation method at atmospheric pressure is currently the most widely used method for wood dyeing. It is a simple, mature technology; however, some problems exist, such as the capillary action from roots to the branches of the tree, wherein the wood is dyed during the process. However, because wood is nonhomogeneous, penetration of the dye solution into the wood is limited and

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increasing the dye penetration is difficult. Several types of dyes are applied in wood staining, such as acid dyes, disperse dyes, and basic dyes. The specific application depends on the material and purpose of dyeing. Ultrasonic treatment was used to stain inferior wood to improve its decorative value. The effects of ultrasonic power, dye concentration, dyeing time, and treatment temperature on the dye uptake, chromatic value, crystallinity, thermal stability, chemical structure, and microstructure of the dyed wood veneer were investigated. It was reported that the dye uptake, chromatic value, and dyeing rate were improved through ultrasonic-assisted treatment. The dyeing performance improved with increased ultrasonic power, dye concentration, dyeing time, and temperature. Further, it was reported that the dyed wood properties, such as lignin degradation, crystallization, and thermal stability, slightly decreased after ultrasonic treatment, and parts of the wood microstructure, such as the pit membrane and parenchyma cells, were mechanically damaged. Ultrasonic-assisted treatment was reported to enhance the permeability of wood by creating new fluid channels and sorption sites. Wood dyeing involves soaking wood in a dye solution and includes the processes of dye solution wetting, dye adsorption at the wood surface, internal diffusion, infiltration, and solidification into the wood structure. With large wood samples, the movement and infiltration of dye molecules into the wood limit the dye from penetrating deeply into the wood. UHP processing is a new technology recently applied to rapidly growing woods for densification and improving mechanical properties. Previous studies have suggested that UHP processing can result in compression and compaction of wood and improve the density and mechanical performance of poplar and Paulownia wood. UHP processing also enhances the hygroscopic behavior and permeability characteristics of wood, which can lead to better dye staining and absorption.

6.2 Ultra-High-Pressure Densification of Wood UHPD is shown in Fig. 6.1. The wood product, already sealed in its final package, is introduced into an UHP cylindrical vessel and subjected to a high level of isostatic pressure (up to 600 MPa) transmitted by water. In this technology, the applied pressure is uniformly distributed throughout the entire sample, whether in direct contact or in a flexible container. This method is a new alternative to the classic thermomechanical ones. With no thermal pretreatment needed, this process allows for very short treatment times and makes it very efficient with possible industrial-scale applications. Plastic compressive strains occur predominately in UHP-densified wood, and the delayed elastic strain is very small. This advantage makes UHP-densified wood attractive for long-term indoor use when the environment’s relative humidity (RH) is low and the climate is relatively stable. The results show that UHPD could generally increase the densities of various low-density wood by 2–3 times. The typical cross section of densified wood is shown in Fig. 6.2. Compared with the control sample (with an oval cross section), the cross section of densified wood

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Fig. 6.1 Schematic illustration of the UHP treatment process. Reprinted from Ref. [1], used with permission

Fig. 6.2 Cross sections of Chinese fir wood: control (left) and densified wood at 50 MPa for 5 min (right). Heartwood and sapwood of Chinese fir had identical compression. After densification, the cross section represented an irregular shape (minimal deformation in the longitudinal direction) after densification. Reprinted from Ref. [2], used with permission

represented an irregular shape (minimal deformation in the longitudinal direction). The diameter of densified wood was notably reduced compared to control. Moreover, heartwood and sapwood were found to be identically compressed after densification. During UHPD, steel plates can also be used to shape the planks, and the positioning method of the steel plates is shown in Fig. 6.3. Firstly, a test specimen board [150 mm (longitudinal) × 70 mm (tangential) × 30 mm (thickness)] was sandwiched between two stainless steel plates (the area and thickness of each plate were 70 mm × 150 mm and 5 mm, respectively, and matched the two parallel surface areas of the board), as shown in Fig. 6.3. Then, the specimen board and the two steel plates were wrapped

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Fig. 6.3 Schematic diagram of poplar board fixing method. Poplar board was fixed between two steel plates and then wrapped in a fat polyethylene pouch and vacuum-packed. Reprinted from Ref. [1], used with permission

in a flat polyethylene pouch (16 silk) and vacuum sealed using a vacuum package machine before UHPD. The width of the steel plates was less than that of the wood specimen because the width of the wood specimen would shrink after UHP treatment. Effects of UHPD (30 s) at different pressure levels on poplar boards are shown in Fig. 6.4. UHPD significantly reduced the thickness of test specimens and gradually progressed with increasing pressure levels. Figure 6.4 also confirms that UHPD with the poplar board sandwiched between two steel plates resulted in more uniform radial densification of the boards, which could not be obtained in previous studies. The shaping tool used in this research is simple and reasonable. However, this simple shaping tool cannot create uniform thickness as traditional hot-plate compression methods do. UHPD offers advantages for wood densification—it requires no heat and is completed in a short treatment time of 30 s, saving time and energy. After the pressure treatments, the steel plates were slightly bent (~4 mm). Hence, more effective shaping tools and packaging methods may be needed and further explored.

Fig. 6.4 Photograph of a cross section of poplar wood after various pressure treatments for 30 s. Reprinted from Ref. [1], used with permission

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6.2.1 Mechanical Properties The ability of wood to resist external mechanical forces is called the mechanical properties of wood. The mechanical properties of wood include elasticity, plasticity, hardness, toughness, strength potential index, and process properties. The elastic property of wood means external forces that deform the wood within a certain stress range. When the external forces are removed, the deformation disappears and returns to its original state. The plasticity of wood is a property of wood to be permanently deformed by external forces under certain conditions. In addition to the wood structure, the factors affecting the mechanical properties of wood are related to factors such as moisture, density, action time, temperature, and wood texture. For example, the density of wood greatly influences hardness; the greater the density, the greater the hardness.

6.2.1.1

Elastic and Plastic Properties

• Measurements of plastic and elastic strains Wood is compressed to obtain a higher density, and it will sustain the higher density if the plastic proportion of the strain at compression is high. Research has shown some elastic strain recovery in densified wood when stored under high humidity conditions, which decreases density by volume expansion and limits its application. Hence, paints and other special coatings have reduced this elastic recovery. It is worth exploring how the plastic and elastic strains vary when the wood is compressed at different pressure levels and holding times. In addition, it is important to study the density changes in wood at different pressure levels and holding times not only before and after pressure release but also while under pressure. Thus, a new method was developed to measure the plastic and elastic strains. Wood had a mean moisture content of 13% with a standard deviation of 1%. The samples were plain-sawn to 60 mm (axial) × 55 mm (tangential) × 60 mm (radial), and the angle between the annual ring and the side of the sample was 90°, as shown in Fig. 6.5. For measuring the elastic strain, a 20-mm-diameter hole was drilled in the middle of each sample along the radial or tangential direction. The device used to measure the maximum compression consisted of a steel tube and a steel plug. One end of the plug was fitted with an O-ring to be secured inside the steel tube. The plug moved within the tube when pushed under pressure; however, it did not retract when pressure was relieved. This device was secured tightly inside the drilled hole for measuring deformation. Two steel plates covering the hole were placed on both sides of the sample to uniformly distribute the pressure in the tangential or radial direction, as shown in Fig. 6.5. A string was used to secure the steel plates to the sample. As described above, the method used for measuring elastic strain was convenient and effective, as determined by preliminary studies. However, the accuracy of the method depends on certain factors that may not be immediately apparent.

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Fig. 6.5 Poplar wood samples a before and b after UHPD. Schematic diagrams of the sample and measuring device c before and d after UHPD. L 0 and L 2 are the thicknesses of the sample in the radial or tangential directions before and after UHPD, respectively, and L 1 is the length of the measuring device after UHPD. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

For example, the hole drilled in the middle of the sample (Fig. 6.5) might slightly reduce the mechanical properties of the wood, leading to a slightly higher deformation during UHPD than that of a solid sample without a hole. Secondly, a large hole could influence the wood’s mechanical properties, while a small hole could interfere with the movement of the measuring device during UHPD. The hole diameter was adjusted to minimize these errors and was optimized at 20 mm. The main deformation of the wood samples during UHPD occurred in the radial and tangential directions. The plastic and elastic strains in the radial and tangential directions were evaluated during UHPD. During pressurization, the length of the measuring device (plug and tube) changed synchronously with the sample in the radial or tangential direction; however, the plug did not have an elastic rebound, such as wood, after pressure release, as shown in Fig. 6.5. Therefore, the elastic strain of the wood sample could be calculated from the length of the measuring device. The elastic, plastic, and total strains of the densified wood were calculated using Eqs. (6.1–6.3): Elastic engineering strain(%) =

L2 − L1 × 100 L0

(6.1)

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Plastic engineering strain(%) = Total engineering strain(%) =

L0 − L2 × 100 L0 L0 − L1 × 100 L0

(6.2) (6.3)

where L 0 is the thickness of the sample in the radial or tangential direction before UHPD, L 2 is the thickness of the sample in the radial or tangential direction after UHPD, and L 1 is the length of the measuring device after UHPD. The L 0 , L 1 , and L 2 values were measured with digital calipers at a minimum resolution of 0.02 mm. The ratio of the elastic strain to the total strain of the densified wood (K 0 ) was calculated using Eq. (6.4). K 0 (%) =

L2 − L1 × 100 L0 − L1

(6.4)

• Results of plastic and elastic strains The radial and tangential plastic strains of wood samples after UHPD at different pressure levels and holding times are shown in Fig. 6.6. For the samples with 1s holding time, the major portion of the plastic strain was formed by the time a pressure of 60 MPa was reached, and the plastic strain of the wood in the radial and tangential directions was stable after 100 MPa. In addition, the wood samples were rapidly compressed and deformed in the radial direction by 15 MPa, and the plastic strain tended to moderate between 30 and 100 MPa. Significant differences in plastic strain were found between the tangential and radial directions. For example, the plastic strain for samples treated at 150 MPa for 300 s was 48.6% in the tangential direction and 18.3% in the radial direction. The plastic strain was, in general, almost three times higher in the tangential direction than in the radial direction, which was mainly due to the anisotropy of poplar wood. The difference in plastic strain between the radial and tangential directions caused irregular deformation of the wood after UHPD. Some studies reported that wood species with low latewood percentages were stronger in the radial direction, while species with high latewood percentages were stronger in the tangential direction. Scots pine was reported to have almost three times higher compression in the radial direction than in the tangential direction during densification with the CaLignum process, which was in contrast with poplar wood deformation during UHPD in this study. Furthermore, the plastic strain of samples obtained in the radial direction through the 5-MPa treatment was not positive, possibly because compression in the tangential direction led to radial expansion. Plastic strain significantly differed between the two holding times for samples treated at 5–30 MPa, as shown in Fig. 6.6. For example, for samples treated at 15 MPa, the tangential strain was 38.1% for 1 s and 43.6% for 300 s, which was as high as that of samples treated at 60 MPa for 1 s. Indeed, wood is a viscoelastic or, more appropriately, a viscoelastoplastic material. The deformation of wood under compression comprises three major components: elastic strain, delayed elastic strain,

6.2 Ultra-High-Pressure Densification of Wood

201

Fig. 6.6 Radial and tangential plastic strains of wood samples at different pressure levels and holding times. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

and plastic strain. Wood is also a porous material. Elastic strain (and elastic recovery) is caused by inner stress formed during low compression levels. UHPD treatment at low and moderate pressures of up to 150 MPa generally leads to a large elastic recovery. The plastic strain of wood during UHPD is caused by bending the cell walls and tracheid cells, as previously reported. The bending of the cell walls reduces the internal porosity of the wood, which results in plastic strain, increasing the density and mechanical properties of the wood. After treatment, higher plastic strain at a longer holding time increases wood density and mechanical properties. However, the plastic strains of samples treated at pressure levels higher than 60 MPa were not significantly different (p > 0.05) at the two holding times, probably because the samples rapidly compressed between 5 and 30 MPa, and the holding time was too short to complete the compression of the samples. Especially in the radial direction, the plastic strains remained slightly different between the two holding times throughout the pressure range. In contrast, they merged at higher pressure levels in the tangential direction. Previous studies have also found that a proper combination of pressure and holding time resulted in significant wood deformation at lower pressures after phenol formaldehyde resin impregnation. The elastic and total strains of samples in the tangential and radial directions after deformation at different pressure levels and holding times are shown in Fig. 6.7. Total strain did not differ significantly for the two holding times in the tangential direction, as shown in Fig. 6.7a. Holding time had a greater effect on elastic strain up to 60 MPa in the tangential direction, where elastic strain significantly declined with increasing holding times at the different pressure levels. This means that increasing

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.7 Elastic and total strain of samples in a tangential and b radial directions at different pressure levels and holding times. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

the holding time could increase the plastic strain proportion of densified wood by reducing the elastic strain. A study reported that the elastic recovery of wood was caused by inner stress created during compression, and the holding time could reduce the inner stress. Increasing pressure increases the plastic strain and elastic strain. Elastic strain in the tangential direction increased with increasing pressure. Still, there was no significant difference in the elastic strain at different pressure levels except for untreated samples, mainly because the mechanical properties of densified samples increased with increasing plastic strain. The elastic strain of samples in the radial direction is shown in Fig. 6.7b. There was no significant difference between the elastic strain and total strain in the radial direction at 5 MPa for 1 s, which means that the principal radial strain at 5 MPa was elastic. In addition, the elastic strain in the radial direction had a sudden decrease at 30 MPa, even though the pressure increased, due to the increase in MOE, which is negatively related to the elastic strain. The elastic strain difference between the radial and tangential directions was less than that for plastic strain above 30 MPa. For example, the elastic strain at 150 MPa for 300 s was 17.3% in the tangential direction and 8.7% in the radial direction. In comparison, the plastic strain at 150 MPa for 300 s was almost three times higher in the tangential direction than that in the radial direction, as shown in Fig. 6.6. K 0 of the densified wood reflects the efficiency of the processing conditions. Particularly, a higher K 0 means lower wood density and mechanical properties after UHPD treatment. The K 0 value decreased with pressure up to 15 MPa in the radial and tangential directions, as shown in Table 6.1. This is because the treatment pressure exceeded the wood’s compressive strength and plastic strain occurred in the radial and tangential directions. However, there was no significant difference in K 0 among the different pressure levels above 30 MPa, which means that the elastic strain was assumed to be proportional to the plastic strain in the radial and tangential directions at pressures above 30 MPa. It was reported that the elastic strain could be proportional

6.2 Ultra-High-Pressure Densification of Wood

203

Table 6.1 Ratio of elastic strain to total strain (K 0 ) of densified wood at different pressure levels. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers Pressure (MPa)

1s

300 s

Radial

Tangential

Radial

Tangential

5

103.1 ± 1.7a

41.6 ± 1.8a

83.2 ± 2.6a

30 ± 3.4a

15

74 ± 1.2b

30.9 ± 1.1b

59.6 ± 0.5b

23.1 ± 0.7a

30

50.2 ± 0.4c

31.9 ± 2.9b

37.4 ± 0.7c

24.5 ± 1.7a

60

44.7 ± 1.7c

29.6 ± 0.7b

36.7 ± 5.3c

23.5 ± 0.4a

100

44.9 ± 4.5c

24.7 ± 1.3b

40.1 ± 2.9c

27.3 ± 3.3a

150

42.4 ± 5.1c

26.2 ± 0.5b

32.4 ± 6.1c

26.4 ± 0.4a

Mean

45.5 ± 4.6

28.1 ± 3.3

36.6 ± 5.1

25.4 ± 2.4

Values are mean ± standard errors. Means in the same column followed by different letters are significantly different (p < 0.05).

to the plastic strain during semi-isostatic compression because there is a positive correlation between MOE and some strength properties. Therefore, the mean K 0 at pressures of 30 MPa and higher could be used as a constant for the ratio of elastic strain to total strain, as shown in Table 6.1, indicating that increasing pressure does not decrease the proportion of elastic strain. In contrast, the holding time could reduce the proportion of elastic strain at pressures below 60 MPa, especially in the radial direction, because holding time could increase the plastic strain by decreasing the elastic strain. Figures 6.8 and 6.9 show the MOE and MOR of poplar wood treated under different conditions. The MOE dramatically increased with increasing pressure (ranging from 50 to 150 MPa), whereas it notably declined when the pressure exceeded 150 MPa. The highest level of MOE was obtained at 150 MPa, with an increase of 162% compared to control. A similar trend was observed in MOR, as shown in Fig. 6.9. The UHP treatment markedly improved the MOR of hybrid poplar wood, and there were no significant differences (p > 0.05) among tested specimens treated at 50, 100, and 200 MPa. Density is an important property of wood as it is closely related to its mechanical properties. In densified wood, the strength is generally positively correlated with the density; thus, changes in the trends of mechanical properties followed that of its average density. It is an abnormal phenomenon that both MOE and MOR values of specimens treated at 200 MPa were lower than those at 150 MPa. However, no significant difference in average density was observed between 150 and 200 MPa treated specimens. This might have been caused by the destruction of the wood structure during UHP treatment. Previous microscopic studies showed that compression caused numerous cracks and fractures in the cell wall of densified wood, especially

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.8 Modulus of elasticity (MOE) of poplar wood samples treated under various conditions. The error bars indicate the standard deviation. Different letters above the columns indicate significant differences (p < 0.05). Reprinted from Ref. [4], used with permission

Fig. 6.9 Modulus of rupture (MOR) of poplar wood samples treated under various conditions. The error bars indicate the standard deviation. Different letters above the columns indicate significant differences (p < 0.05). Reprinted from Ref. [4], used with permission

6.2 Ultra-High-Pressure Densification of Wood

205

at a high CR. These cell deformation defects negatively impact the mechanical properties of densified wood. The more damaged the wood structure is, the less strength the densified wood has at the same density level. • Measurements of mechanical properties Unlike uniaxial compression, wood is compressed in all three directions (axial, radial and tangential) under UHP. However, as mentioned earlier, the deformation of wood in the axial direction during UHPD could be ignored. The elastic strain in the radial and tangential directions was calculated using Eqs. (6.5) and (6.7). ( εR = ( εT =

σ σ·ν − ER ET σ·ν σ − ER ER

) = ) =

σ E T ·E R E T −ν·E R

σ E T ·E R E R −ν·E T

=

σ KR

(6.5)

=

σ KT

(6.6)

where ε is the true strain, σ is the stress, ν is the Poisson’s ratio, and E is the MOE of wood. Subscripts R and T represent the radial and tangential directions of the wood, respectively. K i (i = T, R) can reflect the compressive elastic properties of densified wood in the i direction. The bulk modulus is a physical quantity that demonstrates the relationship between the volume strain and mean elastic stress (i.e., the average of the three principal elastic stresses). The bulk modulus (K) reflects the macroscopic properties of a material and is calculated using Eq. (6.7). K =

σm T2 · R2 · σm σm = = εν T2 · R2 − T1 · R1 (ν2 − ν1 )/ν2

(6.7)

where σm = mean stress; εν = volume strain; ν1 and ν2 = sample volume under compression and after compression, respectively; T 1 and R1 = length of measuring device after UHPD with tangential and radial compression, respectively; T 2 and R2 = sample thickness after UHPD in the tangential and radial directions, respectively; The pressure value can reflect the compressive strength of densified wood in two different directions. • Results of mechanical properties The elastic properties of densified wood can be represented by K i (i = T, R), where a larger K i indicates better compressive elastic properties. The K i values of wood after different UHPD treatments are summarized in Table 6.2. Longer holding times (at pressure levels up to 60 MPa) resulted in marginally higher K i values for densified wood at a given pressure, demonstrating that the longer holding time increased compressive elastic performance, which was closely related to wood

44.8 ± 5.1a

123.2 ± 2bc

221.3 ± 19.5d

430.4 ± 13.6f

630.1 ± 68.2

937.5 ± 1.2 h

37 ± 2a

96.4 ± 3.7ab

176.6 ± 14.4 cd

350.2 ± 7.1e

676.8 ± 23.1 g

962.9 ± 9.7 h

5

15

30

60

100

150

1839.8 ± 248e

940 ± 80.3d

741.8 ± 86.4 cd

420.7 ± 34.5bc

124.7 ± 0.9ab

49.4 ± 0.2a

300 s

K (MPa)

141.1 ± 15.3f 369.9 ± 12.4 g

369.2 ± 9.2 g

165.4 ± 4.2e

91.4 ± 4.2d

44.6 ± 0.1c

19.6 ± 1.1ab

300 s

248 ± 2.2f

147 ± 3.7e

78.3 ± 3.6d

39.6 ± 0.9bc

18.2 ± 0.2a

1s

Values are mean ± standard errors. Means in the same column followed by different letters are significantly different (p < 0.05)

1484.5 ± 118e

922.5 ± 86.2d

656.6 ± 12.6 cd

325 ± 24.3ab

113.2 ± 5.6ab

46.9 ± 2.6a

K R (MPa) 1s

300 s

K T (MPa)

1s

Pressure

Table 6.2 Values of K T , K R , and K of densified wood after different pressures and holding time treatments. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

206 6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

6.2 Ultra-High-Pressure Densification of Wood

207

density. There was no statistically significant difference between the holding times, except at 60 MPa. Irrespective of the holding time, the K i value of densified wood increased significantly with pressure. Related studies have also found that the applied pressure level increased the MOE and compressive strength of poplar wood. This is because the wood density under pressure was greater than that of the cell walls, and all the pore spaces collapsed while the sample was still under pressure. Particularly, further wood deformation was achieved during UHPD, resulting in compression of the cell walls rather than a collapse in the cellular voids. However, as measured by nanoindentation, the elastic modulus of the poplar wood cell walls was ~16.9 GPa, which is much higher than the elastic modulus of wood. Therefore, the increase in the elastic strain was limited despite the rise in pressure above 100 MPa. The difference in compressive elastic properties between the tangential (K T ) and radial (K R ) directions of the wood after UHPD is apparent, as shown in Table 6.2, because wood is an anisotropic material. In contrast, the compressive strength of densified wood in the radial and tangential directions was almost the same, irrespective of the anisotropy, because the wood samples were subjected to pressure from all directions during UHPD. This indicates that some normal anisotropic properties of the samples were eliminated by UHPD, which is a significant advantage of UHPD over uniaxial compression. The longer 300-s holding time resulted in a higher plastic strain than the shorter 1-s holding time in 0–60-MPa UHPD, as discussed earlier, which means that the compressive strength of densified wood would be lower after 1-s holding time. Hence, a longer holding time would help improve the compressive strength of densified wood. The K of porous materials depends on the material matrix properties, pore size and distribution, and saturation state. In addition, the K of a material increases as the pores decrease. The bending of the cell walls caused plastic deformation of wood under UHPD. Bending of the cell walls reduced the internal porosity of the wood, which increased its density and mechanical properties. Hence, the K of the wood increased with increasing pressure. Longer holding time could also increase K at pressures below 60 MPa, with the same trend as the plastic strain. The densities of Chinese fir after various UHP conditions for 5 min are presented in Table 6.3. The densities of wood samples were significantly enhanced by different UHP treatments, with an increase of 126% (at 50 MPa), 163% (at 100 MPa), 154% (at 150 MPa), and 143% (at 200 MPa) compared to the control density. The highest density was achieved with UHP treatment at 100 MPa (0.92 ± 0.02 g/cm3 ), while treatments from 50 to 100 MPa resulted in ~ 25% increase in density. Pressure values beyond 100 MPa decreased the density of treated wood. This could be related to the oil/moisture discharged from the wood when the pressure values were higher than 100 MPa. Such events decrease wood properties due to mass loss. The compression of heartwood and sapwood was uniform, and the directional effect dominated, indicating that the voids in Chinese fir were uniformly compressed without a density gradient. In contrast, the density gradient effect is typical for common methods, in which higher density is generated near the surface and lower toward the center of the board. These may be related to the much higher-pressure levels used in UHP treatment (pressure levels in previous studies were mostly below

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Table 6.3 Densities and mechanical properties of densified and untreated wood. Reprinted from Ref. [2], used with permission Pressure (MPa)

Density (g/ cm3 )

Hardness values (kN) Radial face

Tangential face

0.1(control)

0.35 ± 0.01a

1.16 ± 0.09a

1.27 ± 0.06a

MOE (GPa)

9.81 ± 1.49a

MOR (MPa)

65.9 ± 7.73a

50

0.79 ± 0.01b

5.42 ± 0.31b

6.32 ± 0.75b

14.52 ± 2.16b

124.62 ± 15.32b

100

0.92 ± 0.02d

8.59 ± 0.49d

9.17 ± 0.25d

18.47 ± 1.86d

179.53 ± 28.51d

150

0.89 ± 0.02c

8.03 ± 0.35c

8.75 ± 0.17c

17.68 ± 1.69c

162.98 ± 21.34c

200

0.85 ± 0.03c

7.85 ± 0.26c

8.42 ± 0.23c

16.73 ± 2.03c

160.25 ± 19.86c

Measured values are represented as mean ± standard errors of five replicates; letters represent different levels

10 MPa). Another distinct advantage of UHP compaction in this study is the relatively short processing time (5 min) compared with traditional methods (0.5–5 h). Furthermore, most conventional methods require long treatment times at elevated temperatures (mostly above 150 °C), indicating that the UHP compression of wood for densification saves time and energy (heating is not required during processing). As expected, the hardness values of the tangential and radial faces were significantly increased by densification. The average hardness of the radial and tangential faces for untreated wood specimens were 1.16 ± 0.09 and 1.27 ± 0.06 kN, respectively. In contrast, the hardness of the radial and tangential faces for samples treated at 50, 100, 150, and 200 MPa were significantly increased by 367, 468, 419, and 404%, respectively, for the radial face and by 350, 464, 431, and 405%, for the tangential face, respectively. The hardness values of the radial and tangential faces of wood densified at 100 MPa were significantly higher than those for the other pressure levels, showing a similar trend to density. For untreated and treated wood, the hardness of the tangential face was higher than that of the radial face. This hardness increase in densified wood could open new opportunities, for example, in the flooring industry. The MOE and MOR values of densified wood at various pressure levels are also shown in Table 6.3. The UHP treatment could significantly increase the MOR and MOE of Chinese fir wood. At 100 MPa, MOE increased by 88.3% and MOR increased by 172%, compared with untreated Chinese fir. At pressures greater than 100 MPa (150 and 200 MPa), MOE and MOR decreased with increasing pressure. The MOR and MOE values have been significantly increased with density. In this study, for the wood densified at 50, 100, 150, and 200 MPa, MOE increased by 48.0, 88.3, 80.2, and 70.5%, respectively, and MOR increased by 89.1, 172, 147, and 144%, respectively. These MOE and MOR increments after UHPD were higher than

6.2 Ultra-High-Pressure Densification of Wood

209

that of traditional compression wood, which proved that the densification by UHPP significantly improved the mechanical properties of Chinese fir. • Densification effects on thickness, density, and mechanical strength Quantitative data on board densification effects are detailed in Table 6.4. After densification treatment, the thickness of poplar boards was reduced by increasing pressure and the thickness factor decreased from 35.4% at 25 MPa to 50.9% at 125 MPa. CR increased by decreasing the thickness of poplar boards, as shown in Table 6.4. CR was between 36.8 ± 1.05% and 51.9 ± 0.92% under various UHPD. The CR increase was minimal, with UHPD at 100 MPa and above. As expected, with a decrease in board thickness and an increase in CR, the density of the treated board increased significantly (p < 0.05) by 55.6, 86.7, 91.1, 102, and 113% at 25, 50, 75, 100, and 125 MPa, respectively. Our preliminary experiments decreased density when the pressure level exceeded 150 MPa. These results indicate that UHPD for wood improves compression effects up to a certain pressure level by gradually compressing the voids in the boards, after which the degree of densification will be small. Further increase in pressure level could decrease the degree of densification and eventually may result in board rupture. The appropriate optimal pressure levels could depend on the tree species. In traditional hot-plate methods, CR is an important processing parameter. CR primarily influences density; both average and peak density are improved when the CR gets enhanced. However, CR impacts the width of the density peak and distance of the peak from the surface. • Measurements of delayed elastic strain Wood samples were conditioned at 20 °C and 65% RH after UHPD. Thicknesses were recorded at 0, 1, 24, 48, 72, 96, 120, 144, and 168 h. Delayed elastic strain Table 6.4 Average compression ratio and density of wood samples after various UHPD for 30 s. Reprinted from Ref. [1], used with permission Pressure (MPa)

Original thickness (mm)

Thickness after densification (mm)

Compression ratio (%)

20

29.7 ± 0.11a

18.8 ± 0.53b

36.8 ± 1.05a

Density (kg/m3 ) Before densification

After densification

450 ± 50a

700 ± 60b

50

29.7 ± 0.11a

17.6 ± 0.72c

40.7 ± 0.89b

450 ± 50a

840 ± 90c

75

29.7 ± 0.11a

16.4 ± 0.41d

44.8 ± 1.12c

450 ± 50a

860 ± 80d

100

29.7 ± 0.11a

15.8 ± 0.21e

46.7 ± 1.02d

450 ± 50a

910 ± 60e

125

29.7 ± 0.11a

14.3 ± 0.10e

51.9 ± 0.92d

450 ± 50a

960 ± 20e

Measured values are represented as mean ± standard errors of 15 replicates. Means followed by the different letters (a, b, c, d, and e) are significantly different among original thickness and thickness after densification (p < 0.05). Means followed by the different letters (a, b, c, and d) are significantly different among CR (p < 0.05). Means followed by the different letters (a, b, c, d, and e) are significantly different among density (p < 0.05)

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

(DES) was calculated using Eq. (6.8), where T d is the thickness (mm) at different times. Delayed elastic strain (%) =

Td − Tc × 100. T0 − TC

(6.8)

where To and Tc are the thickness of specimens (mm) before and immediately after densification. • Delayed elastic strain of densified wood DES results of poplar samples treated under various conditions are shown in Fig. 6.10. Wood is a type of viscoelastic material; therefore, the cell deformations caused by densification can result in internal stresses stored in the microfibril matrix, which might lead to a partial spring-back of densified wood after the pressure release. In addition, there was no significant difference (p > 0.05) in DES among UHP-densified specimens (except 50 MPa-30 s), indicating that the holding time and pressure level did not affect the DES of compressed wood when determined at 20 °C and 65% RH. All the subsequent experiments were conducted after the DES swelling test.

Fig. 6.10 DES of poplar wood treated under various conditions; the error bars indicate the standard deviation, and the different letters above the columns indicate significant differences (p < 0.05). Reprinted from Ref. [5], used with permission

6.2 Ultra-High-Pressure Densification of Wood

6.2.1.2

211

Hardness and Abrasion Resistance

• Measurement of hardness Each test board was polished using sandpaper before the following test. A section of 50 mm (longitudinal) × 50 mm (tangential) × thickness (radial) (different thicknesses after various pressure levels densification) was cut from the center of each specimen for hardness measurement (Fig. 6.11). In total, 15 specimens for each group were prepared and measured. Hardness was measured according to the static hardness standard ISO 3350:1975 with a minor modification. In this method, a semicircular steel ball of 5.64 mm radius is pressed into the wood surface with an average speed of 6 mm/min to create an indentation of 5.64 mm depth. The hardness is calculated using Eq. (6.9).

H12 = K P

(6.9)

where H 12 is the hardness of the wood specimen at 12% moisture content, P is the applied force (N), and K is the coefficient of radius for the steel ball indenter at a depth of 5.64 mm, which is 1. • Hardness of densified wood Hardness significantly increased following the densification treatment (Fig. 6.12). The average hardness for untreated samples was 1,140 N while those for the densified samples at 25 and 125 MPa were 1,540 and 2,240 N, respectively. This result suggests that hardness could be significantly enhanced even by the 25-MPa treatment for 30 s, nearly doubling at 125 MPa. The hardness of wood is generally measured by the ability of a steel ball to penetrate the wood surface. Slightly different approaches for evaluating hardness are adopted in various standard test methods. Thus, care should be taken when

Fig. 6.11 Illustration of the sampling of one specimen: DM denotes hardness measurement, and denotes equilibrium moisture and set-recovery experiments. Reprinted from Ref. [1], used with permission

212

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.12 Average hardness of specimens: control (0.1 MPa), 25 MPa treatment, 50 MPa treatment, 75 MPa treatment, 100 MPa treatment, and 125 MPa treatment. (Error bars represent standard deviations, and letters represent significantly different levels at p = 0.05). Reprinted from Ref. [1], used with permission

comparing hardness results on nonhomogeneous materials when different indentation techniques are used in the test. According to EN 1534 (2000), Brinell hardness was calculated by measuring the 10 mm-diameter metal ball indentation and the applied force. Our study calculated hardness values by measuring 5.64 mm indentation depth and applied force (according to ISO 3350:1975). Similar experiments showed that Brinell hardness (BH) of semi-isostatic densified radiata pine wood at 140 MPa improved nearly three times as compared to untreated wood. These reports supported our results. Excluding the differences in measurements, hardness significantly increases during wood densification in various studies. BH is a practical mechanical property to assess the resistance of wood. The BH results on the tangential surface of the control and UHPT wood specimens are presented in Fig. 6.13. The BH value of nondensified specimens was only 1,150.5 N; however, the values increased by 49, 61, 67, and 55% after treatments at 50, 100, 150, and 200 MPa, respectively. Statistical analysis demonstrated that UHP compression enhanced the hardness of poplar samples (p < 0.05). However, no significant change (p > 0.05) was found among samples compressed at different pressure levels. The increasing density and possible destruction of the wood structure also revealed the variation in the hardness with increased pressure. As expected, a similar phenomenon was also found in Paulownia. UHP treatment significantly improved the surface hardness of the Paulownia boards (Fig. 6.14). The hardness values increased with increasing pressure. The highest hardness values were obtained for the boards densified at 80 and 100 MPa, while the lowest values were associated with the control group. The hardness values of the UHPT boards were improved by 84, 121, 137, 160, and 173% for 50, 100, 150, 200, and 250 MPa, respectively, compared to control.

6.2 Ultra-High-Pressure Densification of Wood

213

Fig. 6.13 Brinell hardness (BH) of poplar wood samples treated under various conditions. Error bars indicate the standard deviation. Different letters above the columns indicate significant differences (p < 0.05). Reprinted from Ref. [4], used with permission

Fig. 6.14 Surface hardness of control and various UHPT Paulownia boards. According to Duncan’s multiple range test, letters indicate statistically different groups at p < 0.05. Reprinted from Ref. [6], copyright 2018, with permission from American Society of Agricultural and Biological Engineers

214

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Altogether, these strength results indicate that UHP treatment was analogous to traditional hot-pressing compression methods (e.g., TH, THM, and VTC), which can greatly improve the mechanical properties of low-density wood to substitute for harder species. Compared with traditional densification methods (processing time ranges from 0.6 to 3 h), the UHP treatment used in this study had a relatively simpler compression procedure and a shorter pressing time of 30 s, which can greatly promote production efficiency. • Measurements of abrasion resistance Abrasion tests were conducted following Chinese Standard GB/T 18,102–2007 using a Taber rotary platform abrasion tester (MGL-5, Jinan Precision Testing Equipment Co., Jinan, China). Two boards (50 mm × 100 mm × 15 mm) were adhered together to form the standard size of 100 mm × 100 mm × 15 mm for abrasion tests. Samples attached to steel plates were rotated at a steady speed of 60 rpm while a load (5 N) was applied in the form of two wheels rotating in opposite directions for exactly 100 rotations. The mass loss of the samples was determined at the end of the test. Three samples were run for each group. The test was designed to assess abrasive wear resistance. • Abrasion resistance of densified wood Generally, the abrasive wear of wood relates closely to the surface hardness. It is generally accepted that the abrasion loss is minor when the hardness value is high, which relates to the wood’s density. Figure 6.15 shows the mass loss of the control and UHPT wood samples at the end of the Taber abrasion tests. UHP treatment significantly reduced the mass loss compared to control. The mass loss values of UHPT boards were 40.7% (20 MPa), 47.5% (40 MPa), 54.6% (60 MPa), 68.3% (80 MPa), and 75.3% (100 MPa) lower than control. Thus, the UHPT boards showed higher abrasion resistance.

6.2.1.3

Strength Potential Index Analysis

• Measurements of strength potential index The effects of densification were further analyzed by calculating the ratio between the performance before and after densification. The density index (ρd /ρo ) and strength index ( f d /f o ) reflect the increase in density and strength. The strength potential index proposed by previous authors was used to estimate the increased strength of densified wood relative to that in nondensified wood based on the increase in density and was measured using Eq. (6.10). It should be noted that constant “b” given in Table 6.5 is obtained from Eq. (6.10), which indicates the relationship between density and strength. ln f = ln a + b ln ρ

(6.10)

6.2 Ultra-High-Pressure Densification of Wood

215

Fig. 6.15 Mass loss values in abrasion resistance tests for control and various UHPT Paulownia boards. According to Duncan’s multiple range test, letters indicate statistically different groups at p < 0.05. Reprinted from Ref. [6], copyright 2018, with permission from American Society of Agricultural and Biological Engineers

Table 6.5 Constants a and b describe the relationship between density (ρ) and strength ( f ). Reprinted from Ref. [4], used with permission

b

Density range (kg/m3 )

Strength property

ln a

Modulus of rupture (MOR)

−3.46 1.25

300–1,000

Modulus of elasticity (MOE)

−5.88 1

300–1,000

Brinell hardness (HBtang)

−12.9 2.14

200–1,000

Coefficients a and b given in the literature for functions according to the model: ln f = ln a + b ln ρ

• Strength potential index of densified wood The compression of solid wood causes a general collapse in the cell structure, possibly resulting in compression defects (e.g., breaking and cracking). This may have a negative effect on wood’s strength properties. Then, the strength usually increases relatively less than the density. The more damage the wood structure suffers during densification, the worse its strength is at a given density after densification. Many researchers reported the relationships between density and several strength properties among nondensified woods. Mostly, the strength-to-density relationship takes the form f = aρb or ln f = ln a + b ln ρ, where constants “a” and “b” differ among mechanical properties (Table 6.5). Considering the same strength property, the parameters “a” and “b” are also different. This may be caused by the differences in wood species and individuals. It was reported that the mechanical properties of semi-isostatic densified woods varied with their density, and no apparent change

216

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

was found in the strength-to-density relationship between densified and nondensified wood, except for coefficient “a.” Therefore, the relationship between density and mechanical properties can be used to diagnose the extent of damage to the cell walls by densification. The strength of densified wood relative to that of nondensified wood with the same density is used to denote the “strength potential index.” The strength potential index (ad /ao ) of the UHPD for different strength properties is given in Table 6.6. For MOR and MOE, the value of ad /ao was close to 1, indicating that the UHP-compressed wood was not negatively affected by the compression in the axial direction. However, the ad /ao of hardness was very low, showing that the densified wood was substantially weaker than expected from its density along the compression direction. This appears to be associated with the deformation of rays during compression. Rays are important reinforcements and affect the strength of nondensified wood in the radial direction. Additionally, previous studies showed that 40% of immediate elastic strain accrued in the semi-isostatic densified Scots pine when pressure was released, which caused the wood to become rubbery. Therefore, the hardness of the densified wood was lower than the hardness theoretically assumed. It is noteworthy that the lowest ad /ao of all strength was found in 200 MPa treated specimens, reflecting that 200 MPa of treatment further pressed the wood samples and caused more compression defects on wood cells than that of other UHP treatments. ad /ao was also used as an indicator to compare the UHPD with other thermal densification methods (e.g., TM, THM, and VTC) in the destructive level of wood cells. The ad /ao for traditional compression methods was calculated based on the Table 6.6 Ratios between densified (d) and nondensified (0) woods for density (ρ), strength ( f ), and strength potential index (a = f /ρ b ). Reprinted from Ref. [4], used with permission ρd /ρo

Material

MOR f d /f o

MOE ad /ao

f d /f o

BH ad /ao

f d /f o

ad /ao

Poplar, 50 MPa

1.71

1.72

0.88

2.05

1.20

1.49

0.48

Poplar, 100 MPa

1.96

1.85

0.80

2.41

1.23

1.61

0.38

Poplar, 150 MPa

2.07

2.00

0.81

2.62

1.27

1.67

0.35

Poplar, 200 MPa

2.01

1.84

0.77

2.31

1.15

1.55

0.35

Poplar, 40% CRa

1.60

1.42

0.79

1.52

0.95

1.64

0.60

Poplar, 50% CRa

1.82

1.45

0.69

1.70

0.93

1.85

0.51

°Cb

1.97

2.25

0.96

2.29

1.16

3.34

0.78

Poplar, 180 °Cb

2.04

2.25

0.92

2.33

1.14

2.72

0.59

Poplar, 160

Poplar, 200 °Cb

2.10

2.21

0.87

2.81

1.34

2.79

0.57

Poplar, 220 °Cb

2.06

2.05

0.83

3.00

1.45

2.35

0.50

1.63

1.32

0.72

1.37

0.84

–

–

Hybrid poplar 38% CRc CRc

1.98

1.66

0.71

1.84

0.93

–

–

Hybrid poplar 58% CRc

2.32

2.02

0.71

2.29

0.99

–

–

Hybrid poplar 50%

MOR (b = 1.25), MOE (b = 1), and BH (b = 2.14) for UHP-isostatic densification compared with other densification methods; a TM compression; b THM compression; c VTC compression

6.2 Ultra-High-Pressure Densification of Wood

217

data published in previous studies. As shown in Table 6.6, the ad /ao for MOR and MOE was compared among different densification methods. The ad /ao of UHPT wood was higher than that of TM and VTC, although the ad /ao of UHPT wood was slightly lower than that of THM-treated specimens. This result confirmed that UHP treatment enhanced strength properties more effectively than most thermal compression technologies in the axial direction. However, the ad /ao value of hardness for UHPD was slightly lower than that of all hot-pressing compression methods. Wood hardness reflects the resistance of the wood surface; thus, it is closely related to surface density. The wood treated with traditional densification usually has an uneven density distribution, i.e., the surface density is greater than the internal density. In contrast, UHP-compressed wood has uniform density distribution throughout its thickness. Therefore, traditional densification resulted in higher hardness than the UHP treatment, though the densified wood had a similar average density. The ad /ao results indicate that UHP treatment might have been less destructive than the other thermal compression methods. Noteworthy, small wood specimens were free of visible defects compared to different densification methods. The effects of UHPD on deformation differed between hard and soft structures. More deformation was found in soft structures, which might protect the integrity of the wood cells and prevent densified wood from spreading and checking. UHP treatment is probably the least detrimental wood compression technique, especially when knots and other possible defects of wood samples are concerned.

6.2.2 Moisture Sorption Propertie Changes in the morphological properties of wood generally result from initial changes to its anatomical structure, which can be easily affected by the moisture content and the form in which it exists (free, hygroscopic, and bound water) in the wood. It was reported that moisture content significantly affects the physical and chemical properties of wood. Once the absorbed water is elevated beyond the bound water stage, the wood undergoes swelling, causing dimensional instability, which decreases the density and mechanical strengths and increases thermal and electrical conductivities. Being somewhat hygroscopic, the compressed or densified wood exchanges water molecules with the surrounding environment in a dynamic state, reaching an equilibrium moisture content (EMC) under any given relative humidity (RH) condition. EMC is affected by temperature, RH, species, and wood properties. The moisture–RH relationship at a given temperature can be described using a moisture sorption isotherm (MSI).

218

6.2.2.1

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Moisture Sorption Isotherm

• Measurements of equilibrium moisture content Saturated inorganic salt solutions were used to maintain different constant equilibrium RH. Climate chambers with selected RH 33.1, 54.4, 68.3, 75.5, 85.1, and 94.6% were maintained using specific saturated inorganic salt solutions in a closed environment at 20 °C, as indicated in Table 6.7. The apparatus regulating RH is illustrated in Fig. 6.16, in which a welded wire mesh basket was suspended from the middle of the plug in a hermetically sealed glass jar and then placed in a temperature-controlled incubator to reach equilibrium conditions. First, poplar specimens were placed in a drying oven (103 ± 2 °C) for 12 h until they reached a constant weight (oven drying). During the adsorption experiment, the oven-dry poplar specimens were placed in a welded wire mesh basket (as shown in Fig. 6.16). A static gravimetric method was used in this study to determine the EMC of the poplar wood samples according to Eq. (6.11). Samples were weighed every two days until the change in mass was negligible (less than 0.0050 g). Equilibrium moisture content (%) =

m1 − m0 × 100 m0

(6.11)

Table 6.7 Equilibrium relative humidity provided by different saturated salt solutions at 20 °C. Reprinted from Ref. [5], used with permission 20 °C

MgCl2

Mg(NO3 )2

CuCl

NaCl

KCl

KNO3

33.1%

54.4%

68.3%

75.5%

85.1%

94.6%

Fig. 6.16 Apparatus for determining the equilibrium moisture content of poplar wood samples. Reprinted from Ref. [5], used with permission

6.2 Ultra-High-Pressure Densification of Wood

219

where m0 is the oven-dry mass (g) of the poplar sample and mi is the mass (g) of the polar sample that reached equilibrium humid condition. • Equilibrium moisture content The moisture adsorption isotherms of UHPT and control Paulownia woods at 20 °C are shown in Fig. 6.17, and similar trends were observed at 30 and 40 °C. EMCs increased with increasing RH. At each RH, the EMC values for the test group were different. For example, at an RH of 11.3 and 94.6%, the UHPT samples had higher EMC values than the control, while from 33.1 to 75.5%, the control sample had comparatively higher EMC values than the UHPT groups. This may have been due to the existence of internal stress in the samples caused by the UHP treatment. Wood samples exposed to a high-humid external environment would experience a high driving force of water vapor from the internal pressure deficit, resulting in moisture infusion and consequent swelling of the wood, depending on the compaction level and nature. The residual stress generated during thermocompression was released after moisture absorption and equilibration. The original stable state was generally maintained when the samples were placed in a relatively low-RH environment because the water vapor pressure was small and insufficient to drive the moisture in. This result signified the impact of RH on the moisture absorption of wood samples. The UHPT sample had a comparatively lower EMC in most experimental cases. However, the differences among the treated samples were insignificant, which was a useful result for energy-efficient development patterns for the future large-scale manufacturing of UHP compacted wood products. In addition, at RH below 75%, the EMCs of all UHPT boards were below 12%, which signified a stable condition, especially for construction materials used in indoor applications. The moisture adsorption isotherms of UHPT and control Chinese fir woods at 25 °C are presented as bar graphs in Fig. 6.18. The UHP treatment at different pressure levels reduced the EMCs at 33 and 52% RH, which were at a range of 3.28– 3.48% and 6.53–6.65%, respectively (almost doubled at 52% RH compared with that at 33% RH), with no significant differences from control. However, the EMC increased dramatically (p < 0.05) at 86 and 93% RH, in the range of 16.3%16.7% and 22.4%23.7%, respectively, showing significant increases compared to control. At 67% RH, the UHP treatment at different pressure levels slightly increased the EMC. The EMCs of Chinese fir densified at various pressure levels and stored at various RHs were not significantly (p > 0.05) different, except at 86% and 93% RH. The EMC of Chinese fir compressed at 50 MPa was significantly lower (p < 0.05) than that of other pressure levels. These results indicate that pressure could significantly increase water permeability in the wood, reducing its moisture content at low RH and increasing it at higher RH. Using a conventional compression method, compressive deformation separated and destroyed aspirated pits in the wood, increasing liquid uptake during the recovery process of the deformation because of volumetric pressure. The water absorption test showed that the water absorption of wood samples densified by UHP treatment was two or three times higher than that of untreated wood (data not shown). Combined

220

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.17 Equilibrium moisture content of UHPT and control Paulownia woods at 20 °C. Reprinted from Ref. [7], used with permission

6.2 Ultra-High-Pressure Densification of Wood

221

Fig. 6.18 Moisture adsorption isotherms of UHPT and control Chinese fir woods at 25 °C. Reprinted from Ref. [2], used with permission

with an observation of gas overflow from the wood after UHPD, a reasonable speculation can be derived that the UHP treatment destroyed the structural integrity of the wood by collapsing the cells and expelling the entrapped air from voids. Following the UHP treatment, the hygroscopic sugar polymers (hemicellulose constituents and cellulose) in densified wood may absorb moisture from the environment, increasing degrees of moisture absorption. This process is commonly recognized as hysteresis, contributing to the differences in MSIs between desorption (drying) and adsorption (rehydration) behaviors widely seen in drying applications. MSIs are essential for selecting the optimal processing and storing conditions in the wood industry. The experimental moisture adsorption isotherms of control and UHPT wood samples (0 s and 0.1 MPa, 30 s and 30 MPa, 30 s and 90 MPa, and 30 s and 150 MPa) at 20, 30, and 40 °C are presented in Fig. 6.19. In general, based on the curve trend in the figures, temperature and UHP treatment had a qualitatively similar effect on the equilibrium moisture content. The effect of temperature on the MSI of control and UHPT Paulownia woods was significant. EMC increased with decreasing temperature at each constant RH. In contrast, EMC reduced with increasing temperature during adsorption and desorption. This can be explained by the kinetic energy of the associated water molecules. At a high temperature, water molecules have a higher excitation state and get activated at a higher energy level, making them less stable and reducing the attractive forces among the molecules. The EMC values of the UHPT wood samples were closer to each other than to the control at the three temperatures. This result indicates that UHPT wood samples were more stable to temperature variations than that of untreated samples.

222

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.19 Moisture adsorption isotherms of control and UHPT Paulownia woods at 20, 30, and 40 °C; a 0 s and 0.1 MPa, b 30 s and 30 MPa, c 30 s and 90 MPa, and d 30 s and 150 MPa. Reprinted from Ref. [7], used with permission

Figure 6.20 shows the EMCs of UHPT and untreated poplar woods at 20 °C, which are related to storage at different RHs ranging from 33 to 95%. The EMC of densified samples changed with RH, and all EMC-RH (MSI curves of test samples were inline with the typical wood MSI. In the RH range of 33%–68%, the EMC of UHPT poplar specimens at different pressure levels increased linearly with increasing RH. In comparison, beyond 65%, the extent of EMC increases exponentially, as previously observed. At the same environmental RH, the difference in the EMC of poplar samples treated at different pressure levels was slight. The EMCs of densified samples fluctuated within a small range of 4.86–5.94% (33% RH), 8.19–9.12% (54% RH), 10.39– 11.42% (68% RH), 13.11–14.47% (76% RH), 15.61–17.26% (85% RH), and 21.22– 24.58% (95% RH). There was no trend among different UHP treatments, which indicates that pressure or holding time had little effect on the EMC of poplar. The abovementioned differences in EMC may have been due to normal variability within poplar. Figure 6.20d reveals that during storage at RH ranging between 33 and 68%, the control wood samples yielded a higher EMC than that of all UHPT samples. However,

6.2 Ultra-High-Pressure Densification of Wood

223

Fig. 6.20 Equilibrium moisture content of poplar treated under various conditions; letters on the picture indicate holding time: a 0 s, b 30 s, c 60 s, and d 300 s. Reprinted from Ref. [5], used with permission

there were no significant differences between EMCs of untreated and UHPT samples (p > 0.05) when the RH was above 76%. This may be related to the swelling characteristics of wood material. Our previous studies showed that at an environmental RH of 33.1–75.5%, the UHPT Paulownia wood samples had comparatively lower EMC values than that of the control samples. This is consistent with the present study. Compared with 0-, 30-, and 60-s UHP holding times, the longer holding time (300 s) resulted in denser wood. High environmental RH resulted in volume expansion of the poplar specimens, generating notably denser UHPT samples. The UHPT samples had a small amount of expansion at lower environmental RH, and their structures were still denser, impeding the penetration of water molecules. However, at higher environmental RH, the volume of UHP-densified wood almost recovered to the precompression state; thus, more voids and water channels appeared inside. • Set recovery measurement and equilibrium moisture experiment Uncoated, tung oil (TO)-coated, and epoxy resin (ER)-coated samples were used to measure set recovery in five RH environments at a constant temperature of 25 °C. Five saturated inorganic solutions (MgCl2 , Mg(NO3 )2 , KI, KCI, and KNO solutions) were

224

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

used to maintain five RH (33, 52, 69, 86, and 93%) conditions. These five conditions were created by placing saturated salt solutions on the bottom of the desiccators. Each pressure treatment group used five of these desiccators. Sorption tests were performed in temperature-controlled incubators and maintained at 25 °C ± 1 °C, allowing a precise RH control. The specimens were weighed periodically until the change in mass was negligible (at least 0.05) occurred in treated and untreated woods stored at 33, 52, and 67% RH conditions. At 33% RH, the UHP treatment slightly reduced the radial, tangential, and volumetric dimensions of Chinese fir wood. The effect of different pressure levels on radial, tangential, and dimensional volumetric changes were similar under different RH conditions, except at 93%. Moreover, the radial, tangential, and volumetric dimensional swelling at 50 MPa at 93% RH was significantly lower than that at other pressure levels. Another finding was that the tangential swelling of densified wood was more prominent than radial swelling.

6.2 Ultra-High-Pressure Densification of Wood

235

Fig. 6.26 Dimensional changes in control (0.1 MPa) and UHP-densified Chinese fir wood samples at various relative humidity values at 25 °C after a month when EMCs of wood were reached. The dimensions of all densified and control wood specimens were 20 mm (longitudinal) × 20 mm (tangential) × 20 mm (radial) before the moisture sorption experiments. Reprinted from Ref. [2], used with permission

The recovery rates in radial, tangential and volumetric dimensions were low under low or moderate RH conditions (Figs. 6.26 and 6.27) after nearly a month of balance time. The high recovery rates of UHP-densified wood had a close relationship with high environmental RH. These results indicate that the dimensional stability should be improved by preventing moisture penetration into the wood specimen. Higher CR generally leads to greater immediate spring back because higher inner stresses are created during traditional compression. These observations necessitate further studies involving some surface modification pretreatments for the wood before UHPD or impregnating hydrophobic substances in the wood during the UHP treatment.

236

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.27 a Radial, b tangential, and c volumetric dimensions of UHP-densified and control (0.1 MPa) Chinese fir wood samples at various relative humidity values at 25 °C (the error bars represent standard deviation). Reprinted from Ref. [2], used with permission

6.2 Ultra-High-Pressure Densification of Wood

6.2.3.3

237

Thickness Swelling

• Measurements of thickness swelling After UHP treatment, all samples were conditioned at 20 °C and 60% RH until the thickness became stable. Then, the thickness of the compressed samples was measured for 1, 24, 48, 72, 96, 120, 144, and 168 h after the treatment. The thickness swelling (TS) was calculated using Eq. (6.20). Thickness swelling (%) =

Td − Tc × 100, Tc

(6.20)

where T d is the thickness under air-dry conditions during the swelling test. • Thickness swelling of densified wood Figure 6.28 shows that the TS of the densified woods changed with storage time. Similar trends were observed for all tested specimens. During the first hour after the test, the thickness of the densified wood rapidly increased by 3.5%. Continuous increase trends were observed in all groups until 72 h, and the TS ceased after 96 h. In viscoelastic wood material, the cell deformations by compression can result in internal stresses stored in the microfibrils and matrix of the wood, which is the reason for the springback of densified wood. The average final TSs were less than 10%, similar to hot-pressing compression at the same CR level. In addition, there was no significant difference (p > 0.05) in the final TS among all UHP-densified specimens. All the subsequent experiments were conducted after 168 h of the swelling test. • Measurements of thickness swelling efficiency This test evaluated the dimensional change of UHPT wood under six RH conditions (33.1, 54.4, 68.3, 75.5, 85.1, and 94.6%). Test samples prepared for UHPD were cut into 10 × 10 × 10-mm3 sizes, placed in the oven at 103 °C for 12 h until the wood specimens reached oven-dry condition, and then placed under one of the six different RH conditions (33–95%) until consistent thickness. Then, wood specimens were oven dried at 103 °C for 12 h for the irreversible swelling test. The thickness swelling efficiency was calculated using Eq. (6.21). Thickness swelling efficiency (%) =

T1 − T0 × 100, T0

(6.21)

where T 0 and T 1 are the thicknesses (mm) in oven-dried condition before and after the humidify-condition test. • UHP densification and its effect on thickness swelling efficiency Below the fiber saturation point, changes in the moisture content of wood resulted in deformations, such as shrinkage or swelling. The thickness swelling efficiency (TSE) is used to analyze the progress of swelling compared to the thickness in

238

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.28 Average thickness swelling of UHP-compressed wood samples as a function of time during the swelling test. Reprinted from Ref. [4], used with permission

the oven-dry state (before the moisture absorption/soaking test). It is an important indicator of the dimensional stability of the wood during variable RH. The TSE of UHPT and untreated poplar at different RH (33–95.5%) are shown in Table 6.10. The TSE of the control sample increased with increasing RH owing to the normal water absorption/swelling characteristic of wood. The wood continued to adsorb water when RH increased because water molecules are attracted by the hydrophilic groups of cellulose and hemicellulose to form hydrogen bonds, and the spacing of cellulose molecules is enlarged and causes cell wall expansion. Compared to control, UHPT samples had a significant influence on the dimensional stability of the poplar. The TSE values in the thickness direction increased from 1.38% (control) to 6.06–7.73% at 33% RH, 2.02 to 13.3 to 16.8% at 54% RH, 1.74 to 18.5 to 24.3% at 68% RH, 2.33 to 26.7 to 36.8% at 76% RH, 3.40 to 37.6 to 45.4% at 85% RH, and from 4.47 to 57.4 to 83.5% at 95% RH after UHPD treatment. In addition, the difference between control and UHPT samples enhanced with increasing RH, and the maximum difference reached 79% at 95% RH. The shrinkage and expansion properties of wood are related to its cell walls. The natural wood swelling after compression treatment has been observed in a previous study. It was reported that densified wood usually reverts to the uncompressed condition at high RH, which results in a greater TSE change than in control. As shown in Table 6.10, the change in the TSE of UHP-densified poplar was more complex. There were no clear patterns in the TSE among all UHPT poplars stored at RH values of 33 and 54%, while there were no significant differences at higher RH, except for 50, 50, and 100 MPa-0 s. This phenomenon may have been caused by the following factors. The

150 MPa-30 s

150 MPa-0 s

100 MPa-300 s

100 MPa-60 s

100 MPa-30 s

100 MPa-0 s

50 MPa-300 s

50 MPa-60 s

50 MPa-30 s

50 MPa-0 s

Control

Samples

16.44 ± 1.54abc 15.53 ± 1.24abcd 16.78 ± 1.88abc 16.62 ± 1.92a

6.62 ± 0.91abc

6.61 ± 0.60abc

6.53 ± 1.66abc

2

1

2

7.07 ± 0.36abc

13.27 ± 1.00d

6.16 ± 0.30abc

1

1

15.90 ± 1.59abc 16.65 ± 1.30abc

6.09 ± 0.57c

6.46 ± 0.36abc

1

16.92 ± 2.11abc

7.37 ± 0.68ab

2

2

15.37 ± 0.34bc 16.28 ± 1.93ab

6.39 ± 0.71abc

14.12 ± 0.11bcd

6.81 ± 0.48abc

1

7.73 ± 0.52a

14.04 ± 1.41bcd 17.19 ± 1.81abc

6.25 ± 0.39bc

6.72 ± 0.25abc

1

2

2

17.69 ± 0.53abc

6.16 ± 1.43abc

2

1

17.20 ± 1.32abc

2 16.83 ± 0.51a

15.78 ± 0.96abc

6.06 ± 0.46abc

1

5.59 ± 0.24bc

14.7 ± 0.42abcd 15.77 ± 0.78abc

6.43 ± 0.31abc

5.97 ± 0.42c

1

2

7.00 ± 1.21c

2.02 ± 0.47e 1.29 ± 0.37d

1.38 ± 0.17d

0.64 ± 0.10d

1

2

1

54.4% RH

33.1% RH

19.88 ± 2.13ab

25.89 ± 4.23a

22.08 ± 3.58ab

24.83 ± 0.99a

20.73 ± 0.77ab

27.20 ± 0.87a

23.37 ± 0.56ab

28.56 ± 2.62a

23.89 ± 1.99a

25.66 ± 3.96a

21.34 ± 3.02ab

25.99 ± 4.81a

21.56 ± 4.82ab

28.73 ± 1.55a

24.27 ± 1.21a

22.75 ± 5.49a

18.45 ± 4.49b

23.8 ± 0.39a

19.8 ± 0.46ab

1.82 ± 0.29b

1.74 ± 0.46c

68.3% RH

32.15 ± 0.83abc

36.74 ± 3.33abc

32.81 ± 3.06abc

37.72 ± 1.08abc

33.00 ± 0.79abc

38.97 ± 3.31ab

34.45 ± 2.88ab

36.18 ± 2.50abc

33.07 ± 1.97abc

31.90 ± 3.54bcd

29.56 ± 3.13bc

36.78 ± 5.67abc

32.78 ± 5.03abc

34.57 ± 7.33abcd

32.25 ± 7.55abc

28.83 ± 4.20d

26.77 ± 4.12c

31.0 ± 1.13 cd

26.7 ± 0.70c

2.54 ± 0.42e

2.33 ± 0.52d

75.5% RH

45.41 ± 11.62a

51.57 ± 4.16a

44.83 ± 4.29a

54.60 ± 2.36a

45.29 ± 1.00a

45.54 ± 7.76a

38.53 ± 6.53a

44.53 ± 10.97a

39.22 ± 9.45a

49.65 ± 5.08a

43.08 ± 4.34a

53.74 ± 3.52a

43.52 ± 4.27a

47.75 ± 2.58a

41.98 ± 1.27a

44.52 ± 1.69a

37.60 ± 1.53a

44.6 ± 2.48a

38.3 ± 2.66a

3.09 ± 0.70b

3.40 ± 1.01b

85.1% RH

(continued)

81.42 ± 7.37ab

86.08 ± 2.11ab

78.73 ± 2.03abc

75.41 ± 5.42abc

65.47 ± 4.46 cd

75.21 ± 13.28abc

68.48 ± 13.10bcd

89.78 ± 2.23ab

83.52 ± 2.64a

72.53 ± 17.24 cd

65.62 ± 14.82 cd

82.36 ± 4.85ab

71.08 ± 5.49abcd

85.46 ± 4.10ab

78.98 ± 4.29abc

64.73 ± 4.81c

56.93 ± 4.59d

65.0 ± 5.82c

57.4 ± 4.46d

4.09 ± 0.65d

4.47 ± 0.86e

94.6% RH

Table 6.10 Thickness swelling efficiency of poplar subjected to different UHP treatments under different RH conditions. Reprinted from Ref. [5], used with permission

6.2 Ultra-High-Pressure Densification of Wood 239

200 MPa-300 s

200 MPa-60 s

200 MPa-30 s

200 MPa-0 s

150 MPa-300 s

150 MPa-60 s

Samples

16.20 ± 0.99abc 13.99 ± 0.80bcd 14.85 ± 0.81c 13.43 ± 0.82 cd 16.44 ± 1.20abc

6.06 ± 0.48abc

6.55 ± 0.81abc

6.22 ± 0.33abc

6.07 ± 0.63c

6.71 ± 0.85abc

2

1

2

1

2

2

17.32 ± 3.26abc

15.98 ± 2.67ab

7.06 ± 0.53abc

1 14.71 ± 0.55abcd

14.95 ± 1.60abcd 18.84 ± 1.90a

6.53 ± 0.73abc

7.52 ± 0.83a

1

2

6.30 ± 0.39abc

16.28 ± 0.68ab 17.76 ± 0.43abc

7.54 ± 0.22ab

7.02 ± 0.44ab

1

2

6.68 ± 0.53abc

18.24 ± 2.48ab

6.98 ± 1.47abc

1

54.4% RH

33.1% RH

2

Table 6.10 (continued)

25.90 ± 1.97a

21.56 ± 1.55ab

23.94 ± 5.75a

20.24 ± 5.03ab

23.16 ± 1.11a

19.76 ± 1.18ab

27.47 ± 1.62a

23.03 ± 1.33ab

26.88 ± 3.72a

21.55 ± 2.88ab

24.31 ± 3.03a

19.88 ± 2.82ab

22.96 ± 2.91a

68.3% RH

39.63 ± 3.08a

34.65 ± 2.50ab

35.12 ± 6.22abcd

32.08 ± 6.07abc

37.13 ± 3.65abc

32.18 ± 2.54abc

39.60 ± 2.01a

35.81 ± 3.09ab

38.87 ± 3.80ab

34.49 ± 3.24ab

40.24 ± 2.24a

36.83 ± 2.09a

34.51 ± 0.84abcd

75.5% RH

51.12 ± 4.44a

42.35 ± 2.89a

45.13 ± 8.41a

39.69 ± 7.17a

49.42 ± 7.50a

43.75 ± 6.44a

51.50 ± 5.13a

45.34 ± 4.82a

51.98 ± 3.31a

44.89 ± 2.64a

51.70 ± 1.43a

45.26 ± 1.53a

53.19 ± 13.09a

85.1% RH

83.53 ± 8.82ab

70.91 ± 7.46abcd

79.35 ± 5.88abc

71.23 ± 4.95abcd

83.01 ± 10.04ab

75.41 ± 8.67abc

78.35 ± 10.37abc

71.37 ± 8.62abcd

81.47 ± 11.81ab

68.75 ± 11.21abcd

83.98 ± 8.10ab

76.08 ± 7.36abc

89.14 ± 8.72ab

94.6% RH

240 6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

6.2 Ultra-High-Pressure Densification of Wood

241

shape of densified wood is usually distorted after swelling or shrinking, which affects the consistency and accuracy of data measurement. It was demonstrated that natural pozzolans could stabilize marl soil due to their similar structure, and the increase in volcanic ash content decreases the expansion and ductility of marl soil. This provides research reference for the subsequent studies on wood shrinkage and swelling. When storage RH is low, the expansion of the UHPT sample is small, which leads to more significant relative errors that mask the changing tendency of TSE with pressure or holding time. In addition, the densified wood usually reverts to its original shape at high RH, which causes high irreversible deformation that can be larger than that of the natural wood swelling due to moisture absorption. Therefore, the results above only show that there were no significant differences in the TSE trends between the first and second dry–humid cycles. However, they increased gradually with increasing RH. For example, at 100 MPa-60 s, the increases in TSE were 0.37, 0.76, 3.84, 4.52, 7.01, and 6.73% after repeated humidity cycles at 33, 54, 68, 76, 85, and 95% RH, respectively. While at 150 MPa-0 s, the increases in TSE were—0.46, 1.25, 3.80, 3.94, 6.74, and 7.36%, respectively. The results show that the TSE of UHPT poplar generally increased after repeated humidity cycles; however, the trend was clear only at high RH levels. The compressed wood begins to recover when the environmental RH exceeds a certain value. Then, the resulting irreversible deformation leads to an increase in TSE after the next dry–humid cycle. Therefore, an apparent increase in the set recovery value can be considered the RH threshold, after which set recovery starts to manifest itself. According to the TSE results, the RH threshold for UHP-densified samples appears to be between 33 and 54%. Moreover, traditionally conducting these experiments will only lead to high humidity results, and the valuable information at the lower RH will be lost. • Measurements of irreversible swelling Irreversible swelling (IS) is an important indicator to measure the dimensional stability of wood. It was calculated using Eq. (6.22). Irreversible swelling (%) =

VOD1 − VOD0 × 100, VS − VOD0

(6.22)

where V OD0 is the original volume (mm3 ) under oven-dried condition after UHP treatment, V S is the volume (mm3 ) after the humidify-condition test as described in the thickness swelling test, and V OD1 is the volume (mm3 ) under oven-dried condition after the humidity condition test. • Effect of UHPD treatment on irreversible swelling IS is an important indicator for measuring the dimensional stability of wood and analyzing the permanent deformation during the humidification–dehumidification cycle. Table 6.11 shows the IS of poplar following various UHP treatments. As expected, the IS of UHPT poplar was similar to the TSE discussed above, which

150 MPa-60 s

150 MPa-30 s

150 MPa-0 s

100 MPa-300 s

100 MPa-60 s

100 MPa-30 s

100 MPa-0 s

50 MPa-300 s

50 MPa-60 s

50 MPa-30 s

50 MPa-0 s

Samples

29.26 ± 2.51 16.86 ± 0.95 28.00 ± 1.00 23.46 ± 1.46

5.99 ± 4.82

12.54 ± 1.20

24.09 ± 2.16

5.78 ± 1.56

2

1

2

1

21.30 ± 1.55

2

33.99 ± 4.98

24.70 ± 6.02

8.28 ± 2.32

1

4.75 ± 4.33

32.20 ± 3.38

22.28 ± 7.63

2

15.70 ± 1.68

22.74 ± 2.79

10.96 ± 3.91

1

1

30.44 ± 1.99

15.71 ± 2.45

2

2 20.87 ± 2.41

19.75 ± 3.44

8.43 ± 2.80

1 31.48 ± 2.91

37.08 ± 0.22

21.46 ± 1.93

2

8.98 ± 1.82

26.40 ± 2.11

13.66 ± 2.54

1

10.07 ± 2.89

31.68 ± 3.37

5.74 ± 2.45

2

1

22.53 ± 8.51

2

32.70 ± 0.62

24.09 ± 1.08

11.29 ± 7.18

1

5.22 ± 5.70

33.59 ± 0.84

12.39 ± 6.90

2

11.03 ± 1.50

24.9 ± 1.78

11.6 ± 5.15

1

54.4% RH

33.1% RH

1

25.04 ± 4.24

29.45 ± 7.89

19.61 ± 5.56

35.73 ± 3.51

23.70 ± 6.06

41.03 ± 2.61

32.04 ± 1.76

37.33 ± 1.95

25.53 ± 2.46

38.35 ± 2.79

30.09 ± 3.73

39.11 ± 2.89

28.47 ± 3.74

43.57 ± 1.89

33.87 ± 0.85

36.35 ± 3.44

27.36 ± 3.15

39.49 ± 1.03

30.79 ± 0.60

37.63 ± 1.38

27.9 ± 2.21

68.3% RH

34.74 ± 0.30

41.14 ± 0.71

30.58 ± 1.48

44.13 ± 5.34

35.60 ± 4.87

52.17 ± 0.83

42.55 ± 1.76

49.11 ± 2.77

39.88 ± 3.18

42.72 ± 4.38

31.69 ± 5.39

41.99 ± 3.26

29.51 ± 4.68

53.78 ± 1.15

43.79 ± 0.51

44.92 ± 2.37

33.27 ± 1.26

44.81 ± 1.05

32.02 ± 3.17

43.03 ± 0.97

30.0 ± 1.11

75.5% RH

37.04 ± 0.99

54.89 ± 4.97

42.09 ± 3.76

49.02 ± 1.28

38.86 ± 1.39

51.80 ± 2.01

48.67 ± 3.60

53.40 ± 2.83

41.58 ± 3.23

48.09 ± 1.79

37.62 ± 1.97

51.41 ± 1.51

40.92 ± 1.48

56.62 ± 1.04

56.30 ± 3.44

50.26 ± 2.89

40.90 ± 2.23

51.27 ± 2.65

39.97 ± 1.55

50.19 ± 1.15

40.1 ± 0.24

85.1% RH

(continued)

55.30 ± 2.33

62.20 ± 1.68

56.05 ± 0.62

63.69 ± 3.21

56.80 ± 2.96

62.53 ± 4.85

55.70 ± 1.45

62.13 ± 3.06

56.34 ± 2.10

63.36 ± 1.93

57.53 ± 2.51

61.30 ± 0.82

55.36 ± 2.84

66.37 ± 2.07

67.98 ± 4.27

64.04 ± 3.21

61.04 ± 1.33

67.60 ± 1.58

60.81 ± 4.02

66.44 ± 2.41

57.3 ± 2.65

94.6% RH

Table 6.11 Irreversible swelling of poplar treated by different UHP treatments under different RH conditions. Reprinted from Ref. [5], used with permission

242 6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

200 MPa-300 s

200 MPa-60 s

200 MPa-30 s

200 MPa-0 s

150 MPa-300 s

Samples

30.70 ± 3.10 24.35 ± 1.76 34.33 ± 0.80

9.80 ± 2.08

13.43 ± 3.07

1

2

2

5.34 ± 3.93

18.22 ± 1.98

9.62 ± 2.62

1

2

30.01 ± 3.56

5.32 ± 1.52

2

25.81 ± 1.39

21.88 ± 3.01

4.64 ± 1.66

1

26.28 ± 1.36

36.66 ± 1.93

34.23 ± 2.38

2

9.42 ± 4.43

29.41 ± 0.62

16.44 ± 0.32

1

12.77 ± 3.42

31.96 ± 1.41

9.92 ± 3.73

1

54.4% RH

33.1% RH

2

Table 6.11 (continued)

42.16 ± 2.12

35.27 ± 2.39

32.98 ± 3.20

23.12 ± 1.65

32.22 ± 4.93

22.99 ± 3.95

37.40 ± 1.92

27.71 ± 2.96

44.01 ± 1.71

40.06 ± 5.49

35.53 ± 3.67

68.3% RH

52.73 ± 0.32

44.19 ± 1.30

43.77 ± 2.55

33.60 ± 2.01

45.72 ± 1.92

42.01 ± 3.17

46.53 ± 1.69

38.33 ± 1.72

53.53 ± 1.76

43.73 ± 2.56

46.37 ± 1.27

75.5% RH

53.43 ± 1.84

50.85 ± 5.57

46.10 ± 4.72

33.92 ± 5.40

47.67 ± 2.52

35.67 ± 1.23

42.18 ± 9.27

36.76 ± 1.94

60.24 ± 3.12

45.54 ± 3.09

48.16 ± 1.05

85.1% RH

62.81 ± 1.44

64.94 ± 1.76

60.95 ± 2.11

56.84 ± 1.57

64.31 ± 0.99

58.19 ± 1.64

60.53 ± 3.40

52.97 ± 3.67

61.57 ± 1.21

63.38 ± 3.36

61.28 ± 2.05

94.6% RH

6.2 Ultra-High-Pressure Densification of Wood 243

244

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

increased with increasing RH. IS of all UHP-densified wood samples varied irregularly within the range of 4.75–16.4% at 33% RH. Similarly, no apparent trends were found between the first and second dry–humid cycles. These results indicate that the IS of compressed wood at 33% RH may have possible measurement uncertainty errors. However, above 54% RH, the IS of treated samples irreversibly swelled (15–85%). Therefore, the RH threshold for UHP-compressed poplar was between 33 and 54, which was different from another study that reported the RH threshold of hot-pressed-densified wood was between 65 and 75% RH. It should be noted that high temperatures can damage the hydroxyl groups of wood polymers, which can decrease their hygroscopic properties and increase the RH threshold of compressed wood. In the hot-pressing treatment, samples needed to be treated at a high temperature for several hours, while UHP technology compressed wood at room temperature. In addition, the extent of IS increased rapidly when the RH exceeded the threshold value. The IS values for 50 MPa-30 s were 22.5, 27.4, 33.3, 40.9, and 61.0% after the first dry–humid cycle test at 54, 68, 76, 85, and 95% RH, respectively. Finally, the set recovery of UHP-densified poplar increased quickly at extremely high levels (38–85%) when RH was between 85 and 95%, which is highly undesirable. Comparing the samples treated at the same pressure, it was found that the pressure holding time also affected the IS of compressed wood at the same RH between 54 and 95% (Table 6.11). The IS value of poplar samples with 300-s holding time was higher than those of the other treatment groups. In contrast, no noticeable difference was observed in the IS values for shorter holding times between 0 and 60 s. The increased UHP holding times decreased the RH threshold. Additionally, the IS value of all treated specimens significantly increased when the environmental RH increased from 85 to 95%, regardless of holding time. This could mean that the holding time did not affect the RH threshold for controlling the extent of set recovery. Furthermore, the IS value of all densified wood samples was increased after repeated dry–humid cycles at 54–95% RH. However, a previous study has shown that the densified poplar samples usually slightly increased air-dry conditions after UHP treatment. Then, the wood sample thickness became stable at the end of the TS test. The difference between these two phenomena may have been related to the change in hydrogen bonding during the dry–humid cycle test. The water molecules that were adsorbed by wood directly combined with the hydroxyl groups of wood’s cellulose and hemicellulose to form hydrogen bonds, generating swelling stress in the interior of the wood. When the effect of swelling stress reached a certain level, the metastable state of compressed wood was destroyed, resulting in permanent deformation. Therefore, the set recovery appeared to be significantly higher after the second dry–humid cycle test, which yielded one more moisture absorption and expansion process. In addition, the effect of wood’s mechanosorptive creep can explain the phenomenon above. The natural swelling deformation of wood increases abnormally under load when its moisture content increases with environmental RH. It was reported that a swelling stress formed in the interior of the cell wall when the compressed wood absorbed moisture, similar to wood subjected to an external load, resulting in a simple effect of mechanosorptive creep. Additionally, the mechanosorptive creep of wood under different load conditions has been studied previously. The creep of wood

6.2 Ultra-High-Pressure Densification of Wood

245

formed when wood was subjected to an external load that absorbed moisture from the outside; however, the creep could not fully recover under no load during desorption. It is expected that partial creep deformation will retain in the wood. Thus, the IS value continued to enhance after each dry–humid cycle test.

6.2.4 Others 6.2.4.1

Density

• Measurements of density profile For the density profile measurement, sections sized 50 mm × 50 mm were cut from the nondensified and densified wood specimens, while the thickness remained constant at a compressed state. The density profile of the sections was measured using a cross-sectional X-ray densitometer (DENSE-LAB X, EWS, Germany) with an interval of 0.2 mm through the thickness. Three replicates were tested for each group. • Density profile of densified wood The density profile that characterizes density through the wood thickness is an important attribute because it affects the physical and mechanical properties of wood. As shown in Fig. 6.29, the nondensified wood exhibited almost uniform density with respect to the thickness direction. This was expected since hybrid poplar is a relatively homogeneous hardwood. Additionally, the tested samples had no defects (e.g., knots), developing a constant density distribution. Moreover, the density profile of all UHP-compressed specimens differed from that of previous hot-press compression methods. The N-shaped density gradient across the thickness showed the lower density region on both sides and a higher plateau region in the middle. This phenomenon was observed in all UHPT samples, most probably caused by the combined effect of UHP treatment and spring-back. It was reported that shear deformation from hydrostatic pressure caused some overlaps between latewood and earlywood, decreasing the density heterogeneity for both spruce and cherry. It has also been reported that a softer structure was more easily densified than the harder one under semi-isostatic compression, which made the wood density more homogenous. A decline in the density profile was observed in the surface regions of the UHPT wood. This was most probably caused by the springback, as discussed before. Additionally, the low density on the surfaces of densified wood might be a measurement error because of slightly uneven surfaces. • Measurements of density The mass of the wood remained constant throughout UHPD; therefore, the density of wood under pressure (ρ 1 ) could be calculated by the volume change during the elastic strain of the densified wood. The elastic strain of samples in the tangential or

246

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.29 Density profile of poplar wood samples treated under various conditions: a control density profile; b 50 MPa compressed density profile; c 100 MPa compressed density profile; d 150 MPa compressed density profile; e 200 MPa compressed density profile. Reprinted from Ref. [4], used with permission

radial direction was proportional under compression. Therefore, the density of wood immediately after pressure release was calculated using Eq. (6.23). ρ1 =

V2 T2 × R2 × L T2 × R2 × ρ2 = × ρ2 = × ρ2 , V1 T1 × R1 × L T1 × R1

(6.23)

where ρ1 and ρ2 are the sample densities under compression and after compression, respectively; V 1 and V 2 are the sample volume under compression and after compression, respectively; T 1 and R1 are the lengths of the measuring device after UHPD with tangential and radial compression, respectively; T 2 and R2 are the sample thickness after UHPD in the tangential and radial directions, respectively; and L is the sample thickness in the axial direction. • Density of wood after UHPD Improvement in wood density is the primary purpose of UHPD because wood density is closely related to its mechanical properties, such as hardness, Young’s modulus, and bending strength. The development of wood density after UHPD treatment with pressures up to 150 MPa was analyzed using nonlinear regression. Separate analyses were performed for the two holding times. The relationship between treatment pressure and wood density after treatment is shown in Fig. 6.30. After UHPD treatment, wood density increased with increasing pressure. The sample density increased

6.2 Ultra-High-Pressure Densification of Wood

247

Fig. 6.30 Relationship between treatment pressure and resulting wood density. Each dot represents the average of one sample. The functions were based on data for each segment. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

rapidly up to 30 MPa; however, the increasing rate decreased after 60 MPa. Holding time significantly affected density in the 0–60-MPa pressure range, similar to the trend for plastic strain discussed earlier. The difference in wood density after UHPD treatment between the two holding times was significant until 60 MPa but insignificant at higher pressure levels. The density of samples treated at 30 MPa for 300 s was close to that of samples treated at 100 MPa for 1 s. In addition, compared with UHPD at 100 MPa for 1 s, UHPD at 30 MPa for 300 s was more effective because the latter treatment had lower elastic strain at compression, which is desirable. There was a considerable difference in wood density during UHPD treatment compared with the density after treatment due to the elastic recovery that occurred immediately after pressure release, as shown in Fig. 6.31. This density difference increased with increasing pressure. For example, the wood densities during and after treatment at 150 MPa were 1,719 and 1,024 kg m−3 , respectively, demonstrating ~70% higher density under pressure. The corresponding values at 100 MPa were 1,670 and 962 kg m−3 . Experimental variation of these values was very low, with r 2 of 2.5%–3.0%. Wood is a porous material with very low density (400–450 kg m−3 ) before densification because of porosity. The density of cellular materials is relatively high and has been reported to be ~1,500 kg m−3 . The maximum density of crystalline cellulose has been reported to be 1,589 kg m−3 . Hence, the higher densities of wood (1,670–1,719 kg m−3 ) observed at 100 and 150 MPa could be erroneous. However, there could be some uncertainties in the measured volume and density.

248

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.31 Wood density at different pressure levels with 300-s holding time and after pressure release. Reprinted from Ref. [3], copyright 2020, with permission from American Society of Agricultural and Biological Engineers

The difference between the recognized maximum density of dry crystalline cellulose and the observed density values in this study could be due to the moisture in the wood, which was 13%. The density difference between moist and dry samples can be approximated using the following relationship: m oven−dried wood m moist wood = ρoven−dried wood ρmoist wood

(6.24)

where moven-dried wood and mmoist wood are the masses of dry and moist samples, respectively; ρoven-dried wood and ρmoist wood are the densities of dry and moist samples, respectively. This relationship indicates that wood density with 13% moisture would be ~13% higher. By multiplying the maximum cellulose density of 1,589 kg m−3 by 1.13, the resulting maximum density under pressure can be obtained as 1,795 kg m−3 , nearly the same as the density observed at 150 MPa. At 100 MPa, the maximum compression may not have been realized; hence, the observed density was relatively lower. In addition, based on SEM, many pores were found inside wood after UHPD treatment. In contrast, the density of oven-dried wood was close to that of cellular material when the pressure exceeded 100 MPa, indicating that almost no pores were left in the wood sample. Therefore, the density of oven-dried wood after pressure treatment could be further increased by reducing the elastic recovery immediately after pressure release. The mechanical properties of densified wood could also be further improved. It has been found that the mechanical properties of poplar wood

6.2 Ultra-High-Pressure Densification of Wood

249

(including strength, toughness, and ballistic resistance) could increase more than 10 folds when wood density increased to 1,300 kg m−3 . The average air-dry density of control and densified woods was calculated based on an almost consistent density profile, and the results are shown in Table 6.12. The average density of nondensified control specimens was 484.15 kg/m3 , while remarkable increases of 71, 96, and 107% were observed at 50, 100, and 150 MPa, respectively. However, there was no noticeable change (p > 0.05) in average density as the pressure increased to more than 150 MPa. Additionally, average density did not significantly vary at pressure levels of 100 and 200 MPa. Remarkably, the variation in density became smaller when the pressure exceeded 100 MPa, and the growth percentages fluctuated ~100%. This abnormal trend may result due to the reduction of voids in pressurized wood under the pressure conditions of more than 100 MPa. The increase in the wood density was realized by reducing its volume as the compression technique was based on the viscoelastic nature of the wood.

6.2.4.2

Microstructure

• Measurements of scanning electron microscopy Thin sections were prepared by a sliding microtome from air-dried specimens. The obtained sections were adhered to a silver specimen holder with double-sided conductive carbon tape. Then, they were coated with a thin film of gold (10 nm) in a vacuum evaporator through ion sputtering (E-1010, HITACHI, Japan). The surfaces of the wood samples were investigated using a SEM (TM3000, HITACHI, Japan) at 15 kV to analyze changes in the microstructure by densification. • Scanning electron microscopy analysis SEM was performed to examine the anatomical changes caused by UHP treatment. The SEM micrographs of cross sections of the untreated and UHPT specimens at different magnifications are shown in Fig. 6.32. Figure 6.32a shows the anatomical structure of the control; hybrid poplar is a diffuse porous hardwood with thin-walled vessels and libriform fibers. There are no apparent differences between the earlywood and latewood, which explains the relative uniform density distribution of the control wood samples. According to Fig. 6.32a–e, UHP treatment distinctly changed the microstructure of hybrid poplar wood. Transects of the wood treated at 50 MPa showed vessels collapsed and flattened, while the libriform fibers were slightly deformed. Microscopic examination showed that UHP treatment at low pressure (50 MPa) made the thin-walled vessels collapse, resulting in a high CR. Furthermore, higher pressure degraded the cell wall and reduced the cell lumen volume. After 100-MPa treatment, only a small part of the cell cavity remained open. The most severe deformation was observed at 150 MPa, where the vessel and fiber lumens were almost completely closed. This explained the increase in CR with increasing pressure of up to 150 MPa. However, the compression deformation recovered at 200 MPa. A slight increase

100 947.97 ± 46.08 b

150 999.98 ± 25.55 a

200 971.00 ± 11.91 ab

All values are expressed as mean ± SD. Sample means with different lowercase letters in the same column are significantly different (p < 0.05). Note: the average swelling was calculated based on the stable state (ranging from Day 4–Day 7)

50 826.18 ± 10.81 c

Control

484.15 ± 6.82 d

Pressure (MPa)

Density (kg/m3 )

Table 6.12 Density of treated poplar wood. Reprinted from Ref. [4], used with permission

250 6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

6.2 Ultra-High-Pressure Densification of Wood

251

Fig. 6.32 SEM images of poplar wood samples treated under various conditions at different magnifications: a control cross section; b 50 MPa-compressed cross section; c 100 MPa-compressed cross section; d 150 MPa-compressed cross section; e 200 MPa-compressed cross section; f shows the applied pressure direction of tested wood samples during UHP treatment. Reprinted from Ref. [4], used with permission

in the void volume was found compared with samples treated at 150 MPa, which caused a slight decrease in CR and density at 200 MPa. As shown by the SEM images derived from a larger area (200 μm/100× magnification), the collapsed vessels and deformed cells were evenly distributed inside the UHP-compressed wood. Therefore, a relatively uniform density profile was acquired. As shown in microscopic images (20 μm/1,500× magnification), all UHPT wood specimens were free of evident compression destruction (such as fracture, rupture,

252

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

and fragmentation) in the cell wall, except for some small cracks in the cell wall of 200-MPa-compressed wood specimens. The mechanical properties of the densified wood were determined by the destruction degree of the cell wall. In general, the less amount of destruction occurred in the cell wall, the better the mechanical properties of wood (e.g., MOE, MOR, and BH) will be obtained. Thus, small cracks in the cell wall explained the reduced strength of UHPT wood samples at a pressure level of 200 MPa. Additionally, the unbroken cell structure verified that UHP treatment is a less detrimental wood compression technique and could effectively enhance the mechanical properties of low-density wood. Figure 6.32f describes the simulation chart of the UHP processing applied to the tested wood. The water acted as a pressuretransmitting medium in this process, transferring pressure fast and evenly. Then, the tested wood sample was compressed by uniform pressure from all directions. The soft structure in the wood was more easily deformed by UHP treatment than the hard structure, which reduced the compression defects and protected the integrity of the wood cell. Altogether, the morphology of the UHP-compressed samples was consistent with that of previous studies on optimized thermal compression. The results confirmed that UHP processing is a less destructive compression method for wood. • Cell wall percentage and porosity percentage The cell walls percentage (V C ) and porosity percentage (V H ) of the test specimens were calculated using the following equations: Do × 100, DC

(6.25)

VH (%) = 100 − VC ,

(6.26)

VC (%) =

where Do = oven-dry density of test specimens (g cm−3 ). Dc = oven-dry density of cell walls (1.5 g cm−3 ). As shown in Table 6.13, the V c of test specimens increased significantly from 19.6% ± 0.28% (control) to 45.6% ± 0.96% (100 MPa), and the porosity percentage (V H ) decreased significantly from 80.4% ± 0.27% (control) to 54.4% ± 0.83% (100 MPa). This was likely owing to the elimination of cell lumina from the wood structure. Paulownia wood grown in China has a low density because of its rapid growth rate, low cell wall percentage (19.6% ± 0.28%), and high porosity percentage (80.4% ± 0.27%). Increasing the cell wall percentage reduces the porosity percentage and increases the wood density.

6.2 Ultra-High-Pressure Densification of Wood Table 6.13 Cell wall percentage (V C ) and porosity percentage (V H ) of test specimens. Reprinted from Ref. [6], copyright 2018, with permission from American Society of Agricultural and Biological Engineers

253

Treatment

V C (%)

V H (%)

Control

19.6 ± 0.28 a

80.4 ± 0.27 a

20 MPa

31.9 ± 0.57 b

68.1 ± 0.57 b

40 MPa

42.2 ± 0.87 c

57.8 ± 0.96 c

60 MPa

43.2 ± 1.02 cd

56.8 ± 1.01 cd

80 MPa

45.4 ± 0.82 d

54.6 ± 0.74 d

100 MPa

45.6 ± 0.96 d

54.4 ± 0.83 d

Means in the same column followed by different letters are statistically different according to Duncan’s multiple range test at p < 0.05

6.2.4.3

Surface Roughness

• Measurement Ten samples were tested from each treatment group. Locations for roughness measurements were randomly selected on the surfaces of the test samples. Roughness was measured parallel and perpendicular to the grain. A surface roughness tester (Baoleng JB-5C stylus-type profilometer, Zhongheng Instrument Co., Shanghai, China) was used for the surface roughness tests. The JB-5C stylus unit consisted of main unit and pick-up model TkE. The pick-up had a skid-type diamond stylus with a 5 μm tip radius and a 90° tip angle. The stylus traversed the surface at a constant speed of 1 mm s−1 over a 15 mm tracing length, and the vertical displacement of the stylus was converted into an electrical signal. Three roughness parameters, namely, mean arithmetic deviation of the profile (Ra ), mean peak-to-valley height (Rz ), and maximum roughness (Rmax ), have been used in previous studies to evaluate the surface characteristics of wood and wood composites. • Result The three roughness parameters—Ra , Rz , and Rmax —of the UHPT boards are summarized in Table 6.14. The surface roughness values of the UHP-treated boards decreased significantly with increasing pressure. All UHPT boards had lower surface roughness values than that of the untreated control boards. Above 80 MPa, UHP treatment did not show any further reduction in roughness. The Ra , Rz , and Rmax values of UHPT boards were reduced by 30–67%, 42–52%, and 30–59%, respectively, compared to the control. This improvement in surface smoothness can again be due to the elimination of cell lumina. Solid wood with a rough surface requires more sanding, which wastes the raw material by decreasing its thickness and increasing overall production costs. In addition, the surface roughness affects the contact area between the wood and any liquid glue or paint materials. Smoother surfaces facilitate more accessible application of pressure and gluing or coating processes. UHPD process, which results in

254

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Table 6.14 Surface roughness of the UHP-densified Paulownia boards after various UHP treatments (holding time at 30 s). Reprinted from Ref. [6], copyright 2018, with permission from American Society of Agricultural and Biological Engineers Treatment

Roughness parameters (μm) Ra

Rz

Rmax

Control

10.1 ± 1.17 a

70.1 ± 6.41 a

82.6 ± 6.89 a

20 MPa

6.38 ± 0.29 b

40.9 ± 2.66 b

57.8 ± 3.03 b

40 MPa

5.69 ± 0.19 c

37.0 ± 1.07 c

48.6 ± 1.96 c

60 MPa

4.36 ± 0.14 d

35.0 ± 0.81 c

44.4 ± 0.95 d

80 MPa

3.96 ± 0.22 de

31.7 ± 1.60 cd

41.6 ± 1.16 de

100 MPa

3.02 ± 0.53 e

26.2 ± 3.58 d

34.0 ± 5.76 e

All measured values are presented as mean ± standard deviations. Means in the same column followed by different letters are statistically different according to Duncan’s multiple range test at p < 0.05.

more uniform surfaces, would improve the commercial quality of the boards while minimizing the need for sanding and improving the application of coatings.

6.3 Ultra-High-Pressure Dyeing of Wood The prepared wood blocks were divided into five groups: one for hot dip dyeing and four for UHP dyeing at four pressure levels. For the hot dip dyeing treatment, 0.05% acid red aqueous solution was placed in a temperature-controlled kettle (DKS-24, Jiaxing Zhongxin Medical Instrument Co., Ltd, Jiaxing, China) and maintained at 85 °C. The selected wood blocks were completely soaked in the dye solution (volume ratio 1:10) for 9 h. The kettle was covered to minimize water evaporation. After dyeing, the blocks were removed from the kettle, and their surfaces were washed until the rinse water was no longer red. The selected wood blocks were immersed in 0.05% acid red aqueous solution for each UHP treatment in a heat-sealed polyethylene bag at room temperature. Then, they were packaged in a second polyethylene bag in case of possible leakage and then transferred into the UHP treatment chamber (UHPF-750, Kefa, Baotou, China). UHP treatment was applied at the specified pressure level: 40, 70, 100, or 130 MPa. Treatment included a pressure increase time (80–130 s) depending on the pressure level. Once the chamber reached the specified pressure, the pressure was immediately released without additional holding time. This process constituted a pressure pulse, i.e., pressurization to the desired level and immediate depressurization. Hence, there was no holding time, unlike most other UHP applications. The depressurization time was ~5 s.

6.3 Ultra-High-Pressure Dyeing of Wood

255

After dyeing, the wood blocks from the hot dip treatment (control group) and four UHP dyeing treatments were oven-dried at 65 °C to a constant weight. All experiments were triplicated, and the values were averaged.

6.3.1 Performance of Dyeing Treatments • Color measurement The color parameters were recorded at 10 locations on each wood block using a spectrometer (CM-600D) with a D65 illuminant according to the CIELAB color system. The CIELAB color system is a uniform, three-dimensional color space that characterizes color in terms of three parameters: L* is the lightness (0 = black and 100 = white), a* is the red/green coordinate (–60 = green and 60 = red), and b* is the yellow/blue coordinate (–60 = blue and 60 = yellow). The values of hue angle (H°) and total color difference (ΔE*) were calculated using the following equations: ΔE ∗ =

[(

L ∗1 − L ∗0

)2

◦

)2 ( )2 ] 21 ( + a1∗ − a0∗ + b1∗ − b0∗ ,

H = tan

−1

(

) b∗ , a∗

(6.27) (6.28)

where L 1 *, a1 *, and b1 * are the L*, a*, and b* values of the wood blocks after dyeing (by either UHP or hot-dip treatment), respectively, and L 0 *, a0 *, and b0 * are the L*, a*, and b* values of the wood blocks before dyeing, respectively. • Result Changes in the color parameters (ΔE*, ΔL*, and H°) after the UHP and hot dip treatments are shown in Fig. 6.33. Positive values indicate an increase in the chromaticity coordinates, and negative values indicate a decrease. There were several significant differences between the two treatments, which are discussed below using color-specific parameters. The ΔE* values increased gradually with increasing pressure up to 100 MPa, increasing from 56.9 at 40 MPa to ~5% at 70 MPa and by another 5% at 100 MPa (p < 0.05), and then remained between 60 and 63 in the 100–130-MPa pressure range (p > 0.05). However, the ΔE* value was 51.2 after a 9-h hot-dip treatment, which is much lower than the ΔE* values associated with the UHP treatments (p < 0.05). It has been reported that the ΔE* value of poplar wood veneer was 51.5 after impregnation dyeing at atmospheric pressure for 2 h, and the ΔE* value increased to 58.4 after ultrasonic-assisted dyeing for 1 h. It has been reported that the ΔE* value of bamboo was 68.8 after microwave-assisted dyeing for 9 min. The changes in the ΔL* values at different treatment pressures were similar to those for the ΔE* values. The ΔL* values of UHP-treated wood at 70 MPa were 7.9

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.33 Differences in a color (ΔE*), b brightness (ΔL*), and c hue angle (H°) of wood samples dyed with different treatments. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

6.3 Ultra-High-Pressure Dyeing of Wood

257

Fig. 6.33 (continued)

lower than those at 100 MPa. The ΔL* values of UHP-treated wood were significantly higher than those of hot dip-treated wood. The L* value indicates the brightness of the sample surface; therefore, it shows that the UHP-treated wood surfaces were brighter than the hot dip-treated wood surfaces. There were no differences in the H° values for wood treated at 40 and 70 MPa as well as at 100 and 130 MPa. However, the H° values were ~ 12% higher at higher pressures (100 and 130 MPa) than at lower pressures (40 and 70 MPa). Further, the H° values were distinctly higher (20–40% higher) for UHP-treated wood than that for hot dip-treated wood. Overall, the results indicate that wood dyeing by UHP treatment resulted in deeper and more desirable changes in color characteristics than the hot dip treatment. Among the different UHP-treated samples, dyeing at 130 or 100 MPa resulted in superior color compared to dyeing at 40 or 70 MPa.

6.3.2 Fractal Color of Wood • Measurement Images (4,000 × 6,000 pixels) of the wood surfaces were captured in the RGB color space with a digital camera (Nikon D5300 SLR camera) and transformed to the L*a*b* color space. Using the box-counting method, an algorithm written in MATLAB R2013b was used to calculate the fractal dimension (FD). Briefly, for an image size of M × M, the image was divided into a nonoverlapping square

258

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

lattice of size s × s, where s can have any value between 2 and M/2. The original two-dimensional image was regarded as a set G(x, y) in three-dimensional space, where (x, y) represents the pixel position of the original two-dimensional image. In contrast, the third dimension (G) represents the grayscale level. Therefore, in a twodimensional plane, the original image was divided into a square lattice of size s × s. In three-dimensional space, the proportion of the gray value correspondingly reduced the segmentation, with the three-dimensional space divided into a corresponding volume of s × s × h square boxes, where h is the height of a single box whose value satisfies G/h = M/s. If the (i, j) area of the image is contained in a grid of maximum and minimum gray levels inside l and k boxes, respectively, the number of boxes required to cover the (i, j) area is n r (i, j ) = l − k + 1,

(6.29)

and the total number of boxes required is Nr =

Σ

n r (i, j ).

(6.30)

For different values of r, corresponding values of N r are calculated. FD is the slope of log(N r ) relative to log(1/r) using the linear fitting method of minimum mean square error. FD =

log(Nr ) . log(1/r )

(6.31)

When calculating FD, selecting the size of the subimage window and the FD scale is necessary. An excessively small subimage window will lose important color characteristics. In contrast, an excessively large subimage window contributes to the edge pixels mixing with other pixels in the image area, which affects the selection of color features. Hence, a step size of 1 and box sizes of 2–11 were chosen. • Result Because the surface properties of dyed wood are affected by the dyeing process, dye uptake, or other techniques, the surface distribution of chromaticity is usually complex. To adequately describe surface color characteristics, the fractal color dimension was introduced. In this method, the distribution of color points on the surface of a solid matrix is expressed in a statistical sense, similar to the growth characteristics of natural organisms. The fractal color dimension measures the distribution homogeneity of color data. Generally, a lower fractal color value indicates a more uniform color distribution. In our study, the computed FD of wood treated at 40, 70, 100, and 130 MPa decreased as the treatment pressure increased and was markedly lower than the FD value of hot dip-treated wood, which had a mean value of 2.92 (Fig. 6.34). This shows that the color distribution of the UHP-treated

6.3 Ultra-High-Pressure Dyeing of Wood

259

Fig. 6.34 Fractal dimension of wood dyed with different treatments. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

wood was more uniform than that of the hot dip-treated wood. The decrease in FD with increased treatment pressure indicates more uniform dyeing at higher treatment pressures. Overall, the FD of UHP-treated wood was significantly better at 100 and 130 MPa than at 40 and 70 MPa and far better than that of hot dip-treated wood.

6.3.3 UV Accelerated Aging Test • UV accelerated aging test The UV accelerated aging test was conducted in a dark chamber using a 10-W, 254nm wavelength UV lamp. The L*, a*, and b* values of the aging wood blocks were recorded at 0, 1, 2, 5, 10, 20, 40, 60, 80, and 100 h. Total color differences between aged and nonaged wood blocks were calculated using the following equation: ΔE ∗ =

[( )2 ( )2 ( )2 ] 21 , ΔL ∗ + Δa ∗ + Δb∗

(6.32)

where ΔL*, Δa*, and Δb* are the differences in L*, a*, and b* before and after aging at different times, respectively. • Result The ΔE* of the wood surfaces increased with increasing aging time for all treatments (Fig. 6.35). The ΔE* increased rapidly during the first 40 h under UV conditions, and

260

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.35 Color change during aging on the surfaces of wood dyed with different treatments. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

the rate of increase then slowed. Similar results have been reported and concluded that the color change during aging was induced by the degradation of some unsaturated functional structures in the lignin and dye and the relative concentration of carbonyl groups, resulting in a rapid color change during the first 40 h. After 60 h, the ΔE* of hot dip-treated wood was remarkably higher (p < 0.05) than that of wood treated at 40, 70, and 100 MPa. This indicated that the resistance to aging of UHP-treated wood is superior to that of hot-dip-treated wood. However, the ΔE* of wood dyed at 130 MPa was statistically similar to that of hot-dip-treated wood. This unexpected outcome could result from experimental variability or it may be due to interactions of the wood and the Acid Red 73 dye, extensive physical adsorption, and color instability on the wood surface, leading to fading of the dyed wood. Specifically, dye molecules absorbed onto the wood cell walls are more likely to discolor rapidly during UHP treatment at 130 MPa. This result will be explored in more detail in future studies.

6.3.4 Dye Uptake • Measurement Dye uptake was defined as the absorption percentage of dye from the dye bath to the wood blocks. The maximum absorption wavelength (λmax ) was found at 510 nm

6.3 Ultra-High-Pressure Dyeing of Wood

261

Table 6.15 Dye uptake of poplar wood by different UHP treatments and hot dip treatment. Values followed by different letters are significantly different. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers Treatment

Dye uptake (%)

40 MPa

1.71 ± 0.70b

70 MPa

1.78 ± 0.47b

100 MPa

3.32 ± 0.56a

130 MPa

3.52 ± 0.46a

Hot dip

1.54 ± 0.48b

for Acid Red 73, and the absorption at λmax was measured using a UV–visible spectrophotometer. Dye uptake was calculated as follows: Dye uptake(%) =

A0 − At × 100, A0

(6.33)

where A0 and At are the absorbance of the dye solution at λmax before and after dyeing, respectively. • Result Table 6.15 illustrates the dye uptake by poplar wood following the UHP and hot dip treatments. As with the FD values, the dye uptake gradually increased with an increase in applied pressure. This is probably because the higher pressures produced a larger pressure difference and contributed to higher permeability of the wood structure; therefore, more dye could penetrate the internal wood. It was found that the dye uptake increased from 0.6 to 4.0% with ultrasonic treatment as the power increased from 0 to 300 W, which is similar to the trend observed in this study with UHP dyeing. There was no significant difference in dye uptake between 40 and 70 MPa and between 100 and 130 MPa; however, the difference between the 40–70-MPa and 100–130-MPa treatments was significant, with the UHP range contributing almost twice the amount of dye uptake. A minimum threshold pressure must be applied to the wood structure to achieve increased porosity and dye uptake. In contrast, dye uptake was 56.25% lower using the hot dip treatment than that with the 130 MPa treatment.

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6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

6.3.5 Color Distribution of Interior Wood • Measurement Cross sections in the RT plane were obtained by cutting wood blocks in the middle of their thickness. Images were taken using a digital camera (Nikon D5300 SLR camera) to observe the color distribution of the interior wood. • Result The color distributions in cross sections of UHP- and hot dip-treated wood samples are shown in Fig. 6.36. The UHP-treated wood color is uniformly red, while the hot dip-treated wood did not take up any dye, and the interior color is the same as that of untreated wood. There is also a large inadequately dyed area in the 40-MPa treated wood. For the 100- and 130-MPa treatments, the red-colored area dominates the wood and the color distribution is homogeneous, suggesting that the color became deeper, darker, and more uniform with increased treatment pressure. The dye solution could penetrate more deeply into the wood at higher pressure levels because of the greater pressure difference and increased permeability. This phenomenon is consistent with the dye uptake data in Table 6.15. However, in the hot-dip treatment, the dye remained predominantly on the surface and scarcely reached the wood interior. It has been reported that cross and radial sections of Populus tomentosa were rarely colored after dyeing at 90 °C for 9 h at atmospheric pressure, which is consistent with our results. Even after 9 h, the hot-dip treatment could hardly penetrate the wood, while the UHP treatment achieved deep and uniform dyeing results within a short treatment time (The pressure boost time was 80 to 130 s, the pressure holding time was 0 s, and the pressure relief time was not more than 5 s). This result shows that UHP treatment can achieve better dyeing with improved efficiency.

Fig. 6.36 Color distributions in cross sections of wood dyed with different treatments. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

6.3 Ultra-High-Pressure Dyeing of Wood Table 6.16 Sulfur, nitrogen, and sodium contents in different woods. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

Wood sample

263

Elemental composition Sulfur

Nitrogen

Sodium

Natural

0

0

0

100-MPa-treated

0.13

0.94

0.03

Hot-dip-treated

0

0

0

6.3.6 EDX and SEM Analysis • Measurement Wood samples were cut into 1-mm slices at their smoothed transverse surfaces using a sliding microtome. The slices were attached to a specimen holder using double-sided conductive carbon tape and then coated with a thin film of gold (10 nm) in a vacuum evaporator. A SEM (SU8010, Hitachi, Japan) equipped with an energy-dispersive xray detector (EDX; X-MaxN 80, Oxford Instruments, UK) at an accelerating voltage of 3 kV was used to characterize the micromorphologies of dyed wood samples and the elemental distribution of dye within the wood cell walls. • Energy-dispersive X-ray analysis Table 6.16 lists the elemental composition of natural wood, 100-MPa-treated wood, and hot-dip-treated wood. Sulfur, nitrogen, and sodium are characteristic of Acid Red 73, and only the UHP-treated wood contained these characteristic elements. It can be concluded that dye molecules filled the internal structure of the UHP-treated wood but not the hot-dip-treated wood. This shows that the dye solution penetrated the wood with the UHP treatment, while no dye penetrated the wood with the hot dip treatment. • Scanning electron microscopy analysis Figure 6.37 shows the morphologies of natural wood, UHP-treated wood (at 70 and 100 MPa), and hot-dip-treated wood. Compared with the natural wood, the internal spaces of wood treated at 70 and 100 MPa were notably compressed and destroyed, while no major changes were observed in hot-dip-treated wood. From a macroscopic perspective, a 6% reduction of the wood thickness was estimated after 100-MPa treatment, while there were no measurable dimensional differences in the hot-dip-treated wood. Placing the wood in the dye solution, treating it at UHP for a certain time, and then releasing the pressure immediately resulted in partially destroying the wood structure. The destroyed wood structure creates passages for intercellular liquid, allowing the dye solution to penetrate more easily into the wood. It was reported that UHP treatment could densify the Paulownia wood blocks, indicating that UHP treatment produced compression destruction in the wood cell walls. Those findings are consistent with the results of this study. SEM shows that the wood treated at 100 MPa suffered more damage and compression than the wood treated

264

6 Ultra-High-Pressure Modification of Wood: Densification and Dyeing

Fig. 6.37 SEM images of a untreated wood, b 70 MPa-treated wood, b 100 MPa-treated wood, and d hot-dip-treated wood. Reprinted from Ref. [8], copyright 2019, with permission from American Society of Agricultural and Biological Engineers

at 70 MPa. Higher pressures caused more damage; however, increasing damage was no longer apparent when the pressure increase was insufficient or when the pressure had reached a certain threshold value. Therefore, a distinct difference in wood dyeing was observed between the results at 40–70 MPa and those at 100–130 MPa. It was reported that steam explosion treatment could improve the permeability of wood. UHP treatment is consistent with steam explosion treatment, and their mechanisms for increasing permeability may be similar. UHP treatment can damage the wood structure and increase the structural porosity. This may have contributed to the increased concentration of dye molecules and improved the evenness and color saturation of the UHP-treated wood.

6.4 Summary UHP treatment has great potential for plantation wood densification applications. The density and mechanical properties of wood samples could be significantly increased by two or three times within a short treatment time using UHPD. The optimum

References

265

compression pressure was 100 MPa. Higher pressure treatment decreased the density and mechanical properties of wood. Pressure treatment significantly increased the permeability of wood, making it more susceptible to dimensional and water exchange activities when stored in various RH environments. These results indicate significantly low recovery rates at low or moderate RH levels but extreme and dimensionally unstable conditions at high RH values. Compared with hot-dip dyeing, UHP dyeing resulted in a more profound and uniform distribution of color in the treated wood. The fractal color dimension can be used for exterior surfaces to indicate color uniformity. The surface color distribution of UHP-treated wood was more homogeneous, as demonstrated by increased chromaticity (Δ L* and Δ E*) and hue angle with the red color. For internal dye penetration, UHP treatment resulted in greater dye uptake and the color in the internal cross sections was more intense and more uniform. No dye penetration into the internal structure was observed using the hot-dip treatment. This indicates that UHP dyeing can produce a more uniform and deeper coloring of the wood structure. Among the UHP treatments, higher pressures (100 and 130 MPa) had better dyeing performance than lower pressures (40 and 70 MPa). Considering the dyeing performance, overall cost, time, and energy efficiency, 100 MPa could be the best choice for developing industrial applications. The microstructure of UHP-treated wood showed compressive deformation, and the damaged structure may have contributed to the increased permeability.

References 1. Yu Y, Li A, Yan K, Ramaswamy HS, Zhu S, Li H (2020) High-pressure densification and hydrophobic coating for enhancing the mechanical properties and dimensional stability of soft poplar wood boards. J Wood Sci 66(1):45 2. Li H, Zhang F, Ramaswamy HS, Zhu S, Yu Y (2016) High-pressure treatment of Chinese fir wood: effect on density, mechanical properties, humidity-related moisture migration, and dimensional stability. BioResources 11(4):10497–10510 3. Li A, Yan K, Ramaswamy HS, Zhu S, Yu Y (2020) Plastic and elastic strains in poplar wood under high-pressure densification. Trans ASABE 63(6):2021–2028 4. Yu Y, Zhang F, Zhu S, Li H (2017) Effects of high-pressure treatment on poplar wood: density profile, mechanical properties, strength potential index, and microstructure. BioResources 12(3):6283–6297 5. Yan K, Zhang F, Du Y, Ramaswamy HS, Zhu S, Hu L, Yu Y (2020) Delayed elastic strain and set-recovery evaluation in high-pressure densified hybrid poplar wood—new assessment considerations. BioResources 15(2):2691–2707 6. Li H, Jiang X, Ramaswamy HS, Zhu S, Yu Y (2018) High-pressure treatment effects on density profile, surface roughness, hardness, and abrasion resistance of Paulownia wood boards. Trans ASABE 61(3):1181–1188 7. Yu Y, Jiang X, Ramaswamy HS, Zhu S, Li H (2018) Effect of high-pressure densification on moisture sorption properties of Paulownia wood. BioResources 13(2):2473–2486 8. Yu Y, Yan K, Ramaswamy HS, Zhu S, Li H, Hu L, Jiang X (2019) Dyeing of poplar wood through high-pressure processing: performance evaluation. Trans ASABE 62(5):1163–1171

Chapter 7

Conclusion and Prospects

Abstract This book focuses on the nontraditional applications of high pressure technology on Chinese liquor, brown rice and wood. The methods and theories in this book are designed to inspire and help the reader to better understand high pressure technology and to be able to think outside the box to expand more nontraditional applications of high pressure technology. Keywords High pressure treatment · Chinese liquor · Brown rice · Wood · Nontraditional applications

This book focuses on the nontraditional applications of UHP technology on Chinese liquor, brown rice and wood. Chinese liquor is a distilled spirit unique to China, with annual sales of Chinese liquor in China at around 560 billion, yet the traditional aging process needs to be improved and enhanced. Aging enhances the combination of alcohol molecules and water molecules, accelerates esterification, condensation, redox and other reactions, accelerates the escape of hydrogen sulfide, acetaldehyde and other components that may be present from the liquor, eliminates the spicy taste of the liquor, and allows the liquor to reach its optimal state both in terms of taste and aroma. Traditional aging techniques include high temperature, light, microwave, and electric field, but these traditional aging processes take months or even years and take up a lot of human and material resources. Chapter 4 in this book systematically evaluates the effect of UHP in accelerating the aging of Chinese liquor qualitatively and quantitatively by exploring the patterns of changes in volatile components during natural aging, after UHP treatment and during storage after UHP treatment. Overall, UHP can change the quality of Chinese liquor by affecting the volatile components of Chinese liquor. The results of this book achieved a small increase in the ageing of Chinese liquor and improved the researcher’s understanding of the applications related to the modification of Chinese liquor quality by UHP treatment, providing more theoretical basis and technical support for the application of accelerated aging of Chinese liquor by UHP. Combining the findings and shortcomings of this study,

© Zhejiang University Press 2023 Y. Yu, Nontraditional Applications of Ultra-High-Pressure Technology in Agricultural Products Processing, Advanced Topics in Science and Technology in China 69, https://doi.org/10.1007/978-981-99-3776-9_7

267

268

7 Conclusion and Prospects

the authors offer the following perspectives on research related to exploring new technologies to accelerate aging of Chinese liquor: (1) Exploring the mechanism of aging of Chinese liquor. The mechanism of natural aging of Chinese liquor is still unclear. Before exploring the technology of accelerated aging of Chinese liquor, it is important to find out as much as possible the main changes that occur during the natural aging of Chinese liquor, and to establish a link between these changes and the quality of Chinese liquor, so as to explore the processing methods that can cause these changes, which will make the process of exploring new technology more targeted; (2) Exploring methods for the detection of volatile components in Chinese liquor. Due to the limitation of the detection method, only the main volatile components of Chinese liquor were investigated in this study, but in fact, some small trace components may also have a very important contribution to the quality and taste of Chinese liquor. Further exploration of the detection method of volatile components of Chinese liquor can characterize the changes of trace components, so as to collect more comprehensive information on the changes of components of Chinese liquor during natural aging, and also to characterize the influence of processing technology on the components more comprehensively; (3) More research on different categories of Chinese liquor. This study investigated the changes of the main volatile components of Chinese liquor during natural aging and after UHP treatment, but the composition of volatile components of Chinese liquor varies greatly among different aromatic types and brands, and the patterns of component changes are also very different, therefore, using different categories of Chinese liquor as research objects can enrich the research basis in the field of accelerated aging of Chinese liquor. Compared with traditional white rice, brown rice is more comprehensive and richer in nutrients, and is gradually becoming more popular among consumers. At present, brown rice products are still rarely seen on the market, and brown rice with comprehensive and balanced nutritional value has not been able to become a staple food for people. The main reason is that brown rice has problems such as not easy to cook, poor taste, easy to produce undesirable flavor, not easy to store, and short shelf life. Since 2010, our team has focused on the research of novel healthy and palatable brown rice, aiming to improve the taste of brown rice to the greatest extent through pure physical processing technology such as UHP, while achieving the full retention of the nutritional value of brown rice. In 2013, we made a major breakthrough in research and developed the first set of “Germinating -UHP healthy and palatable brown rice production technology”. By exploring the mechanism of nutritional quality retention and taste improvement of brown rice whole grain produced by this technology, two new technologies such as “novel Germinating—parboiled healthy and palatable brown rice production technology” and “novel freeze–thaw cycle healthy and palatable brown rice production technology” have been developed in 2016. Chapter 5 investigates and compares the advantages and disadvantages of three brown rice processing techniques: UHP, GB, and FTC, in terms of improving texture, digestibility and storage stability. The modification effects of the

7 Conclusion and Prospects

269

three techniques on different physicochemical properties of brown rice and the influence patterns are systematically explored. Combining the conclusions and findings already obtained, it is considered that subsequent studies can be carried out in the following aspects; (1) Non-thermal processing is a hot topic in food processing. The FTC and UHP processing in this book are both non-thermal processing techniques and have their own advantages in improving the texture, digestibility, and storage stability of brown rice, and subsequent attempts can be made to combine the two to further optimize brown rice processing technology; (2) The mechanism of modification by UHP technology should continue to be explored in depth through more molecular perspectives, such as the chain length and branching of straight-chain and branched-chain starches, the binding forms of the two, and the mechanism of their interaction with other components; (3) The subsequent digestibility experiments can further analyze the release of various functional substances (e.g., vitamin and glutamate) after different processing treatments to improve the analysis of the nutritional value of different brown rice products; in addition, the actual utilization of nutrients after the brown rice has passed through the human digestive system can be investigated by establishing a human in vitro digestion model. Wood is a consumable product that generally takes 30, 40 years or more to mature and cannot be cultivated quickly in a short period of time. As quality hardwood resources continue to be depleted, the amount available is becoming less and less, far from being able to meet the increasing demand of the market. The huge contradiction between supply and demand is driving the search and development of “high quality” alternatives to wood. Species such as poplar, pine and eucalyptus have a short growth cycle and can quickly become timber within a decade or so, causing widespread interest. However, fast-growing forests are too fast-growing, resulting in soft texture, low density, poor dimensional stability and mechanical properties, and cannot directly replace natural forests for applications in furniture, flooring, decoration and other industries that urgently need high-quality wood. How to achieve quality improvement and optimization of softwood plantation forests and give full play to their resource advantages to solve the contradiction between the rigid growth of demand for quality wood and its supply constraints has been a hot research topic for forestry researchers. At this stage, modification techniques such as heat treatment, cross-grain compression and impregnation can improve fast-growing forest properties in a targeted manner to a certain extent. However, all these modification treatments have the defects of cumbersome process and unsatisfactory product performance, which limit their industrial application. Therefore, there is an urgent need to develop a new wood modification technology to improve the usable value and expand the use of fast-growing forests and to alleviate or solve the problem of shortage of high-quality wood. With UHP technology as the core, it can realize the rapid strengthening of plantation wood to the material of high quality wood and functional modification such as rapid dyeing. Chapter 6 investigates the effects of UHP treatment on the mechanical properties, moisture absorption properties, and

270

7 Conclusion and Prospects

dimensional stability of wood. The results show that UHP plays a good role in densification, resulting in a substantial increase in the mechanical properties of wood, while its equilibrium moisture content decreases under a range of humidity conditions. Chapter 6 also improves the common cutting methods for wood based on the deformation laws of densified wood and increases the effective utilization of densified wood. In addition, the combination of high-temperature heat treatment and ultra-high-pressure densification significantly improved the dimensional stability of densified wood. UHP enhances the liquid permeability of the wood and enables fast dyeing of the wood. Due to time and other constraints, there are some shortcomings or areas that could be further investigated in this book, and the following are some prospects based on the findings of this book: (1) The differences in compression deformation of earlywood and latewood should be explored in depth to make the investigation of anisotropy of deformation of ultra-high-pressure densified wood more comprehensive; (2) Attempts can be made to combine the softening pre-treatment of wood and ultrahigh-pressure densification treatment to reduce the elastic rebound of wood in ultra-high-pressure densification treatment, improve the densification efficiency, and further improve the mechanical properties such as density of ultra-highpressure densified wood; (3) The temperature gradient of high-temperature heat treatment should be studied to optimize the parameters of high-temperature heat treatment conditions and to find a balance between improving the dimensional stability of densified wood and reducing its mass loss. The methods and theories in this book are designed to inspire and help the reader to better understand UHP technology and to be able to think outside the box to expand more nontraditional applications of UHP technology. For example, at present, the research on UHP technology has gradually penetrated into the field of ultra-high pressure and sub-zero temperature. Water has a variety of ice crystal forms under ultra-high pressure. By regulating the water form and ice crystal phase transformation in food through the environmental parameters of pressure and temperature, micro ice crystal refrigeration and preservation of agricultural products can be realized. Therefore, more nontraditional applications of UHP technology in agricultural processing need to be expanded and developed.