Distilled Spirits 0128224436, 9780128224434

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DISTILLED SPIRITS

DISTILLED SPIRITS Edited by

Annie Hill Professor, International Centre for Brewing and Distilling, Heriot-Watt University, United Kingdom

Frances Jack Senior Scientist, Scotch Whisky Research Institute (SWRI), Edinburgh, Scotland, United Kingdom

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-822443-4 For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Nikki P. Levy Acquisitions Editor: Nancy J. Maragioglio Editorial Project Manager: Czarina Mae S Osuyos Production Project Manager: Rashmi Manoharan Cover Designer: Matthew Limbert Typeset by Aptara, New Delhi, India

Contents Contributors ix Preface xi Acknowledgments

2.2.6 Charcoal & filtration 51 2.2.7 Other purification methods 2.3 Quality 53 2.4 Future 55 References 56

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1. Whisk(e)y Alan G. Wolstenholme

52

3. Rum and cachaça

1.1 Introduction 1 1.1.1 History 1 1.2 Raw materials 3 1.2.1 Cereal 3 1.2.2 Water 7 1.2.3 Yeast 8 1.2.4 Commercial enzymes (non-Scotch) 1.3 Whisk(e)y production 8 1.3.1 Malting 8 1.3.2 Mashing 10 1.3.3 Fermentation 12 1.3.4 Distillation 16 1.3.5 Maturation 21 1.4 Blending 29 1.4.1 Single Malts 30 1.5 Prebottling processes 31 1.6 Quality 31 1.6.1 Flavor wheel 31 1.7 Future 32 References 35

Aline Marques Bortoletto

3.1 Introduction 61 3.2 Raw materials 62 3.2.1 Sugarcane 62 3.3 Yeast and other micro-organisms 62 3.4 Production process 63 3.4.1 Must preparation from sugarcane juice or molasses 63 3.4.2 Fermentation process 64 3.4.3 Distillation 66 3.4.4 Aging and maturation process 66 3.5 Filtration, standardization, and bottling 68 3.6 Quality 68 3.6.1 Flavor profile 68 3.6.2 Sensorial properties 71 3.7 Future 71 References 72

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4. Gin Matthew S.V. Pauley and Jan Hodel

2. Vodka 4.1 Introduction 75 4.1.1 History 75 4.1.2 Genever 75 4.1.3 Market 76 4.1.4 Regulations 76 4.1.5 Other styles of gin 76 4.2 Raw materials 78 4.2.1 Ethyl alcohol 78 4.2.2 Water 79 4.2.3 Botanicals and their flavors 4.3 Gin production 89

Jan Hodel

2.1 Introduction 37 2.1.1 Markets and volume 37 2.1.2 Definition and regulations 2.2 Production 40 2.2.1 Overview 40 2.2.2 Raw materials 41 2.2.3 Brewhouse operation 42 2.2.4 Fermentation 42 2.2.5 Distillation 46

37

v

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Contents

4.3.1 Steep infusion method 90 4.3.2 Multishot 90 4.3.3 Vapor infusion 91 4.3.4 Vacuum distilling 93 4.3.5 Distillation summary 97 4.3.6 Compound gin 97 4.4 Quality 97 4.4.1 Flavor defects 97 4.5 Future 98 References 98

5. Baijiu Jun Zhang, Shuang Chen, Yan Ping L. Qian and Michael C. Qian

5.1 Introduction 103 5.1.1 The origin of Baijiu 103 5.1.2 The value of Baijiu 103 5.2 Production of Baijiu 105 5.2.1 Baijiu and brewing techniques 105 5.2.2 The unique application of Jiuqu 105 5.2.3 Solid-state fermentation progress 107 5.3 Quality: Flavor chemistry of Baijiu 110 5.3.1 Analysis of chemical composition 110 5.3.2 Characteristics of flavor compounds 114 5.4 Future 125 References 125

6. Soju Taewan Kim

6.1 6.2 6.3 6.4 6.5

Introduction 131 Distilled spirit industry in Korea 132 Raw materials 132 Soju production process 134 Pot distilled Soju 134 6.5.1 A treasure trove of microbial diversity, Nuruk (Guk) 135 6.5.2 Traditional pot distillation still, Sojut-gori 136 6.6 Diluted (blended) Soju 139 6.6.1 Large-scale rapid fermentation of new-make spirit for diluted Soju 139 6.6.2 Continuous distillation of diluted Soju 139 6.6.3 Activated carbon filtration for purifying of new-make spirit in diluted Soju production 140 6.7 Quality 141 6.7.1 Soju flavor compounds 141

6.8 Future 142 6.8.1 Development of characteristic strains and formulation technologies on demand 142 6.8.2 Energy efficiency of distillation stills 142 6.8.3 Development of maturation materials using soil and forest resources 142 6.8.4 Development of matured pot distilled Soju, tradition to modern 142 References 143 Website Links 143

7. Shochu Ichiro Moda, Toshikazu Sugimoto and Akira Wanikawa

7.1 Introduction 145 7.2 What is shochu? 147 7.2.1 General overview 147 7.2.2 Definition according to Japanese Liquor Tax Regulations 147 7.3 What is koji? 148 7.3.1 Shochu koji 148 7.3.2 Enzymes in shochu koji 149 7.3.3 Flavors originating from koji 150 7.4 History and geography of shochu production 7.4.1 History 150 7.4.2 Shochu-making regions 152 7.5 Shochu market 153 7.6 The shochu production process 154 7.6.1 Raw ingredient processing 154 7.6.2 Koji-making 156 7.6.3 Multiple parallel fermentation 157 7.6.4 Distillation 160 7.6.5 Purification 161 7.7 Quality: Flavors of shochu 162 7.7.1 Sensory analysis 162 7.7.2 Volatile components 165 7.7.3 Musty off-flavors 167 7.8 Future 168 References 168

150

8. Mezcal and Tequila Gary Spedding

8.1 Introduction: A culturally historical family of agave-based distilled beverages 173 8.2 Of Mexico, maguey-agave, and mezcal 174

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Contents

8.3 Agave storage carbohydrates: Fructans, agavins, and a note on methanol 182 8.3.1 Fructans 182 8.3.2 Methanol 182 8.4 The production of mezcal 183 8.4.1 Mezcal, artisanal, and ancestral mezcal 191 8.4.2 Fermentation 195 8.4.3 Distillation processes in mezcal production 195 8.4.4 Summary 196 8.4.5 The worm in the bottle 196 8.5 Tequila – Introduction and Production 197 8.6 The production of tequila 198 8.6.1 Cooking and the hydrolysis of carbohydrates 198 8.6.2 Diffuser 199 8.6.3 Fermentation 200 8.6.4 Distillation 200 8.6.5 Maturation of tequila 201 8.6.6 The valorization of waste residues from the mezcal process 202 8.7 A mess of mezcals—A few other better-known types: Bacanora, raicilla, and introducing sisal 202 8.7.1 Bacanora 202 8.7.2 Raicilla 203 8.7.3 Sisal 204 8.8 Microbiology and mezcal/tequila 204 8.9 The maturation of tequila and other mezcals 206 8.10 Quality 209 8.10.1 Mezcals, including tequilas–volatiles and their flavors and markers of authenticity 209 8.10.2 Authenticity of mezcals 211 8.10.3 Sensory evaluations and expectation of consumers 212 8.11 Summary 213 8.12 Future 213 References 220

9. Brandies, grape spirits, and fruit distillates Hugh R. Holds

9.1 Introduction 229 9.1.1 A brief history of brandy

229

9.2 Un-matured grape spirits: Eau de vie, grappa, pisco, and neutral spirits 231 9.2.1 Eau de vie and pisco 231 9.2.2 Grappa, Marc, and Orujo 231 9.2.3 Neutral spirits 232 9.2.4 Fortifying spirit 232 9.3 Matured grape spirits: Cognac, Armagnac, and world brandies 234 9.3.1 Cognac 234 9.3.2 Armagnac 235 9.3.3 Spain 236 9.3.4 Australia 237 9.3.5 South Africa 237 9.3.6 United States of America 239 9.3.7 Cut “brandies” 239 9.4 Fruit brandies 240 9.4.1 Calvados and apple/pear brandies 240 9.4.2 Stone fruit brandies and schnapps 241 9.4.3 Cashew Feni 241 9.5 Quality: analytical and technical considerations for fruit distillates 242 9.5.1 Aroma and flavor development 242 9.5.2 Processing and food safety considerations 243 9.5.3 Analysis of brandy products 244 9.5.4 Viticultural parameters for grape distillates 245 9.6 Future 246 References 247

10. Quality control: Methods of analysis Shona Harrison

10.1 Introduction 251 10.2 Compliance 252 10.2.1 Alcohol strength 252 10.2.2 pH, acidity, extract, and color 255 10.2.3 Metals 256 10.2.4 Anions and cations 257 10.3 Safety 257 10.3.1 Methanol 257 10.3.2 Ethyl carbamate & hydrocyanic acid 259 10.3.3 Phthalates 260 10.3.4 Particulate matter 261 10.4 Flavor 261 10.4.1 Major volatile congeners 261

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Contents

10.4.2 Maturation-related congeners 262 10.4.3 Sugars 265 10.4.4 Trace congeners by GC-MS 266 10.4.5 Phenols 268 10.4.6 Taints and off-notes 268 10.5 Authenticity 269 10.5.1 Application of flavor and safety methods 270 10.5.2 Denatured alcohol 271 10.5.3 Carbon dating 271 10.5.4 Stable isotope ratio techniques 272 10.6 Summary 272 References 273

11. Quality control: Sensory evaluation Leah Gruenig

11.1 Background 277 11.2 Sensory perception and modalities 279 11.2.1 Taste—Basics tastes 279 11.2.2 Smell—Aromatics 279 11.2.3 Chemesthesis—Mouthfeel 279 11.3 Best practices 280 11.3.1 Regulations 280 11.3.2 Evaluation environment 281 11.3.3 Gold standard 281 11.4 Sample presentation 281 11.4.1 Sample temperature 282 11.4.2 Serving containers 282 11.4.3 Dilution 282 11.4.4 Nosing vs. tasting 284 11.4.5 A worked example using the formula 284 11.4.6 How to use the recommended serving volume table 285 11.4.7 A worked example using the recommended serving volume table 286 11.4.8 Order of samples 286 11.4.9 Blinding codes 287 11.4.10 Incentives 287 11.5 Understanding acceptable liquid variation 287 11.5.1 Corrective actions 287

11.6 Sensory assessors, screening, training, and calibration 288 11.6.1 Screening 288 11.6.2 Training 289 11.6.3 Calibration 291 11.7 Methods 291 11.7.1 Consensus vs. statistical analysis 291 11.7.2 In-out test 291 11.7.3 Discrimination 291 11.7.4 Scaling and descriptive analysis 292 11.8 Evaluation points in the process 294 11.8.1 Raw materials 295 11.8.2 Fermentation 295 11.8.3 Distillation 295 11.8.4 Maturation 295 11.8.5 Bottling or final packaging 296 11.9 Looking to the future 296 11.10 Additional resources 296 References 296

12. Sustainable distilling: CO2 emissions, energy decarbonization, and by-products Jane S. White

12.1 12.2 12.3 12.4 12.5

Introduction 299 Sustainability: goals and ambitions 300 Life cycle assessment of distilled spirits 305 Greenhouse gas emissions 310 Energy decarbonization and net zero emissions 314 12.6 Sustainability and by-products 321 12.7 Sustainability, product quality, and conclusions 325 References 327

Distilling glossary 333 Index 337

Contributors Aline Marques Bortoletto INOVBEV Research and Development of Drinks, Piracicaba, São Paulo, Brazil

Michael C. Qian Department of Food Science & Technology, Oregon State University, Corvallis, OR, United States

Shuang Chen State Key Laboratory of Food Science & Technology, Key Laboratory of Industrial Biotechnology of Ministry of Education & School of Biotechnology, Jiangnan University, Wuxi, Jiangsu, China

Yan Ping L. Qian Department of Crop and Soil Science, Oregon State University, Corvallis, OR, United States Gary Spedding Brewing and Distilling Analytical Services, Kentucky, United States

Leah Gruenig Gruenig Consulting, Louisville, Kentucky, United States Shona Harrison The Scotch Whisky Research Institute, Edinburgh, Scotland

Toshikazu Sugimoto Technology Development Center, The Nikka Whisky Distilling, Co. Ltd. Kashiwa, Chiba, Japan

Jan Hodel Product Owner – Product Development, Pilz Global Distillery Business Unit, Cork, Ireland

Akira Wanikawa Technology Development Center, The Nikka Whisky Distilling, Co. Ltd. Kashiwa, Chiba, Japan

Hugh R. Holds Department of Wine Science, School of Agriculture, Food and Wine, University of Adelaide, SA, Australia

Jane S. White International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh, Scotland

Tae Wan Kim Korea Food Research Institute, Iseo-myeon, Wanju-gun, Jeollabuk-do, Republic of Korea

Alan G. Wolstenholme Honorary Professor, International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh, Scotland

Ichiro Moda Developmental Laboratories for Alcoholic Beverages, Asahi Breweries, Ltd. Moriya, Ibaraki, Japan

Jun Zhang State Key Laboratory of Food Science & Technology, Key Laboratory of Industrial Biotechnology of Ministry of Education & School of Biotechnology, Jiangnan University, Wuxi, Jiangsu, China

Matthew S.V. Pauley International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh, Scotland

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Preface Distilled spirits are fascinating! We may be biased, but it is difficult for anyone not to be impressed by the diversity, intensity, and complexity of spirits that may be achieved from the same starting point—fermentation of natural plant materials—and from extremely low concentrations of flavor-active compounds. This text aims to explore the range of raw materials and production methods used in potable spirit production around the world. There are several spirit categories that historically have not had accompanying research institutes or research that has been made publicly available including many Asian spirits such as Baijiu, and South American spirits including Mezcal. Expanding our knowledge of the flavor and aroma compounds that may be produced through fermentation and distillation will hopefully serve as inspiration to experiment further and create novel spirit drinks. A second impetus for this text is to promote quality. It is a term that has a range of definitions, but in this context includes the promotion of best practices and ensuring product consistency, while identifying flavor defects and developing methods to mitigate against their production. The importance of including appropriate methods to determine product quality regardless of spirit type is highlighted specifically within chapters on sensory evaluation and methods of analysis. Sustainable production is essential within the distilling industry and methods to maintain quality and expand production whilst lowering carbon footprint are also covered within this text. It is hoped that all distillers, from those with just a budding interest to those employed in large operations, will be encouraged to explore further and to continue a journey of discovery of the world of distilled spirits. Annie Hill and Frances Jack

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Acknowledgments With the arrival of a global pandemic during the commissioning of this text and the consequent disruption to our working and family lives, the completion of each chapter has been a significant task. There have been job moves, home relocations, new-born babies, and many other events to cope with amongst the authors, and we would like to say a very heartfelt thank you to everyone involved.

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C H A P T E R

1 Whisk(e)y Alan G. Wolstenholme Honorary Professor, International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh, Scotland

1.1 Introduction Whisk(e)y is potable spirit produced from cereal grain, the saccharified starch of which is fermented by yeast into a wash (or beer) prior to being distilled and the ethanol and volatile compounds (congeners) collected as a spirit. Normally, this spirit then undergoes a period of maturation in wooden casks at least for sufficient time that the spirit is positively modified in terms of flavor and color. Opinions on what defines quality in whisk(e)y vary, but it should meet the generally accepted parameters organoleptically and visually expected by the customer. It should be described honestly and transparently as to its composition and provenance while meeting the relevant legislation or rules of the product as laid down by the controlling authority. Taste and aroma are highly subjective, varying among individuals, cultures, and even generations. Ultimately, the popularity of whisk(e)y depends on these characteristics, and other intangible aspects associated with the product. Unless otherwise indicated, Scotch whisky is being referenced throughout this chapter. Scotch is, if not the biggest volume of potable spirit worldwide (that honor going to Baijiu), the most valuable financially with exports exceeding £3.8 billion in 2020 (Scotch Whisky Association, 2021).

1.1.1 History The word whisky or whiskey (the different spellings indicating nothing other than comparatively recently adopted convention) is an anglicization and contraction of the Irish/Scots Gaelic words Uisge baugh meaning “water of life”. The equivalent Latin phrase of “aqua vitae” was also widely employed reflecting the religious roots of the science and old industry documents often refer to sales of “aqua” within the established trade. Among the general population, “uiskie” or “whiskey” was the usual terminology and that was the description for cereal-derived spirits taken with emigrants from both countries when

Distilled Spirits. DOI: https://doi.org/10.1016/B978-0-12-822443-4.00008-6

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c 2023 Elsevier Inc. All rights reserved. Copyright 

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1. Whisk(e)y

they colonized first North America and subsequently elsewhere. The rise in popularity of whisky among late 19th-century English markets fueled an international sales boom and global recognition. It is at the start of the 19th century though that we find an early and elegant elegy to the illicit predecessor of the product. The arrival in Edinburgh of King George IV in 1822 and his insistence (encouraged by Sir Walter Scott) that he and his retinue drink only “pure Glenlivet” led to a severe shortage. Elizabeth Grant of Rothiemurchus was told by her father to send all her personal stock which, as instructed, she did. She bemoaned her loss of her “pet bin, where there was whisky long in wood, long in uncorked bottle, mild as milk, and the true contraband goût in it”. This is an early reference to the importance of time spent in wood (maturation) being key to improving the quality of whisky, a topic that will be covered later in the chapter (Grant, 2006). The quality of Scotch whisky is to a perhaps unexpected degree influenced by the legislation on the product, although this situation is not unique to the United Kingdom. The ongoing interest of the legislators has been driven far more by attempts to maximize revenue from the product and to this end tax on malt has been imposed and removed, still dimensions imposed and removed, and an enormous effort expended on recording alcoholic strength, output and monitoring stocks while controlling sales outlets. The unintended consequence of this has been to encourage, discourage, or drive underground various sections of the industry at various times (Daiches, 1969; Moss & Hume, 1981). In the UK (including at that time all of Ireland), legislation following the Royal Commission of 1909 settled the issues of the ingredients (malt and grain) and the equipment (both batch and continuous stills) which could be used. The minimum maturation period (initially 2 years, increased to 3, in 1915) was motivated by wartime Temperance opportunism rather than a desire to improve the quality of the product though, ironically, that was the lasting outcome. That legislation has been revisited and updated throughout the past century, most recently in the 2009 Scotch Whisky Act, to reflect changes of opinion, practices, and priorities among the various stakeholders of revenue gatherers, producers, industry bodies, and special interest groups, albeit not generally consumers. Scotch whisky benefits from qualifying as a product of Geographic Indication status conforming to the EU Quality Regulation approach and having, as well as legislation (HMSO; Scotch Whisky Regulations, 2009), a supporting Technical Document (HMSO, Scotch Whisky Technical File, amended 2019) and a verifying authority in the form of His Majesty’s Revenue and Customs (Cormack, 2014). This lays down the regulations as to what can or cannot be legally called Scotch whisky. As a result, the legislative elements of the definition are now extremely detailed, covering, for example, geographical areas (Highland, Speyside, Lowland, Islay, and Campbeltown), distillery names, and even lettering sizes of script on labels. His Majesty’s Revenue and Customs now has a new role as verifiers operating the Geographic Indicator scheme (Cormack, 2014). Specific requirements for Scotch include that only endogenous enzymes may be used for saccharification, and only oak barrels for maturation, as well as the requirement that the product must be both made and matured in Scotland. This means that Scotch whisky is the most completely defined, legislated for, and scrutinized spirit anywhere in the world, giving customers a high level of confidence when purchasing it. Legislation, particularly in the UK and North America, has helped shape the individual whisk(e)y variants and will continue to shape them, albeit some outcomes of legislation may be unintended. For example, with Bourbon whiskey, the 1935 legislation that only new barrels

1.2 Raw materials

3 FIGURE 1.1 Scotch malt whisky production (Produced by Hannah Proctor using BioRender, The International Centre for Brewing and Distilling, Heriot-Watt University, 2021).

can be used for maturation has had a major impact on the subsequent quality of the product, ensuring a pronounced wood derived character (Gollihue, 2021). This chapter discusses whisky in the context of quality, what that is perceived to be, what it must be and how that is achieved. Broadly following the sequence of production steps (Fig. 1.1), the sources of flavor constituents, their modification and removal, at the various stages of production will be examined.

1.2 Raw materials 1.2.1 Cereal Cereals are of the grass order Gramineae and include maize (corn), wheat, barley, rye, oats, millet, rice, triticale, sorghum, and others. Cereal’s role in whisky production is to provide as much starch as possible having been as easily and economically grown as possible. For Scotch malt whisky only malted barley (Hordeum vulgare) is used while wheat (Triticum aestivum) is predominant in Scotch grain whisky. In North America, corn (Zea mays) is by far and away the most used cereal though rye (Secale cereale), barley and wheat are common too. The starch stored in the seeds by the cereal plant is intended to fuel a slow release of energy for growth of the next generation of plants. This is repurposed by distillers into fermentable sugars either by malting and mashing, or solubilization and subsequent conversion. Any one cereal or a mix of two or more could be the basis of a whisk(e)y “mashbill” as the recipe of cereal proportions in a mash is known.

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TABLE 1.1 Cereal varieties typically used in Scotch Whisky production (Scotch Whisky Association Technical Note, August 2012). Cereal

Use (Scotch)

Type

Name

Barley

Malt Spirit

Spring

Laureate LG Diablo KWS Sassy

Barley

Grain Spirit

Spring

Fairing (For High DP malt)

Wheat

Grain Spirit

Winter

Elation KWS Jackal LG Skyscraper

Barley will be examined in more detail in the subsequent Malting section of this chapter, but may also be used unmalted, rarely in Scotch grain whisky but always in Irish Pot Still Whiskey. Agu et al. (2006) showed that cereals are extremely varied in their properties and yield potential, and that very different processing conditions will be necessary to achieve maximum yield. In the selection process, processing ease, for example, low viscosity, tends to receive more attention than flavor potential.

1.2.1.1 Cereal developments Bodies such as the Maltsters Association of Great Britain and the Institute of Brewing and Distilling together with Home Grown Cereals Authority and Scottish Agricultural College (all in the UK) have long sought to identify cereal varieties which would meet the needs of both growers and users (Table 1.1). Plant breeders have used traditional (nongenetical engineering) methods to produce varieties with “better” features, including starch content, yield per acre, reduced fertilizer requirement, disease resistance, stress tolerance, etc. In terms of congener development, flavor potential is not normally a particular focus but potential to produce ethyl carbamate is. This compound of concern was ultimately traced back to epiheterodendrin in certain varieties of barley. This characteristic can now be identified at an early stage and only barleys that don’t produce this compound will advance to the approved lists. Modern methods employed by the James Hutton Institute have improved the time taken to bring new varieties to the field. Utilizing molecular genetics technology, it has been possible to speed up the time to market and increase probability of suitable traits in new cereal varieties (Bringhurst, 2015). Starch content maximization is the desired outcome, whereas nutrients resulting in unnecessary stalk growth may be wasted, especially if lodging (stalk collapse) results. Indirectly this may lead to mold growth (mycotoxins) and result in flavor taints such as geosmin (Martinez & Woloshuk, 2012). The identification of varieties of barley which will both grow economically and reliably in the field and also display desirable and consistent behavior during the malting process is given much attention by plant breeders. Generally, for malt whisky, barley with low nitrogen uptake and content are desirable as is negligible, or at least predictable, dormancy, when barley resists germination immediately after harvest. Typical varieties used in Scotch whisky production are given in Table 1.1.

1.2 Raw materials

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When quality is discussed in relation to barley for malting it is generally implying predictable processability rather than flavor potential (Bringhurst, 2015). Maximizing yield (both in field and distillery) and cost control have received more attention than the quality of the end product, which tended to be assumed as a given. This has led to the selection of larger grained cultivars (two rows) with lower soluble nitrogen ratios over the small-grained cultivars which have a propensity to produce malt with a high total soluble nitrogen. There was concern that yeast metabolism might be affected by nitrogen content, increasing production of esters and higher alcohols. However, in practice, little difference has been noted (Bathgate, 2019). There has been recent growing interest regarding the use of heritage varieties (such as Bere barley, a six-row variety) as a means of (re) introducing desirable flavor or other properties albeit at a yield and cost penalty (Martin et al., 2008). Golden Promise barley, the first modern variety which had agronomic properties to allow extensive cultivation in Scotland, continues to be a sought-after ingredient by some distillers despite a 10% yield penalty relative to more modern cultivars, as it is held to deliver better flavor, although Bathgate (2019) suggests this may be due to other factors. There has been ongoing debate as to the extent that “Terroir,” is a factor in whisk(e)y quality, either from the cereal itself or malted barley. It is highly likely that the same cereal, and even same variety of cereal grown on different soils, in different locations may have different taste or other properties. However, unlike wine, other than in extremely rare circumstances such as the controlled experiment conducted by Waterford Distillery in Ireland (Kyraleou et al., 2021), it is unlikely to be a significant feature across the industry generally.

1.2.1.2 Grain whisky Grain whisky is made from predominantly unmalted grain the starch of which is converted during mashing by the enzymes from a smaller proportion of malted cereal (usually but not always barley) or other sources. Scotch grain whisky is produced from a mashbill of about 10%–20% malted barley and a balance of unmalted cereal such as wheat or maize. The malted barley is slightly different from that used for malt whisky being less concerned with starch content and more with enzyme content. The barley used to produce it tends to be thinner and higher in nitrogen level compared to malt whisky varieties. The previous practice of using green malt (unkilned) with undegraded enzymatic levels has largely given way to the convenience of use of kilned malt, even though the continuous distillation removes vegetal notes which would occur in pot distillations (Robson, 2007). The starch in the unmalted cereal must be made accessible to breakdown from those enzymes. This is done by solubilizing the starch in water by varying cooking regimes involving milling (usually), heat, pressure, and time. Milling entails dust losses and more particularly explosion risk so some producers employ whole grain mashing, or just slightly crack the grain to obviate this, while maximizing yield. If unmilled, extreme conditions in a pressure cooker at several bar pressure for up to 2 or 3 hours may be necessary to solubilize the starch. At the other end of the scale, low temperature mashing regimes have been installed in a couple of Scottish grain distilleries where finely ground grist is mashed at sub boiling point temperatures. Wheat, a temperate crop, requires lower temperature processing to achieve optimal yields than maize (Agu et al., 2006; Green, 2015), introducing variable process conditions to the innate tendency of different cereals to contribute different volatile components (Biernacka and Wardencki, 2012). Different unmalted cereals do introduce different flavors with variable

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amounts of esters, acetals, and higher alcohols detectable after fermentation. Many of these would be removed in an efficient (continuous) distillation regime and, after maturation, it may be extremely difficult to detect differences in flavor. Little sensory work has been published in this area but Phetxumphou et al. (2020), looking at US “straight” matured whiskies of varying mashbills, concluded: “no specific attributes were driven by mash bill- ryes and bourbons were not discriminable in this study”. Having however personally nosed rye, corn and wheat new make spirits produced under otherwise identical circumstances it is undeniable that subtle but detectible differences in odor and flavor exist in new make spirit. This personal observation is supported by the published findings of Wilson and Pryde (2014). These widely varying temperature and pressure regimes potentially introduce quality variables due to differing production of Maillard compounds and furfural type products, but the stringent distillation conditions remove almost all trace of these. Recycled spent wash (backset) is sometimes used as a percentage of the mashing liquor for water economy, environmental, energy, pH reduction and carbohydrate recovery reasons. The drop in pH should logically suppress bacterial activity but again the effect on the quality of the spirit appears slight. After the appropriate cook, and depressurization if required, the starch slurry is mixed with the ground malted barley slurry in a mash tun or mixing tank. As in malt whisky production the grain wort is not sterile but bacterial activity is significantly suppressed by a partial pasteurization type effect as the wort resides in the mash tun at 63°C. Nowadays all seven Scottish grain distilleries operate on an “all grains in” fermentation, and, with one exception, all the grain husk material also passes through the first distilling column. “All grains in” procedures allow maximum yield to be achieved as no starch particles are lost through adherence to husk particles remaining in a mash tun. No spirit quality impact has been reported from the use of this extremely cloudy wort. A couple of alternative processing procedures involving continuous (as opposed to batch) cooking and/or mashing in tubular or continuous batch vessels encountered issues related to excessive build ups of malt derived bacteria impacting on fermentation performance by inhibiting ongoing enzymatic activity. This arrangement is perfectly satisfactory if commercial (fungal) enzymes rather than malt is being utilized and is common internationally.

1.2.1.3 Pot still whiskey Now seen as exclusively an Irish style, pot still whiskey reflects the common practice of commercial (as opposed to illicit) distillers at the beginning of the 19th century, pre continuous stills, to eke out expensive malted barley with cheaper unmalted grain such as barley, oats or rye. Historically, ratios were probably freestyle determined by availability of feedstock, albeit with sufficient malt to convert all starch, but are now specified in (Irish) legislation. The spirit quality has been described as being between Scotch grain and malt in flavor (Hills, 2002). 1.2.1.4 US whiskey US straight whiskey is categorized by mashbills of 51% of for example corn (for Bourbon), wheat or rye. “All grains in” fermentation with fully ground material is normal and although malted barley is usually used, so are exogenous enzymes. Light whiskey usually has a mashbill of predominantly corn, unless otherwise stated.

1.2 Raw materials

7

Some US craft distilleries adopt a brewing style approach with boiled clear wort often combined with attemperation throughout fermentation, and distillation in pots stills which feature a few reflux plates.

1.2.2 Water A high quality and abundant supply of water is a prerequisite for the manufacture of any whisk(e)y. In Scotch Whisky Regulations, it is specifically mentioned as one of the permitted ingredients. The potential sources of water are varied such as surface (river, stream, or loch), ground (borehole or well), or mains supply (municipal or city). Water is used throughout the process in various manners including as process liquor, as a “once through” heat transfer medium, as recycled and upgraded heat exchange fluid or in a discrete recirculating cooling water system. It is also vital as a diluent to reach a desired cask or bottling strength. It is used in cleaning processes or as service water for steam raising, though in both these cases, the distiller must ensure no chemical cross contamination occurs. Water for distilling should be free of taint due to geosmin or bacterial metabolites. Water may be treated to remove either organic and/or mineral components prior to use. The water used for adjusting alcoholic strength prior to bottling must meet potable standards and may be purified by distillation, demineralization or reverse osmosis. Use of hard water is risky as mineral salts may result in the formation of an irreversible floc (precipitate) in the bottled product. While process and diluting water actually form part of the product (although most process water leaves as a component of one of the various effluent streams) cooling water can also impact spirit quality. This particularly relates to the various techniques of spirit cooling where the temperature regime experienced by the spirit vapor as it leaves the spirit still will affect its organoleptic character due to the impact on copper contact as mentioned elsewhere in this chapter. Much has been written, usually in marketing copy, but also in the Scotch whisky technical file, about how water (usually soft) flowing over granite stone or peatland makes it particularly suited for the production of quality Scotch whisky. In Bourbon production, the local limestone rich supplies tend to hardness though this may be offset by the use of acidic backset. The subject of how water type might impact whisky quality has been reviewed in an unpublished work by Dr. K. Inatomi (Inatomi, 2022) who examined the sparce reputable research on the topic, but including material by Dolan (2003), and particularly Wilson (2008). This lattermost work concluded that although Speyside, Lowland, and Islay whiskies displayed different sensory character, this did not correlate with water composition, under the laboratory conditions employed. Wilson did however find that there was an effect on flavor from the use of brown “peaty” water impacting on fermentation though it did not give a peaty character. What is certain is that this apparently vital aspect of Scotch whisky production has received virtually no (published) research by the industry and Wilson’s findings require corroberation and elaboration. Water also impacts whisky quality as a result of its physical properties and varying interaction with ethanol (and congeners) depending on which liquid is more prevalent, or if there is an approximately equal amount. While most evident in maturation, when the compounds eluted from the cask vary with strength, there are also noticeable variations to density, volume and viscosity with packing efficiencies due to hydrogen bonding and polarity effects.

8

1. Whisk(e)y

Morishima et al. (2019) examines further the topic of “clustering” in maturing whisky, identifying large clusters which form immediately reducing irritation and smaller ones which form gradually and correlate with improving quality (Morishima et al., 2019).

1.2.3 Yeast Yeast for distilled spirits production is almost exclusively a strain of Saccharomyces cerevisiae. Usually, a “high attenuation” strain is chosen which can ferment a broad spectrum of sugars and tolerate high ethanol concentrations. In Scotland, yeast is bought in from specialist manufacturers in dried, paste or slurry form. Elsewhere, including the United States, it is much more common for large distillers to follow the practice of many large breweries and propagate yeast themselves from proprietary cultures which may give more varied congeners.

1.2.4 Commercial enzymes (non-Scotch) If malted barley is not the source of amylolytic enzymes to break down the amylose and amylopectin of the starch into fermentable sugar, fungal based enzymes will be required to fulfil the task. Amyloglucosidase and dextrinases frequently are employed to achieve higher degrees of saccharification than malted barley alone can achieve. Prohibited under Scotch Whisky Regulations, elsewhere they are sometimes used exclusively or in combination with malt to enhance yield.

1.3 Whisk(e)y production 1.3.1 Malting The first step in whisk(e)y production is mobilization of starch in the cereal grains. Malting is the process of “tricking” the cereal into starting its growth cycle, steeping it in water to initiate the growth of roots and shoots. More importantly enzymes migrate from the embryo into the starch, converting the solid matrix into smaller discrete packages. The detailed work by Palmer on the routes by which this happens in barley and how it could be enhanced by abrasion is useful in understanding the process (Palmer, 1987). Following steeping and subsequent germination, whether on a floor, in a drum or in a multipurpose vessel, the sprouted barley will be sent for kilning, unless it is that very small percentage which might be used immediately as “green malt” (unkilned) for grain whisky production. Heat, from a variety of possible sources, is applied to arrest the growth of the malt at the point of optimal conversion thus preventing unnecessary plant growth. The drying process must be carried out cautiously to prevent “stewing” of the enzymes early in the process. Later, once moisture has dropped after the break point, the temperature may be raised to reduce the moisture level to 4% or 5%. Temperatures are kept low on the bed (∼30°C), while moisture is initially removed, rising to the low 60s °C later on, in order to minimize temperature sensitive enzyme degradation. Despite the comparatively low temperatures, amino acids and reducing sugars undergo heat degradation (known as Maillard reactions; Fig. 1.2) and compounds are generated which are important contributors to final flavor.

1.3 Whisk(e)y production

9

FIGURE 1.2 Summary of the processes involved in Maillard (browning) reactions (redrawn from Bringhurst et al., 2003).

These include:

r Heterocyclic nitrogen containing compounds such as pyrazines which contribute a nutty aroma, and pyrroles which give a corn or bread-like aroma.

r Heterocyclic sulfur containing compounds such as thiazoles. r Other heterocyclic compounds such as furans and furfural, giving caramel or vanilla aromas, and maltol which contributes a toffee aroma.

r Other sulfur compounds such as dimethyl sulfide and polysulfides will contribute cabbagelike aromas. The fuel of choice changed over time from peat (see below) and anthracite to heavy fuel oil and subsequently natural gas, albeit still direct firing. The discovery that the lack of sulfur in gas fired maltings encouraged the presence of undesirable nitrosamines (NDMA) led to either indirect firing with gas or steam usage as an intermediary heat transfer agent (Duncan, 1994). Depending on the fuel, the burning of sulfur to reduce NDMA may still be practized. This compound of concern is now well understood, and processing techniques ensure that it no longer features in new make spirit. Nowadays the vast majority of malted barley utilized in Scotch manufacture is produced in huge facilities, whether by large distilling companies or commercial malting companies. This means that quality is generally high and consistent but the wide variability from distillery to distillery (good or bad) when each distillery made its own malt no longer exists. The changes in malting practice over the last half a century or so are well recorded and explained by Bathgate (2019). In what will become a critical reference work, he refers to seven major changes in malt whisky production, one of which is from each of mashing, fermentation and distilling but four relate to malting practice.

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1.3.1.1 Peat-kilned malted barley Peat, traditionally, was used as a fuel for maltings in Scotland. The smoky flavor commonly associated with Scotch whiskies is largely peat derived and is frequently used by whisky producers elsewhere who wish to adopt this flavor characteristic. Peat is often said to contribute phenol, and although that is commonly used as a marker to determine the degree of “peatiness,” guaiacol, furfural, and the various isomers of cresol and p-ethylphenol are also important, with lower threshold values. Historically, each distillery operated its own maltings meaning practices, flavor, and quality would have varied widely, particularly due to the variability of peat composition (Bathgate, 2019). Interestingly, the “Shackleton” time capsule whisky revealed that Glen Mhor, a Highland Whisky, utilized Orkney peat so it would be erroneous to assume a local source (Pryde et al., 2011). Peat, although no longer used as primary fuel, can still be burned for its flavor addition, during the kilning process. Rather than being dry on burning, it is damp and smolders like a bonfire of wet leaves early in the drying cycle while the malt is still comparatively high in moisture though not actually wet. Now, with indirect firing, special arrangements such as smoke blowers have been devised to ensure the characteristic taste expected in Scotch whisky. Hitting an exact degree of “peatiness” (usually expressed as mg/kg phenols) is difficult to control during kilning so a precise parts per million (ppm) level is usually achieved by analyzing a kilned batch exactly and then blending it with unpeated malted barley to achieve low, medium, or high specifications such as 10 mg/kg, respectively. Peat from different areas contributes varying mixtures of compounds and hence flavors, undoubtedly constituting a form of terroir. The differing chemical composition of peats from varying locations across Scotland has been confirmed by the work of Harrison et al. (Harrison et al., 2006). The properties of the malt do not divide along geographical lines but rather relate to the nature of the bog topography and the organic composition resulting from varying plant matter contribution related to the proportions of woody or mossy material. The usage of heavily peat/smoke imbued spirits will be covered later when blending is discussed, though they also have a strong following when used to make single or blended “Islay style” malts.

1.3.2 Mashing Mashing, or saccharification, is the process of converting the starch in the cereals, whether malted or unmalted, into a sugary solution so that it may be utilized by the yeast during fermentation. In a Scotch malt whisky mashing regime, the mash bill consists solely of malted barley. The malted barley, cleaned after kilning of roots and culms (which might introduce a bitter flavor note), is milled in a mill whose purpose is not simply a pulverization. The malted grain is torn apart by fluted rollers so that, by careful setting up of the mill and monitoring, a 70:20:10 ratio of grist, husk and flour is achieved. After collection in a bin this grist is mixed with warm water in a Steel’s type mashing machine where rakes on a rotating shaft within a cylindrical tube produce a well-mixed wort at about 63°C as it enters the traditional mash tun. The bed depth, stirring by rakes or lauter mechanism, and rate of run off are critical factors in determining the nature of the liquid (wort) which will be drawn off from under the perforated floor of the mash tun and, after cooling, sent for fermentation. The wort, drawn off via an underback, may be of varying turbidity (“clear” to “cloudy”), depending on the straining characteristics of the mash bed which has formed (Fig. 1.3).

1.3 Whisk(e)y production

11

FIGURE 1.3 Lauter tun.

Traditional practice was to drain almost all the liquor off from a deep bed and then add a “second water” of hotter liquor at about 70°C to refill the tun, stirring prior to allowing the bed to settle again and run more worts again as a second batch of weaker worts to the washback (fermenter). Current practice is more commonly for the secondary liquor to be applied in a more continual sprayed manner (sparging), gently replacing the worts flow as it is run off. The use of stirring tends to be minimized to avoid increasing lipids and oils in the wort, and also air entrainment (Bathgate, 2019). The lauter arrangement allows for thinner bed depths and faster drainage, particularly in new installations with larger filtration area. Clear worts tend to be preferred, so as not to provide excessive nutrient for yeast metabolism. Nutty as opposed to grassy spirit properties will be largely determined at the mashing stage. The topic of “nuttiness” in spirit was examined by Boothroyd et al. (2014), finding it was linked to lipids in cloudy or turbid wort. They concluded: “The nuttiness aroma of whisky is a complex sensory character both because its congeners are linked to the production of families of compounds generated via Maillard reaction pathways and because the nutty percept has crossover into related characters such as ‘cereal’ or ‘oily’. In separate publications we have demonstrated that wash lipid levels and concentration of longchain esters also appear to play a part in the perceived nuttiness of spirits.” Work by Webb (Webb et al., 2014) correlated this phenomenon with a higher ratio of linoleic to oleate esters and attributed this to cloudy wort, mentioning that short fermentations and fast distillation may also be factors. The temperature of mashing, usually initially around 63°C is optimal for the activity of the alpha and beta amylases to achieve their task of debranching and reducing the amylose and amylopectin starch structures to maltose, glucose, maltotriose, and larger dextrins. At these temperatures, whether as a deliberate mashing “stand” in old style regimes or during the “vorlauf” stage where worts are recycled to the top of the lauter bed to improve clarity, there will be a partial pasteurisation, reducing but not eliminating the natural bacterial population which arrived with the malted barley (Geddes, 1985). The thermolabile enzymes will also reduce but sufficient enzymatic activity will survive and continue during fermentation to increase yield by approximately 15% by the action of limit dextrinase on dextrins (Dolan, 2003). While the yield uplift is most welcome and the focus of much distiller attention it is the remaining bacterial presence from the unboiled worts which will have the major quality impact during fermentation (see later).

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There are a couple of mash filters operating within the Scotch whisky industry, and at least one in Ireland. Here the malted barley is pulverized as the cloths of the plates provide the filtering medium. The equipment will cope with more poorly modified malted barley than a mash tun but has not been widely adopted. They are suitable for processing some materials such as rye, that are more difficult to process due to higher viscosity, but such a product must be termed grain whisky. Given a certain type of malted barley, the resulting spirit will not be better or worse, but it will be different from that from mash tun wort, even if all other variables are held constant. Wort from a mash filter is sometimes described as giving an “oily” note.

1.3.3 Fermentation Fermentation and yeast metabolism is a huge and complicated subject. One cannot attempt to tackle all aspects within one general chapter, so instead will focus on the areas which impact on whisky quality. Those who want more detail should read Iain Campbell’s chapter “Yeast and Fermentation” in Whisky: Technology, Production and Marketing (Campbell, 2003). The character of malt whisky spirit is determined largely by the volatile congeners from yeast metabolism coupled with those resulting from late bacterial action during fermentation. Minor changes or differences in temperature, time or gravity at this stage will impact spirit quality significantly. The wort, cooled to about 20°C, is pumped to the washbacks (fermenters), which traditionally were made out of wood such as Douglas Fir (Oregon Pine) but nowadays longer lasting and easily sanitized stainless steel is often chosen. If thoroughly cleaned, wood can be satisfactory from a hygiene standpoint and also provides a natural insulation to ambient conditions. Stainless steel, as well as being longer lasting, is easier to install attemperation in or on, if the conditions or scale require. Although not usually an issue with malt whisky scale production, for grain it is much easier to scale up vessel size, even by an order of magnitude from a typical 500 hectoliter vessel, using stainless steel. However, as the volume goes up by the cube, the surface area goes up only by the square, affecting the rate of heat loss so that additional cooling becomes much more necessary. Setting declaration temperatures (the temperature the wort is cooled to at the commencement of fermentation) ever lower to allow for the extra heat generation may slow up yeast activity giving infection a chance to take hold. Yeast (Saccharomyces cerevisiae) is pitched into the wort immediately after the initial strong worts are cooled to ensure the inoculation outcompetes bacteria. Distillery wort experiences natural oxygenation which helps multiplication during the lag phase when the yeast acclimatizes and undergoes aerobic growth as budding occurs for a few generations. Once the oxygen is used up the yeast operates under anaerobic conditions and will be unable to synthesize fatty acids for cell membranes, drawing down reserves built up during the aerobic phase (Walker & Hill, 2016). The yeast metabolizes sugars and expires CO2 as it produces energy to sustain itself. Metabolization of sugars occurs sequentially; initially glucose, then maltose and later maltotriose and possibly some larger molecules, if the amyloglucosidase has remained active, will also be metabolized. However, key to flavor and hence ultimately quality of spirit is the yeast’s propensity, due to the variety of metabolic pathways it operates, to produce a small amount of congeneric compounds such as other alcohols, diacetyl, aldehydes and ketones, etc. (Table 1.2). As before, much focus on yeast development for distilling has been related to yield improvement (quantity) while avoiding the introduction of off flavors, rather than positively influencing the quality of the spirit. However, as a positive sign of evolving priorities, Walker and Hill (2016) put “spirit flavor” first in a list of desirable attributes for new distilling yeast strains.

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1.3 Whisk(e)y production

TABLE 1.2 Products of yeast fermentation (from Campbell, 2003). Alcohols

Acids

Esters

Others

Ethanol

Acetic

Carbon dioxide

Propanol

Caproic

Ethyl acetate and any other combination of acids and alcohols on the left

Butanols

Caprylic

Diacetyl

Amyl alcohol

Lactic

Hydrogen sulfide

Glycerol

Pyruvic

Phenylethanol

Succinic

Acetaldehyde

The wort which arrives in the washback differs from that from which beer is made in that viable enzyme activity is ongoing, generating additional fermentable material. It also contains a bacterial population which can, if excessive, compete with the yeast and, by dropping pH, inhibit enzymatic activity. However, the activity of bacteria, e.g., lactobacilli (LAB), has an important positive contribution as well, assisting in the production of desirable flavor congeners such as when lactic acids interact with alcohols to produce esters. Work by Ensor et al. (Ensor et al., 2014) demonstrated the importance of LAB on congeneric levels in new make spirit. The behavior of the yeast is strongly influenced by the environmental factors it experiences including temperature, acidity, gravity (sugar concentration) but also nutrient availability resulting from mashing “efficiency”. Ensuring worts run relatively clear as opposed to cloudy means less nutrient is available and more yeast stress occurs. Yeast stress produces higher levels of congeners, the volatile ones of which (or their reaction derivatives) will appear in the spirit after distillation. These non-ethanol flavor compounds the yeast produces during fermentation, although not large in amount are extremely numerous, approximately 400 metabolites having been recorded (Suomalainen and Lehtonen, 1979). A selection is given in Table 1.2. It is important to note that just because a compound is present in a measurable amount it may not be detectable organoleptically. Conversely, if its aroma threshold is low, tiny amounts of a compound may contribute significantly, whether for good or bad. Additionally, some products amplify or obscure the detection of others organoleptically. The fermentation conditions are enormously important in determining the character of spirit due to contributing:

r Volatile structural components of yeast. r Metabolic products of yeast growth. r Products from the microbial contamination of yeast and malt.

Esters such as ethyl acetate or lactate and isoamyl acetate are also produced during fermentation. The production of esters is related to the recycling of the enzyme co factor coenzyme A (Peddie, 1990). While acetic acid and longer chain fatty acids are key intermediates required in biosynthesis, a proportion are lost to the wash, appearing as flavor congeners in the spirit (Fig. 1.4). Esters comprise two main groups, the acetate esters (made up from the reaction between an acetate and an alcohol), and the ethyl esters (from the reaction of ethanol with a fatty acid). While the former may be present in larger concentrations the latter are particularly important due to their property of being very aromatic in very low amounts due to their low threshold values. Longer chain esters tend to be produced in the later stages of fermentation.

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FIGURE 1.4 Formation of esters and fatty acids by Saccharomyces cerevisiae (redrawn from Campbell, 2003).

TABLE 1.3 Examples of esters and their odors typically produced by Saccharomyces cerevisiae during Scotch Whisky fermentation. Ester

Odor

N-pentyl acetate

Banana

Octyl acetate

Oranges

Ethyl butyrate

Pineapple

Amyl butyrate

Pineapple

Pentyl butyrate

Apricot

Ethyl formate

Rum-like

Producing wash with the correct balance of esters and other congeners is critical to achieving the desired spirit character (Table 1.3). Regimes involving long fermentations, high original gravities and high pitching rates of yeast may be employed to encourage this. However, care must be taken to avoid undesirable solventy notes due to excess higher alcohols or rancid aromas which would arise from excessive amounts of fatty acids being present. Temperature is also a major influencer of congener levels and hence flavor with most fermentations being “declared” at about 20°C and allowed to increase as the fermenting yeast emit heat up to the low or mid 30s°C. “As a general rule, increasing fermentation temperature increases the amount of yeast growth in proportion to the available (alpha) amino nitrogen, resulting in increased production of higher alcohols. Conversely… ester production is reduced” (Campbell, 2003). Rapid fermentation with insufficient wort free amino nitrogen, overpitching or high fermentation temperatures give rise to increased diacetyl which has a buttery/butterscotch odor detectable in spirit at very low levels. Due to its similar volatility to ethanol, it is difficult to distil out, even on continuous columns. “The flavor congeners directly produced are pyruvic acid and acetaldehyde but many other acids, alcohols and esters are produced as by-products of biosynthetic activities of the yeast.

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TABLE 1.4 Lactic acid bacteria and their congeners. Congener(s)

Substrate(s)

Metabolic category

Example

Lactates/acetates

Sugars (including hexose and pentose)

Heterofermentative

Limosilactobacillus fermentum Lacticaseibacillus paracasei Levilactobacillus brevis

Acetates

Hexose and cell walls

Homofermentative

Lactobacillus acidophilus Lactobacillus delbrueckii

However, the majority of these flavor congeners are by-products of biosynthesis of lipids, nucleic acids and proteins” (Campbell, 2003). Volatile compounds from fermentation include higher alcohols (fusel oil) such as isoamyl alcohol, 2-methyl-1-butanol, isobutyl alcohol and 1-propanol. There is a structural relationship between certain amino acids and their corresponding higher alcohols, for example leucine resulting in isoamyl alcohol, and valine giving rise to isobutanol. The particularly low threshold levels of detection of sulfur compounds can cause problems even in column distilling set ups and this will be elaborated upon later. Acrolein may appear due to hygiene issues either as a bacterial metabolite or due to pH impacting yeast behavior. Acrolein may break out if infection builds up, e.g., in a washcharger, and is particularly noticeable on nosing, with a chilli pepper like impact. It derives from glycerol and its precursor β-hydroxypropionaldehyde is formed as the wash is heated during distillation. Length of fermentation has a major impact on congener levels mostly due to the degree of late lactobacilli activity. As mentioned previously the malt wort is not sterile and lactic acid bacteria are active slowly throughout the entire fermentation. The product of this activity is lactic acid and lactones. The lactic acids and the esters they form with the various alcohols present are, if volatile, significant contributors to whisky flavor (Barbour & Priest, 1988; Table 1.4). Higher gravity wort tends to produce higher ester levels in the resulting wash. There is a small pushback against the dominance of the various pure distilling cultures available as those (including the newly established distilleries) who wish to differentiate their spirit to make it stand out from the crowd are willing to sacrifice a small amount of yield and performance (if necessary) in return for more desirable flavor contributions and have been experimenting with diverse yeast strains from various sources. The metabolic mechanisms occurring in all malt fermentations also apply to grain whisky fermentations even though the wort will have a slightly different composition, such as lower nitrogen content. However, it is of less importance due to the more extreme fractionation which occurs during column distillation as, although present, the congeneric organoleptic properties are comparatively slight. Gravities of whisky wash have trended upwards over time but particularly in grain distilleries with more modern continuous stills able to cope with the resultant higher wash strength. Certain fermentation congeners, such as acrolein, diacetyl and sulfur compounds may prove difficult to remove even in column distillation. Due to scale, many grain fermentations are partially (reducing temperature rise) or fully (maintaining the declaration temperature) attemperated by a variety of methods such as

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clamshell jackets or external heat exchangers which alter significantly the climate the yeast experiences.

1.3.4 Distillation 1.3.4.1 Batch distillation The simple copper alembic is widely used for the batch distillation of whiskies. Variations on the basic “onion shaped” still will be encountered such as the ball, raised ball and lantern shape, providing extra copper surface area to encourage more reflux (the partial condensation of spirit vapor) and discourage frothing. The tops of some stills may have optional features in the form of purifiers (which condense and return some of the less volatile elements to the body of the still as additional reflux) and the water-cooled head (internal or external) which also increases reflux. Stills will vary in height and width, as well as volume. As volume increases the ratio of active copper surface area per unit of wash or low wines volume will reduce. All these factors will result in minute, and separately incalculable, changes to the reflux within the head of the still. Other factors will come into play as well including ambient temperature, rate of heating, height of filling wash within the body, thickness and emissivity of copper, which make further minor changes to the internal reflux of the system. This means that, although the pot still is a relatively simple looking arrangement, it is actually subject to a very wide range of factors which can impact on spirit quality, in addition to any variations of wash composition. Double distillation is the common practice in the manufacture of Scotch malt whisky. During the first (wash) distillation step the volatile elements, particularly the ethanol and similar boiling point congeners, are boiled off the fermented wash over a few hours. The wash has an alcohol content of about 8%–10% alcohol by volume (ABV) and its vapor is condensed into a liquid called Low Wines at around 20% ABV. No particular attention need be given to cuts or reflux during this first distillation, though heating is carefully controlled as an over enthusiastic application might result in foam rising up the neck and result in a “foul” distillate. Classically, direct heat was applied from the outside of the concave base of the still, initially burning peat or coal and more recently gas. The intense heat, including hot spots, would cause charring and burning on of organic material from the wash on the inside of the still. Rummager arrangements of rotating chains were employed to remove these deposits in order to expose the copper surfaces and thus maintain heat transfer. The use of steam heat transferred to within the body of the still via coils or pans, or externally via heat exchangers has become predominant in the last few decades. Obviously, the production of steam is fuel independent insofar as impact on quality of spirit. This heating is much more controllable via valves, whether manual, remote or automatic, than ever was the skillful rake of the stillman but many feel that the quality of the whisky has potentially been changed by the reduction in Maillard reactions and compounds such as furfural. Ludford-Brookes and Hancock (Ludford-Brookes and Hancock, 2017) discuss the various options of still heating and how direct firing, particularly for the wash still, might be emulated. The distillation is terminated when there is no more economically recoverable alcohol in the still and the residue is discharged as pot ale for treatment. Ideally the output of one wash still distillation would feed one (slightly smaller) spirit still distillation and all volumes, strengths and temperatures would be identical from run to run to produce identical distillate in a “balanced” arrangement (Nichol, 2003). However, in practice, many stillhouse set ups of wash

1.3 Whisk(e)y production

17

FIGURE 1.5 Spirit still cut points in a typical Scotch malt whisky distillation (credit: International Centre for Brewing & Distilling, HeriotWatt University, Edinburgh).

and spirit stills are not matched and widely varying strengths and compositions of low wines charges to the spirit still may result. The second (spirit) distillation is of the low wines from the wash distillation to which are added the foreshots and tails of the preceding spirit distillation. These combine together to form a “charge” for the spirit still in the low 20s% ABV. Much closer attention must be paid to this distillation as the exact choice of cut points will determine the spirit character and quality. As heat is applied and the foreshots start running they clear any residues from the Lyne arm, pipework and cooler from the preceding distillation. These residues contain long chain fatty acids, higher alcohols, etc., which would taint the spirit. Once an adequate foreshots flow has run “to feints”, whether determined by time, strength, taste/smell or the classic water test to determine cloudiness, the flow will be redirected to the spirit safe from the feints receiver to the spirit receiver. During the spirit run the strength will drop as the more volatile components are succeeded by less volatile elements, including an increasing amount of water. The rate of running must be controlled carefully during this “heart” phase. The decision to terminate spirit collection may be determined by time, volume collected, strength or organoleptic means or by a combination of these. The subsequent condensate flow, now termed feints, will again be directed to the feints receiver and the rate of distillation may well be sped up until the point at which virtually no alcohol remains in the still. The distillation is now complete and the remaining liquid, termed spent lees, is discharged for whatever treatment is necessary. Fig. 1.5 shows graphically the supreme importance of getting the cut points correct. The foreshots will be high in strength, above 70% ABV, and must clean away residues from the previous distillation, as described earlier. Once that is done, however, they should be collected as they contain highly volatile esters that are important contributors to flavor. The heart of the distillation is run slowly and consists primarily of ethanol and congeners of similar volatility. Spirit collection is usually stopped when the strength is in the low 60s% ABV, as the subsequent feints fraction contains less pleasant flavor congeners. The exact “cut points” of commencing and completing the hearts run will determine which flavorsome congeners will be collected. When using peated malt, the cut point is generally delayed to ensure the collection of phenolic compounds, which have a relatively low volatility. Phenol and guaiacol (a very important peat derived congener, which is formed during distillation from cinnamic acid precursors) will be captured which will tend to mask the feinty aromas.

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Reflux will vary due to a variety of operational factors, including wash and low wines strength, still charge volumes and hence surface area above the liquid interface, rate of running, cooling water temperature, etc. Higher reflux rates will increase fractionation and lead to lighter spirit while lowering reflux will lead to heavier spirit character. More reflux allows additional contact with the copper surfaces which act as catalytic sites for sulfur and other reactions. Copper as a material of construction is so important to the quality of whisky that it has been described as the fourth ingredient of whisky (Bathgate, 2019). The available copper surface area for condensing spirit vapor is all important in removing sulfur compounds. Much of the reaction between copper and sulfur takes place in the condenser, the design of which can impact the degree to which removal occurs. Significant change was observed when “worm tub” type condensers were extensively replaced by shell and tube ones. The effect of the resultant increase of surface area of copper in these latter condensers has been to remove more sulfur compounds from the spirit, lightening it considerably. Temperature, whether ambient conditions, or the setting of condenser parameters for optimum energy recovery, will also impact on the degree to which sulfur compounds, unpleasant in excess in spirit, but contributing to depth and fullness in moderation, are removed (Bathgate, 2019). The advanced use of energy reduction technology such as initial condensers run hot for energy recovery and/or thermocompressors has divided opinion. Bathgate says: “The phenomenon (of copper dissolution) may be exacerbated when heat recovery aftercoolers are fitted to the wash still condensers. These obviously work most efficiently when very hot condensate is presented to the heat exchangers resulting in higher copper concentrations in the spirit. More recent research has confirmed that copper pickup is greatest in the wash still condensers and the pot of the spirit still (Harrison et al., 2012). Therefore, systematic changes within the heating and cooling elements of the pot stills can affect copper solubility and hence spirit character”. Inadequate research has been published (though more may have been carried out) on the complex chemistry of the interactions which occur in the feints tank wherein a veritable soup of many alcohols, acids, esters, etc., interact at various temperatures and over multiple cycles. After a maintenance shutdown, several distilling cycles may take place until the spirit attains its usual consistent quality as the congeners regain equilibrium. The “managing” of the feints tank can either consist of ensuring that the entire contents are returned back to the subsequent run or cooling them so that a layer of fusel alcohol and fatty acids forms on top of the contents. This runs the risk that if temperature and/or strength are allowed to go too high the layer containing higher alcohols and long chain fatty acids may solubilize and overload a particular distillation run. This may cause the dreaded “blank run” where the spirit never clears (on the water test) and no product is collected until parameters are brought back into control by cooling and diluting the feints (Nichol goes into commendable practical detail in his chapter of Russell’s work, 2003 (Nichol, 2003). Spirit quality will initially be assessed organoleptically, both for conformance to expectation and for the absence of defects (Table 1.5).

1.3.4.2 Column distillation Various developments to improve batch distilling culminated in the 19th century invention of continuous distillation by Blumenthal and others in France and Stein and Coffey in the UK. The famous Coffey still’s analyzer fulfils the function of the wash still and its rectifier fulfils the role of the spirit still, but on a continuous as opposed to a batch basis. For as long as wash and

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1.3 Whisk(e)y production

TABLE 1.5 Typical malt distillery analyses (g/100L abs. alc.). Speyside

Islay

Acetaldehyde

5.4

7.0

Ethyl acetate

26.3

33.2

Diacetyl

2.1

2.8

Methanol

4.8

9.0

Propanol

40.3

37.6

Isobutanol

80.1

85.6

o.a. Amyl alcohol

44.2

53.1

Iso-amyl alcohol

138.9

170.8

Total higher alcohols

303.5

347.1

Ethyl lactate

3.7

5.3

Ethyl octanoate

1.8

2.7

Furfural

3.6

4.8

Ethyl decanoate

6.0

8.9

b-Phenethyl alcohol

7.2

8.7

Ethyl myristate

0.9

1.2

Ethyl palmitate

2.8

3.3

Ethyl palmitoleate

1.6

2.1

Phenols

0.01

0.11

steam are fed in as inputs, high strength spirit and spent wash (and a little fusel oil) appear as outputs. In batch distillation the strength and congener content will change continually during the duration of the run. In a continuous set up, once started and brought on spirit, the strength and congener concentration vary in different places, particularly as one ascends the rectifier, but stay more or less constant on a given plate for the duration of the run (Fig. 1.6). The majority (about two thirds) of Scotch grain whisky is produced in circular stainless steel (Patent) stills with copper strategically placed for catalytic reasons, particularly in the vapor phase. These distilleries have been built, or entirely re-equipped, since the middle of the 20th century. Thus, there has been a major change in the character and individual qualities of grain whiskies available to the blender as stainless-steel stills, usually with higher reflux ratios than the old Coffey still, tend to produce lighter whiskies while conforming always to the legislative requirements on maximum strength. Column distilled spirit is significantly lighter than batch produced malt spirit, containing relatively more alcohol and less congeners. Scotch whisky legislation defines that it must observe a strength upper limit of (less than) 94.8% ABV ensuring that some congeners and hence flavor is retained, and it is not a neutral grain spirit. Indeed, significant variations in congener content can be detected among various grain spirits and this ensures a significant but variable contribution

20

1. Whisk(e)y

FIGURE 1.6 Rectifier contents during continuous whisky distillation (Robson, 2007).

1.3 Whisk(e)y production

21

to flavor and quality in a blend. The congeneric levels of higher alcohols, aldehydes etc. can be determined in the same way as for malt spirit by analysis. As mentioned earlier in this chapter differences arising from the use of different cereals, such as wheat or maize, are likely to be marginal and are not a major issue for blending after maturation (Wilson & Pryde, 2014). Different whiskies from across the world show varying congeneric levels related to methods of production, but particularly the distillation techniques employed (Table 1.6).

1.3.4.3 Non-Scotch distillation US Bourbon whiskey is usually made with a mashbill of over 51% corn with malt and other cereals and distilled through a beer column which strips the alcohol followed by a doubler or thumper where condensation and re-evaporation occurs. Some premium brands, however, have (re) introduced twin pot still set ups (Robson, 2007). There is also extensive use of the multi column still for continuous style light whiskies, with corn predominating in mash recipes. Canadian whisky follows similar patterns to United States with light (column) and heavy (bourbon like) components, sometimes blended prior to maturation. The Japanese whisky industry has tended to adopt Scottish methodology for distillation with malt and grain whiskies being produced, sometimes with several styles at the same distillery. Irish whiskey may be double or triple distilled in both batch and continuous modes, usually, but not always, using malted barley which has not been peated. Wherever triple (batch) distillation is employed the resultant spirit will be of significantly higher strength (80%) than the equivalent double distilled product (from identical wash) and will typically have lower congeneric levels and thus flavor. An analogy to triple distilling exists in column distilling where an additional column may be added between the analyzer/beer column and the rectifier/product column. This is usually termed a purifier or hydro extractive column which, by reduction of the strength of the alcohol from the first column, uses the relative volatility properties to remove higher alcohols and aldehydes from the top of the column before the final product column rectification (Fig. 1.7). This can produce a lighter than normal grain type whisky spirit while still conforming to legislative strength requirements. This issue of copper and sulfur interactions discussed in regard to malt spirit also can impact grain whisky quality, especially while commissioning new stills, as copper requires time and usage to develop a catalytic patina of various copper salts such as oxides and sulfides. The breakthrough into spirit of the extremely low odor threshold compound methyl-2-methyl-3furan-thiol, which has a meaty aroma that does not mature out, is a major defect. If detected, stills must be shut down for cleaning and aeration. Active copper salts and oxygenation also proved important in initial attempts to reduce ethyl carbamate in grain spirit (Cook, 1990).

1.3.5 Maturation 1.3.5.1 Wood (oak) varieties Although other woods have been used for spirit maturation, such as chestnut and mulberry, oak has predominated both because of its structural properties and because of its positive sensory impact on the spirit held. Not all varieties of oak are equally suitable, with those lacking tyloses having unacceptable resultant losses from the ends of staves. The main varieties used for cooperage are Quercus alba (White) from the United States. Q. robur (pendunculus) and Q. petrea (sessile) from Europe. Q. crispula (mizunara) is also used

22 TABLE 1.6 Major volatile congener concentrations (g/100L abs. alc.) in single samples of Scotch malt, grain, and blended whiskies, and Irish, American, and Canadian whiskies. Acetaldehyde

Methanol

Ethyl acetate n-Propanol Isobutanol

2-methylbutanol

3-methylbutanol

2- and 3-methylbutanol

Ratio 1

Ratio 2

Scotch malt whisky

17

6.3

45

41

80

46

130

176

2.2

2.8

Scotch grain whisky

12

8.5

18

72

68

6

17

23

0.3

2.8

Scotch blended whisky

5.4

8.9

23

55

62

19

53

72

1.2

2.8

Irish whiskey

4.1

10

13

28

15

13

36

49

3.1

2.8

Kentucky bourbon whiskey

15

17

89

28

160

104

281

385

2.4

2.7

Canadian whisky

3.3

7.9

7.1

6.2

6.9

5

11

16

2.3

2.2

Ratio 1= 2- and 3-methylbutanol/isobutanol. Ratio 2= 3-methylbutanol/2-methylbutanol.

1. Whisk(e)y

Whisky type

1.3 Whisk(e)y production

23

FIGURE 1.7 Relative volatility of congeners to ethanol (Robson, 2007).

in Japan. All varieties have closely related species and hybridization is common so not all wood in use may be varietally pure (Gollihue, 2021). The oak wood structure is comprised of cellulose, hemi-cellulose and lignin. It also contains hydrolysable tannins and volatile compounds, containing large numbers of both aliphatic and aromatic volatile oils, and it is these which are of particular interest (Masson & Puech, 2000). Medullary rays and tyloses are primarily seen as being key structures to minimize leakage but are also reservoirs of tannins and lignin. Hydrolysable tannins include compounds such as gallic and ellagic acid (Table 1.7). The growing area, the soil fertility (or lack thereof) and the climatic conditions experienced during growth will affect early and late growth ring size and resultant soluble extract (Gollihue, 2021).

1.3.5.2 US spirit barrel manufacture The manufacture of American spirit and European wine casks follow similar but not identical routes. After-bourbon casks (American Standard Barrels or ASBs) are made exclusively from American Quercus alba (and closely related varieties) and these will be considered first. Suitable trees are identified, felled, and cut into bolts before being sawn into stave blanks which are seasoned and dried in ambient conditions and/or in heated kilns. After drying the blanks are tapered, backed, and hollowed before being raised into barrels with hoops and ends. This entire process could be regarded as a simple sequence of joinery type tasks but the details of how this is carried out will have a major impact on the resultant whisky quality.

24

1. Whisk(e)y

TABLE 1.7 Oakwood components. Component

Percentage

Cellulose

40-50%

Virtually inactive

Hemicellulose

25-30%

Furfural

Almond/grainy

Maltol

Sweet

Coniferyl

Piney

Guaiacol

Smoky

Vanillin

Vanilla

Lactones

cis & trans oak lactones

Spicy/woody coconut

Tannins

Gallo and Ellagitannins

Astringent

Other

Eugenol

Cloves

Hexanal

Grass

2-Octenal

Green leaf

Lignin

Wood Extractives

Sub component

20-25%

5-10%

Selected Compound

Flavor note

Research by Swan at Pentlands Scotch Whisky Research (PSWR, now The Scotch Whisky Research Institute) in the early 80s demonstrated that new (after bourbon) barrels were delivering significantly less color and solubles year on year in maturation compared to those of only a few years before. It transpired that, in order to cut inventory costs and speed up the process, there had been a major shift from air drying to kiln drying the stave blanks (Swan, 1987). This “sterilization” of the timber resulted in a prevention of the natural fungal degradation of the exterior of the wood which gives rise to much of the soluble material and flavor. Subsequent research has identified Aureobasidium pullulans as a key organism whose hyphae growing into the wood help break down the long wood polymers. Oak hemicelluloses, such as methylglucuonic acid can break down into simple sugars such as xylose and rhamnose. Vivas and Glories (1993) concluded: “The natural seasoning of the oak wood process is something more than a simple dehydration process, it is more similar to a maturation process in which the wood modifies its chemical composition, along with its physical properties.” The subsequent severe charring, where the inside of the cask is set alight, will sterilize barrels and degrade many wood components but kiln drying alone results in much less soluble material being available to mature whisky. Charring on a scale of one (light) to four (“alligator”) results in varying levels (depth) of damage to the internal barrel surface. However, higher temperatures and charring do not lead to higher color as the charcoal layer is inert and if charring is carried out too severely this leads to the lactones that have been formed degrading. Hemicellulose will break down to furfural (toast aroma), 5-hydroxymethyl furfural (fudge) and 5-methyl furfural (sweet caramel). Lignin breaks down to a variety of very important flavor components including vanillin, syringaldehyde, coniferaldehyde (a precurser of vanillin), and guaiacol. Moderate toasting may yield 4-methyl guaiacol which is perceived as “spicy” while higher temperatures degrade the guaiacols to phenols which are perceived as “smoky.”

1.3 Whisk(e)y production

25

Cis and trans lactones need special mention giving coconut-like flavors. The proportions of cis and trans vary from species to species, while presence of four isomers indicate artificial oak extracts have been used. Eugenol is an aromatic oak extractive which adds clove or cinnamon notes. Some extractives demonstrate synergistic effects while the active carbon layer formed by charring will remove sulfury notes as will be covered later.

1.3.5.3 European wine casks When European oak is used for casks it is normal to use air drying for extended periods of several years, in part at least to reduce the much higher tannin levels which will be washed out by rain and subject to oxidation processes. European oak bolts tend to be split along the grain rather than sawn, which while less economic in terms of wood usage this may be structurally preferable and give better flavor (Gollihue, 2021). Also, during raising the cask, the wood tends to be lightly toasted by heat from burning shavings, to help with forming the shape rather than being heavily charred from actual combustion. Subsequently, salt water or other treatments may be applied to sterilize the wood. It used to be thought, mistakenly, that the smaller percentage of Sherry casks from Jerez were ex fermentation or Bodega casks made of Spanish or at least European oak. This was incorrect as it now seems that many were so-called “transport casks” made from readily available American white oak lumber imported to Spain. The transport cask no longer exists, and bodega casks rarely come on the market. To achieve the sought-after sherry character, distillers now pay significant sums for bespoke casks to be built and put through a seasoning regime where the cask is used for fermenting grape must for a period of time. Its purpose is to extract woodiness (resin and tannins) from the cask before its use for whisky maturation. A shortage of casks led to a much wider uptake of table wine casks which had not been a feature of the Scottish industry for generations. Initially for many (Glenmorangie’s cask finishes excepted) a resort of some desperation, they are now an established and valued option. Some, particularly red wine casks, had a layer of wood removed internally by a cutting tool. Other blenders did not find the resulting flavor a problem and Katsuki et al. (2017) state “they have a jam-like, sweet, fruity, and sour character… which added complexity to whisky aroma” (Katsuki et al., 2017). The previous contents of casks do not just add flavor themselves but will have interacted with the wood of the cask changing the compounds which subsequent spirit will leach from it. New wood casks, which have not held any other product prior to maturing Scotch have also had a revival in popularity. Careful toasting is used here too to minimize the introduction of “green” wood notes into whisky. The impact of cask components on spirit quality will be covered under “Maturation”. Scotch regulations were recently clarified to allow, with a few specific exceptions, any beer, wine, or spirits casks as well as new wood. 1.3.5.4 Cask rejuvenation Whisky casks are often used multiple times, although each time the wood activity, as most easily demonstrated by color uptake, will reduce until the cask is said to be “spent”.

26

1. Whisk(e)y

A shortage of casks led to experiments, initially crude, to dechar (with metal brushes) and rechar (with a gas burner) the cask to restore, at least to some extent, its activity. As the wood was alcohol infused, rather than “green,” it could burn too easily and introduce acrid notes if used in excess. These regeneration processes are now much more widespread and finely tuned, involving the controlled removal of a layer of depleted wood (Connor et al., 2014). Some wine casks undergo a precise regeneration process resulting in the STR (Shaved, Toasted, Recharred) cask where subtle heat treatment may be applied using infra-red radiation. Others, such as Madeira, Port and Sauternes, are highly valued for the positive contribution they can make to spirit flavor.

1.3.5.5 Warehouse types and conditions 1.3.5.5.1 Filling strengths

The exact amount and nature of material extracted from the cask over time during maturation will vary depending on the alcohol strength of the spirit which has been filled into the cask. Lower strengths will tend to encourage more water soluble lignins and tannins into solution increasing color, while higher proofs will encourage dissolution of lactones and wood oils. Scotch malt whisky tends to be “cut” with soft water to about 63% ABV although some fill casks undiluted at the charge strength off the still. Scotch grain whisky tends to be filled a little higher, up to 70% ABV, but not at charge strength. Despite the attraction for economy of barrel usage and warehouse space, filling strengths above 80% ABV are not practiced as an unfilterable haze of lipids and lignin will elute from the wood. 1.3.5.5.2 Warehousing

Traditionally warehousing took place at individual distilleries with damp earthen floors and dunnage (wooden rails) storage only two or three rows high. This arrangement was superseded by centralized complexes of large buildings with metal racks up to thirteen rows high. In the last few decades, the use of pallets, up to eight high, loaded with barrels, hogsheads or butts standing upright on their ends have become popular for maturation, with no significant quality differences reported. Differences have however been observed between lower and upper situated barrels in (wooden) racked warehouses in Kentucky where temperature fluctuations are more severe (Buffalo Trace, 2014). It was also observed by Connor et al. (2014) that warehouse temperatures and hence evaporative losses are increasing in Scotland due to climate change (Connor et al., 2014). Gonzales-Robles noted “The extraction of wood-derived maturation compounds was favored in general by elevated temperatures (40°C) and higher alcohol content (>63.4% ABV). However, some compounds, such as vanillin and syringaldehyde, were extracted in greater concentrations at lower alcohol levels (40% ABV)” (Gonzales-Robles, 2016). The relative sizes of casks (capacity) and internal surface area are shown in Table 1.8. It demonstrates why a small cask will mature whisky faster than a large one in the same period of time. This has led to some even smaller casks than normal ( 1 tells that the compound i is more volatile than compound j, α ij < 1 means that the compound i is less volatile than compound j, α ij = 1 means it is equally volatile (Decloux and Coustel, 2005). Predicting the vapor–liquid equilibrium (VLE) in fermented beverages is difficult and different models exist (Faúndez and Valderrama, 2004). In beverages, and especially for spirits

47

2.2 Production

(A)

(B) Water Ethyl Acetate Acetaldehyde Acetone

(C)

Water Ethanol Methanol Acetic Acid

(D) Water Isopropyl Alcohol Propanol Isobutanol Isoamyl Alcohol

Water Isopropyl Alcohol Propanol Isobutanol Isoamyl Alcohol

FIGURE 2.2 Relative volatility for congeners in relation to water (Batista and Meirelles, 2011). (A) Relative volatility of the light elements in relation to water, (B) Relative volatility of ethanol and methanol, (C & D) Relative volatility of the higher alcohols in relation to water. Credit: (Batista and Meirelles, 2011).

production, the recommended usage of a heterogeneous approach using the nonrandom two liquid (NRTL) activity coefficient model is common (Renon and Prausnitz, 1968; Valderrama et al., 2012). Distillation column design to separate congeners until only ethanol in the desired concentration remains has received great attention, especially for bioethanol. Multiple studies on neutral alcohol have been carried out over the years (Decloux and Coustel, 2005; Batista and Meirelles, 2011; Valderrama et al., 2012; Batista et al., 2013; Esteban-Decloux et al., 2014). In their work, Batista and Meirelles (2011) calculated the relative volatility of different major congeners derived from sugar cane fermentation. This holds true for other fermentations such as grain mashes, potatoes, grapes, etc. The congeners were segmented into three groups depending on their volatility. Fig. 2.2A represents the most volatile compounds at any given point, where the relative volatility α was greater than 1. Fig. 2.2B represents ethanol and methanol. The fact that the line for ethanol crosses the 1 mark represents the azeotropic point. The figure also shows that methanol has a lower volatility than ethanol (ethanol xm = 0.47) but slightly higher in an ethanol rich environment. Fig. 2.2C and Fig. 2.2D show the relative volatility of higher alcohols from 0 to 1 mass fraction xm ethanol and 0.5 to 1 mass fraction xm ethanol, respectively. The reason for this is the decrease in volatility of the higher alcohols as ethanol in the liquid phase increases. Webb and Ingraham (Webb and Ingraham, 1963) thoroughly discussed the formation and behaviour of fusel oils and list characteristics such as boiling points. All higher alcohols from

48

2. Vodka

Fig. 2.2 possess a boiling point greater than ethanol. Higher alcohols can be classified as intermediate volatile because they also show a greater volatility in a more aqueous environment. It is important to note that volatile flavor compounds do not influence the thermodynamic properties of the system because of their low concentration as reported in previous research (Osorio et al., 2004; Carvallo et al., 2011; Valderrama et al., 2012; Sacher et al., 2013; Puentes et al., 2018). First an industrial distillation process for neutral alcohol shall be discussed. Fig. 2.3 introduces different columns used and labels all streams as quantified by Montevecchi et al. (2021). Differences in plant design for neutral alcohol producing systems have been studied by different researchers (Piggot, 2003; Decloux and Coustel, 2005; Batista et al., 2013); nevertheless, principal structures such as a stripping column, rectifier, as well as ethanol recovery columns are common. The feed, preheated and degassed (removal of CO2 ), is continuously fed into the stripping column. In this column, the solid biomass is removed into the stillage (50 nm diameter) r mesopores (2–50 nm diameter) r micropores (15

Nitrogen content

2.0–2.5

Titratable acids (mL 0.1N alkali/10 mL)

1.5–2.0

Phosphoric acid (mg/100 mL)

600–750

temperature is raised to 70°C after which the diluted molasses is transferred to a settling tank and further diluted to the desired working specific gravity (16–20°Bx). The main carbon source for yeast in molasses is sucrose, followed by glucose and fructose. Molasses also contains a range of organic nitrogen compounds such as amino acids —amino acids are essential for yeast growth but also significant in the formation of flavor compounds— and minerals such as phosphorus, potassium, sodium, calcium, sulfur, and magnesium. A typical must composition is given in Table 3.1. Although suitable for fermentation by yeast, for best success, inorganic nitrogenous compounds such as ammonium phosphate, ammonium sulfate, or urea are commonly added (Bluhm, 1983). A must low in inorganic nitrogen yields high fusel oil due to amino acid deamination and decarboxylation. Pasteurization is also typically carried out on molasses to destroy unwanted microorganisms. The raw material for cachaça is natural and fresh sugarcane juice. As described above, to avoid the loss of cane quality and formation of higher alcohols, sugarcane culms should be processed within 24 hours. Milling is typically carried out using cylindrical compressors followed by filtration of the juice through a fine mesh sieve. The juice extracted during milling should have a sugar concentration ranging from 18% to 22%. For ideal fermentation, however, the must should contain 14–16% of sugars. Pitching undiluted juice will result in slower, incomplete fermentation and the formation of undesired compounds in the final spirit. Therefore, dilution with potable water is usually necessary. Water filtration and dechlorination are also of paramount importance to preserve yeast performance.

3.4.2 Fermentation process The most common way of conducting fermentation to produce sugarcane spirits is in batches fed with recycled yeast cells obtained through decantation. This process recycles the yeast cells decanted in the wine, which occupy from 17% to 20% of the usable volume of the fermentation tank and are left there as the inoculum for the next fermentation cycle. This avoids the need to inoculate with new yeast every cycle. Continuous fed-batch systems are typically carried out in large-scale operations with the addition of nutrients or substrate in small concentrations throughout the fermentation. According to ideal practices, the must pH has to be 4.5–5.5 for cachaça and 5.5–5.8 for rum. The pH influences fermentation and consequently the metabolic products formed. A lower pH yields relatively higher amounts of alcohol, with the formation of a range of congeners, typically increasing at higher pH. Lowering the pH is thought to encourage the growth of

3.4 Production process

65

Schizosaccharomyces yeasts in comparison to Saccharomyces yeasts during fermentation. If a combination of yeast and bacteria are used, as in open or wild fermentation or those involving fermented dunder, the pH should be kept around 5 or higher. The alcoholic fermentation must be conducted under controlled temperature; typically, 28– 32°C for cachaça, and 31–32°C for rum. Heating and cooling systems are important to control the temperature during fermentation and control the production of secondary compounds by yeast (Caruso et al., 2008). In the production of light rums, fermentation generally takes 20–26 hours, with each cycle of fermentation in cachaça production normally 14–24 hours. Spontaneous fermentation, often used in the production of heavier rums and cachaça, can last 2–4 days, or up to 1–3 weeks. Alcoholic fermentation is typically complete within 72 hours with the remaining fermentation time included promoting the formation of flavor compounds. Volatile congeners are produced during fermentation. The excessive formation can be controlled using preventive measures. The content of each volatile congener is measurable only in the final spirit and monitoring procedures are generally not applicable during the fermentation. Therefore, corrective actions are typically taken during the distillation process. Volatile acidity, measured in terms of acetic acid, is a consequence of bacterial contamination during fermentation. Acetic acid bacteria ferment the wine and increase its acidity, while lactic bacteria comprise approximately 76% of the microbiological contaminants frequently found in the sugarcane spirits production process. Acidity is one of the principal causes of sensorial rejection of cachaça by consumers (Odello et al., 2009). The acetic acid formation can be avoided by minimizing acetic bacteria contamination during and after the fermentation step. A cleaning-in-place (CIP) system should be applied to equipment and pumps before and after use. Esters and aldehydes are important components of sensory characteristics in spirits. They are related to viscosity and aromatic attributes (Bortoletto, Corrêa, and Alcarde, 2016). Esters are produced during fermentation by yeast, as well as during the aging process as a result of the esterification of fatty acids with ethanol. During the process of fermentation, they originate from the esterification of ethanol with acetic acid and the amount produced depends on the relative abundance of the corresponding alcohols and acyl-coA radicals involved in yeast metabolism (Janzantti, 2004). Ethyl acetate, the major component of this group, is responsible for the agreeable flavor of aged spirits. Aldehydes containing up to eight carbon atoms have a disagreeable aroma and those containing more than 10 carbon atoms confer disagreeable taste and aroma to beverages. Excess of these compounds affects the aromatic balance of cachaça and is rejected by consumers. Preventive measures include avoidance of autochthonous non-Saccharomyces or wild yeast and multiple yeast recycling. Higher alcohols produced by yeast include n-propyl, isobutyl, and isoamyl alcohol. Their presence is important for the aromatic characterization of cachaça, but when in excess, they cause negative effects. The main strategies for controlling the production of these alcohols are: maintaining the ideal temperature during fermentation (28–32°C), using proprietary yeast, controlling the pH of must (≥4.0), avoiding intense oxygenation in the fermentation tank, and reducing the period between the end of fermentation and the beginning of distillation (Cardoso, 2013). When excessive higher alcohols are formed in spirits, it is impossible to remove them, and rejection of the batch is strongly recommended. The most important safety aspect related to fermentation is to avoid the formation of ethyl carbamate precursors. Ethyl carbamate, or urethane, is considered the main contaminant of

66

3. Rum and cachaça

spirits since it is a potentially carcinogenic compound (EFSA, 2007). Brazilian law establishes the upper limit of 210 μg/L ethyl carbamate in cachaça (Brasil, 2005).

3.4.3 Distillation In relation to wash distillation, small and medium producers conduct it in copper pot stills, while larger producers normally use column stills. In small distilleries, the spirit is usually obtained by simple distillation and the separation of fractions must be controlled. Too small a “heads” fraction during this type of distillation can have a detrimental effect on the quality of the final product. Currently, only a few producers perform double distillation. The first distillation is carried out to recover ethanol from wash, whereas the second distillation is performed with the following cuts in the distillate: “heads” (initial 10% of the distilled volume), “heart” fraction or spirit (60% of the distilled volume), and “tails” (final 30% of the distilled volume). Aldehydes, esters, and methanol are regarded as “heads” components because they are more concentrated in the first fractions of the distillate. Their excess can be easily eliminated by controlling the “heads” cut volume (about 1.5% of the pot still volume). Brazilian law limits the concentration of aldehydes in cachaça. Esters, from ethyl hexanoate to ethyl laurate, are also found in the heads cut, with esters of long-chained carboxylic acid (with the acid part longer than 12 carbon atoms) distilling out towards the end of the run (Nykänen and Nykänen, 1983). Acetic acid and furfural are concentrated in the last fraction of the distillate and can be removed by cutting at about 38% ABV in the collection of potable spirits. The demisting test is used to determine the exact point for the “tails” cut. This measures the point at which fine turbidity is noted in the distillate, due to the presence of a substantial content of higher fatty acid esters (Nicol, 2003). Moreover, controlling the “tails” cut during distillation is essential to reduce the acidity formed in the wine. Double distillation can be a great alternative to remove excessive acetic acid from the spirit. Copper contamination in spirits can occur during the distillation process by dissolving the verdigris formed on the inner wall of the still and internal parts, such as deflegmator and coil of the distilling column. Alcohol and acid vapor can dissolve this compound and contaminate the distillate. High copper contents in the spirit are undesirable because they are potentially harmful to human health. Brazilian law established that the maximum permitted content of copper in cachaça is 5 mg/L (Brasil, 2005). Moreover, high levels of copper in cachaça indicate a lack of hygiene. It is recommended to keep the still and streamers filled with water during the breaks that occur in the dry season period. Water reduces copper oxidation, formation of verdigris, and spirit contamination. Heavy metals can contaminate cachaça and rum during the production process, inasmuch as they are present in equipment and tools, in the water used for filtration and standardization, as well as in contaminated equipment during the bottling process. In food industries, all the equipment must be made of stainless steel to ensure safety and avoid metal contaminants (Brasil, 2005). The presence of methanol in distilled spirits is undesirable because of its toxicity. Methanol is concentrated in the “heads” of the distillate and most of it can be removed by controlling distillation or applying double distillation (Bortoletto et al., 2015).

3.4.4 Aging and maturation process The maturation of distilled beverages in wooden casks is an important step of the production process. A fresh spirit presents aggressive sensory features and a strong alcoholic

3.4 Production process

67

flavor. These negative characteristics can be attenuated by aging in appropriate wooden casks for a certain period of time to refine the sensory profile and improve chemical quality. During the aging process, the use of new wood barrels may be considered to control copper contamination. Numerous physicochemical interactions occur between the wood and the spirit during aging. Several phenomena of migration of volatile and non-volatile compounds of wood to spirit take place. Aging is not a mandatory step for cachaça and rum. Brazilian law establishes that aged cachaça should contain at least 50% of the spirit matured in appropriate wooden barrels (maximum capacity of 700 liters) for a period of not less than one year. Premium and extra premium cachaça are spirits completely aged for one year and three years, respectively (Brasil, 2005). The effects of maturation are mainly influenced by the type of cask wood. The mechanism of spirit maturation is based on the exchange of compounds present in wood and beverage, which can be classified into seven categories: direct extraction of wood compounds; decomposition of macromolecules; reaction between wood compounds and the compounds present in the freshly distilled spirit; interactions specifically involving the wood extract; reactions involving only the compounds present in the distilled beverage; evaporation of volatile compounds through the cask surface; and formation of stable molecules (Chatonnet, 1999; Piggott and Conner, 2003). Oak is the principal wood used for spirits aging worldwide because it actively participates in the beverage flavor due to the extraction of aromatic molecules from the wood. However, in Brazil oak is not a native wood and it is necessary to import it from Europe or North America. Native Brazilian woods can be a viable option for sugarcane spirit producers since they are easily found and compounds from each different type of wood may be transferred to the beverage giving unique characteristics. As Brazil has a vast diversity of flora it is very interesting to study the chemical profiles of sugarcane spirits aged in casks made of different Brazilian wood and compare them with those obtained in oak casks. Although several physical and chemical properties of wood may influence the maturation process of alcoholic beverages, a number of studies report direct and indirect effects of the wood, such as species, methods used to treat the wood and make the casks, thermal treatment, and final cooperage operations (Mosedale, 1995). The most common wood species for aging cachaça are amendoim (Pterogyne nitens Tul.), araruva (Centrolobium tomentosum Guillem. Ex. Benth.), cabreúva (Myrocarpus frondosus Allemão), cerejeira [Amburana cearensis (Fr. Allem.) A.C. Smith], grápia [Apuleia leiocarpa (Vogel) J.F. Macbr.], ipê roxo [Tabebuia heptaphylla (Vell.) Toledo], jequitibá [Cariniana estrellensis (Raddi) Kuntze], jequitibá rosa [Cariniana legalis (Mart.) Kuntze], oak (Quercus sessilis Ehrh. Ex Schur.), and pereira (Platycyamus regnellii Benth.) (Bortoletto and Alcarde, 2013). In Brazil, about 25 types of national wood have been used for cachaça maturation in traditional regions. The Ministry of Agriculture, Livestock, and Supply does not define specific woods for aging cachaça (Brasil, 2005). Several scientific researches in Brazil have assessed the chemical and sensory quality of cachaça aged in barrels made of various national types of wood. Sensory and physicochemical analyses show important differences among wood species from different geographical regions, which typify the notes of wood developed in the spirit due to the extraction of peculiar compounds. Therefore, wood species, its geographical region, wood age, and forest management are relevant parameters when choosing the cask, since they interact to define wood quality and, consequently, the sensory and chemical profile of the resulting aged spirit.

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3.5 Filtration, standardization, and bottling After the aging process, filtration and standardization are important steps to remove any solid particles from the barrels, reduce the alcohol content, and avoid future turbidity in the spirit. Particles deposited at the bottom of the bottle or suspended in the liquid are considered physical contaminants of this product. To prevent these flaws and ensure longer shelf life, it is recommended to use good quality water for standardization. Therefore, water should be filtered using specific mineral and heavy metal filters to prevent turbidity in the final product after bottling. The bottling system must be automatic and the process must be carried out in a specific and separate room. To check physical contamination (glass, machine parts, insects) it is advisable to carry out good visual control.

3.6 Quality 3.6.1 Flavor profile The flavor is a product of all of the substrates and conditions used during manufacture, as described above, as well as the sum and synergy between all of the volatile compounds that make it through to the final spirit (Oliveira et al., 2005). The fermentation stage is responsible for basic flavor formation impacted by the choice of the yeast strain, the presence of bacteria, and the fermentation conditions. Distillation then affects the proportion of congeners recovered, and maturation results in the addition as well as the removal of flavor compounds. Within the rum category, a distinction is made between “heavy”, “light”, or “medium” style products according to their flavor or aroma profile. Cachaça similarly has two varieties: unaged (white or silver) and aged (yellow or gold). Esters are the key compounds in rum and cachaça due to their fruity-like aroma. The ester content of rum varies from 4% to 64% w/v and depends on a range of factors including yeast strain and fermentation conditions as well as the inclusion of yeast in distillation. The main esters are acetate-esters of alcohols, for example, ethyl acetate, and ethyl esters of fatty acids, for example, ethyl decanoate (Table 3.2). Cachaça is similar to rum as it shares a similar concentration of apple-smelling esters and the ketone β-damascenone. Higher alcohols are the most abundant aroma compounds in rum providing alcoholic, winelike, and whisky-like notes (Nykänen and Suomalainen, 1983; Table 3.2). The main factors influencing the formation of higher alcohols include yeast strain and distillation method; typically, larger amounts of higher alcohols are found in rums distilled in pot stills compared to continuous stills. The presence of 2-butanol appears to be related to the action of bacteria during fermentation (Nykänen and Nykänen, 1983). Other flavor compounds significant in both rum and cachaça include organic acids, typically formed by strains of Saccharomyces cerevisiae. Acids are both precursors to esters as well as flavor active in their own right. The total acid content in heavy-bodied rums is about 100–600 mg/L, with acetic acid being the predominant volatile acid. Heavy-bodied rums contain more volatile acids than light rums, as do aged products (Nykänen and Nykänen, 1983). Aliphatic carbonyl compounds including aldehydes, ketones, and acetone are associated with spirit flavor, as are acetals. Aldehydes can total up to 5–9% w/v in Jamaican, Puerto Rican, and Martinique rum, acetaldehyde being the predominant compound (Nykänen and Nykänen, 1983). As many as 43 acetals have been identified in rum, most of which are formed during distillation (Nykänen and Nykänen, 1983).

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TABLE 3.2 Important flavor volatiles in rum (Salo, Nykanen, and Suomalainen, 1972; Burnside, 2012; Pino et al., 2012). Compound

Typical concentration (ppm) Flavor threshold (ppm) Sensory attribute(s)

Esters Ethyl acetate

73–200

3–6.6

Pineapple, fruity, solvent

4.19

0.0095

Pineapple

10

150

Ethereal, fruity, rum-like

5–10

3.4

Banana, estery

Ethyl propionate

50

>4

Ethereal, fruity, rum-like

Ethyl butyrate

220

0.15

Ethereal, fruity, apple, buttery

Ethyl valerate

40

0.0015

Fruity, apple

1–40

0.001

Fruity, winey, apple, banana, pineapple

15–50

0.015

Sweet, cognac, apricot

25–130

0.51

Sweet, fatty, nut, winey-cognac

Ethyl dodecanoate

12–15

2

Oily, fatty, floral

Ethyl hexadecanoate

5–50

>2

Waxy, fruity, creamy, greasy, oily, balsamic

Ethyl linoleate

50

0.45

Fatty, fruity, oily

Ethyl lactate

10

14

Buttery, butterscotch, fruity, artificial strawberry, raspberry, perfumed

Methyl salicylate

0.5–25

0.1

Wintergreen

Ethyl 2-methylpropanoate∗

0.14

0.0045

Ethereal, sweet, alcoholic

Ethyl 2-methylbutanoate∗

0.02

0.0016

Fresh, fruity

2-phenylethyl acetate∗

0.12

0.108

Rose, honey, raspberry

0.11

0.001

Fruity, apple

Acetic acid

10–35

22–26

Vinegar

Hexanoic acid

0.3–15

8

Goaty, fatty, vegetable oil

Butyric acid

0.3–7.5

>2

Rancid

Valeric acid

0.04–7.5

>0.75

Strong, pungent, cheesy

Ethyl

butanoate∗

Ethyl formate Isoamyl

Ethyl

acetate∗

hexanoate∗

Ethyl octanoate∗ Ethyl

decanoate∗

Ketones (E)-β-damascenone∗ Acids

(continued on next page)

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TABLE 3.2 Important flavor volatiles in rum (Salo, Nykanen, and Suomalainen, 1972; Burnside, 2012; Pino et al., 2012)—cont’d Compound

Typical concentration (ppm) Flavor threshold (ppm) Sensory attribute(s)

Propionic acid

0.2–1.5

20

Vinegar, milky

Octanoic acid

4.3–7.5

13–15

Goaty, fatty, vegetable oil, wet dog

1-propanol

7.5–420

3–9

Alcohol

1-butanol

10

>0.5

Malty, solvent-like

Isobutanol

100

75

Malty, alcohol

2-methyl-1-butanol

200–210

0.18

Fish oil, green, malt, onion, wine

3-methyl-1-butanol (isoamylacetate)

860–1000

6.5

Fruity at low levels, unpleasant at high

2-phenylethanol∗

10.40

2.6

Floral

9.36

0.719

Ethereal, green, nutty, earthy, sweet, vegetable

2-methoxyphenol (guaiacol)∗

0.65

0.0092

Smoky

4-methylguaiacol

0.05

0.01

Vanilla, clove

4-Ethylguaiacol∗

0.03

0.0069

Bacon, clove, phenolic

4-Propylguaiacol∗

0.34

0.02

Spicy, sweet

Eugenol∗

0–1.36

0.007

Clove-like

Vanillin∗

0.25

0.1–2

Vanilla

Cis-oak lactone∗

2.41

0.067

Sweet, spicy, coconut, vanilla

γ -nonalactone∗

0.15

0.021

Coconut, creamy, waxy, sweet, buttery, oily

Alcohols

Acetals 1,1-diethoxyethane∗ Phenolic compounds

Lactones

∗

Key odorants in aged rum (Pino et al., 2012).

Finally, phenols, mainly formed during maturation via alcoholic extraction from the oak casks, are compounds present in small amounts but have very low flavor thresholds meaning that they can have a significant impact on the overall flavor. They can impart flavors such as spicy, woody, and vanilla (Table 3.2). Cachaça typically has a more spicy character than rum due to higher levels of eugenol, 4-ethylguaiacol, and 2,4-nonadienal (De Souza et al., 2006).

3.7 Future

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3.6.2 Sensorial properties Sensory tests are important tools in the food and beverage industry, as they represent a multidimensional measure of the product, with important advantages, such as the ability to identify the presence or absence of perceptible differences, quickly describe important sensory characteristics, detect particularities that cannot be detected by analytical procedures and assess product acceptance. The sensory evaluation of cachaça and rum is of great importance to define their quality. The harmony between the components of the drink creates the smooth, pleasant, and characteristic set of aroma and flavor of the distillate. It is necessary that quality control be maintained throughout the beverage production process, whether in a large industry or a small distillery. The identity of cachaça and rum depends on both physical-chemical and sensory characteristics. These parameters allow the distinction between products through three basic aspects: raw material, characteristics of the final product, and its sensorial profile. The sensory profile is improved by aging, which is usually carried out in appropriate wooden containers, where it resides for a period of time. The need for further studies related to the sensorial quality of cachaça is urgent, considering the increase in demand and exportation and expansion of the sector, mainly in Brazil due to more than 40 species of tropical wood used for aging and stockage. The development and standardization of cachaça sensory descriptors are important to expand beverage qualification techniques and drive greater investments in quality. The Sensory Wheel (Fig. 3.2) is an instrument for fast descriptive profile determination in cachaça and sugarcane spirit. The use of the wheel is intended both for experienced tasters who work professionally in sensory analysis of cachaça, as well as for training new tasters and evaluators of the beverage’s quality. Its purpose is to activate sensory memory and provide descriptive, standardized, and organized terms.

3.7 Future As a finished product, distilled spirits need to meet the acceptance criteria of consumers including those of appearance, flavor, and aroma. They should also meet legal and technical specifications and be free from faults. Cachaça has continued to evolve in terms of quality, safety, innovation, presentation, and sustainability. In this trajectory, the Cachaça Production Chain Sectorial Chamber, installed at Ministry of Agriculture, Livestock and Supply (MAPA), published and presented on December 17, 2021, the Strategic Guidelines for Cachaça 2021–2025. With the bold aspiration of being recognized, in 2025, as one of the three main spirits in the world, the cachaça sector fulfills its mission, always with responsibility, looking to the future and focusing on the connoisseur. Tasted pure, in the form of Caipirinha, fruit shakes or other drinks, or even in pairings with dishes of different textures and complexities, cachaça opens up a range of opportunities to enrich gastronomic experiences on a global scale. Thus, cachaça, as a national icon, is understood by Brazilians as a symbol and expression of Brazilianness—an attribute of a shared ancestral heritage, the motto of an entire people—the Brazilian people. Always enjoyed with style, elegance, and moderation (Jairo Martins, cachaça specialist and author).

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FIGURE 3.2 Cachaça Flavor Wheel (Inovbev Beverage Research and Development).

References Arroyo, R., 1945. Studies on Rum. Research bulletin. Agricultural Experimentation Station, University of Puerto Rico 5. Bluhm, L., 1983. Distilled beverages. In: Rehm, H.-J., Reed, G. (Eds.), Biotechnology, a Comprehensive Treatise in 8 Volumes, 5. Verlag Chemie, Weinheim, Germany, pp. 447–475 1983. Bortoletto, A.M., Alcarde, A.R., 2013. Congeners in sugarcane spirits aged in casks of different woods. Food Chem. 139, 695–701. Bortoletto, A.M., Alcarde, A.R., 2015. Assessment of chemical quality of Brazilian sugarcane spirits and cachaças. Food Control. 54, 1–6.

References

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Bortoletto, A.M., Corrêa, A.C., Alcarde, A.R., 2016. Fatty acid profile and glycerol concentration in cachaças aged in different wood barrels. J. Inst. Brew. 122 (2), 293–298. Brasil. Ministério da Agricultura, Pecuária e Abastecimento. 2005. Instrução Normativa no. 13. Aprova o Regulamento Técnico para Fixação dos Padrões de Identidade e Qualidade para Aguardente de Cana e para Cachaça. Diário Oficial da União, Section 1, p. 3, June 30. Available from http://extranet.agricultura.gov.br/ sislegisconsulta/consultarLegislacao.do?operacao=visualizarandid=12386. Burnside, E., 2012. Characterization of volatiles in commercial and self-prepared rum ethers and comparison with key aroma compounds of rum. Urbana, Illinois. Cardoso, M.G., 2013. Sugarcane Spirit Production. = Produção de aguardente de cana. UFLA, Lavras, MG, Brazil (in Portuguese). Caruso, M.S.F., Nagato, L.A.F., Alaburda, J., 2008. Evaluation of alcohol content and secundary compounds in cachaças. = Avaliação do teor alcoólico e componentes secundários de cachaças. Revista Instituto Adolpho Lutz 67, 28–33 (in Portuguese). Chatonnet, P., 1999. Discrimination and control of toasting intensity and quality of oak wood barrels. Am. J. Enol. Vitic. 50, 479–494. Coulombe, C.A., 2005. Rum: The Epic Story of the Drink That Conquered the World. Citadel Press, Brasilia, Brazil. De Souza, M.D., Vásquez, P., Del Mastro, N.L., Acree, T.E., Lavin, E.H., 2006. Characterization of cachaça and rum aroma. J. Agric. Food Chem. 54 (2), 485–488. Duarte, W.F., de Sousa, M.V.F., Dias, D.R., Schwan, R.F., 2011. Effect of co-inoculation of Saccharomyces cerevisiae and Lactobacillus fermentum on the quality of the distilled sugarcane beverage cachaça. J. Food Sci. 76 (9), C1307– C1318. EFSA. European Food Safety Authority, 2007. Ethyl carbamate and hydrocyanic acid in food and beverages. EFSA J. 551, 1–44. Available from http://www.efsa.europa.eu/sites/default/files/scientific_output/files/main_ documents/551.pdf. Gomes, F.C.O., Araújo, R.A.C., Cisalpino, P.S., Moreira, E.S.A., Zani, C.L., Rosa, C.A., 2009. Comparison between two selected Saccharomyces cerevisiae strains as fermentation starters in the production of traditional cachaça. Braz. Arch. Biol. Tech. 52, 449–455. Hill, A.E., Salvesen, O., Russell, G., 2018. Deciphering dunder: an examination of rum fermentation. In: Proceedings of the 6th Worldwide Distilled Spirits Conference, Glasgow, United Kingdom, pp. 103–108. IBRAC 2020. Instituto Brasileiro de Cachaça, https://ibrac.net. Janzantti, N.S., 2004. Volatile Compounds and Quality on Cachaça Flavour. = Compostos voláteis e qualidade de sabor da cachaça (Doctoral Thesis). Faculdade de Engenharia de Alimentos. Universidade Estadual de Campinas, Campinas, SP, Brazil (in Portuguese). Mosedale, J.R., 1995. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 68, 203–230. Nicol, D.A., 2003. Batch distillation. In: Russell, I., Stewart, G., Bamforth, C. (Eds.), Whisky: Technology, Production and Marketing, Handbook of Alcoholic Beverages Series, 1. Elsevier, London, pp. 155–176. Nykänen, L., Nykänen, I., 1983. Rum flavour, in Flavour of distilled beverages: origin and development. Piggott, J.R. (Ed.), Ellis Horwood, Chichester 49–63. Nykänen, L., Suomalainen, H., 1983. Aroma of Beer, Wine and Distilled Alcoholic Beverages, 3. Springer Science & Business Media, Berlin, Germany. Odello, L., Braceschi, G.P., Seixas, F.R.F., Silva, A.A., Galinaro, C.A., Franco, D.W., 2009. Sensory evaluation of cachaça. = Avaliação sensorial de cachaça. Química Nova 32, 1839–1844 (in Portuguese). Oliveira, E.S., Rosa, C.A., Morgano, M.A., Serra, G.E., 2005. The production of volatile compounds by yeasts isolated from small Brazilian cachaça distilleries. World J. Microbiol. Biotechnol. 21 (8), 1569–1576. Piggott, J.R., Conner, J.M., 2003. Whiskies. In: Lea, A.G.H., Piggott, J.R (Eds.), Fermented Beverage Production, 1. Kluwer Academic, New York, pp. 239–262. Pino, J.A., Tolle, S., Gök, R., Winterhalter, P., 2012. Characterisation of odour-active compounds in aged rum. Food Chem. 132 (3), 1436–1441. Salo, P., Nykänen, L., Suomalainen, H., 1972. Odor thresholds and relative intensities of volatile aroma components in an artificial beverage imitating whisky. J. Food Sci. 37 (3), 394–398.

C H A P T E R

4 Gin Matthew S.V. Pauley a and Jan Hodel b a

International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh, Scotland b Product Owner – Product Development, Pilz Global Distillery Business Unit, Cork, Ireland

4.1 Introduction Part of the enduring appeal of gin is that this most cosmopolitan of drinks is able to move and change with the times. The invention of the Coffey still in 1830, or maybe even the lesser known but strikingly similar Cellier–Blumenthal still of France in 1813 (Rothery, 1968; Kockmann, 2014), enabled the production of high quality spirit. Such spirit no longer required the addition of fruit or botanicals to mask faults but was suitable for use as a base for flavor creation. With continued exploration of flavor and innovation in production techniques, the gin category remains buoyant with increasing numbers of more educated producers and consumers.

4.1.1 History Much has been written on the origin of gin, with several characters suggested as the inventor of commercial gin. The most notable was the young chemist Franz de la Boe (Hughes, 1992). This is now considered to be a historic misconception as there are records of gin tax being collected before de la Boe was even born (Van Schoonenberghe, 1999). The true origin of gin as we know it today is slightly lost, or maybe not quite as straightforward as authors would hope, especially considering juniper’s historic use in medicine and food by the Vikings (Teixidor-Toneu et al., 2021) and its wider use is a part of the ancient history of humankind (Ložiene and Venskutonis, 2016).

4.1.2 Genever Genever is the forerunner to modern gin, being made of a beer-like fermented base where the botanicals are added and distilled off in what distillers would consider the stripping run (Buglass et al., 2010), as opposed to the high strength and highly rectified spirit used by conventional distillers of “Young” gin (Van Schoonenberghe, 1999). Genever is a more grainand-malt led distillate with a warmer, sweeter flavor than gin as we know it today. If ever you are trying to convert someone with a whisky-accustomed palate to gin, Genever acts as a handy gateway to botanical led distillates.

Distilled Spirits. DOI: https://doi.org/10.1016/B978-0-12-822443-4.00010-4

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4.1.3 Market Gin was seen in the earliest part of this century as the slightly older, less-edgy character of the spirits world (Serjeantson & IWSR, 2021), with the typical gin drinker being stereotyped as a grandmotherly type character. At one point, the UK’s Queen Mother (Elizabeth Bowes-Lyon) was seen as the stereotypical gin drinker. However, over recent years gin has gone through a renaissance. Total gin consumption is set to rise at +4.5% 2021–2025 Compound Annual Growth Rate (CAGR) according to the IWSR Drinks Market Analysis (Serjeantson & IWSR, 2021), with emerging markets being the main driver of growth for the sector. Pink gin was the main driving force behind growth in 2018 (Hopkins, 2019). It is an undeniable truth that social media has changed the game in almost every sphere of life (Jung and Jeong, 2020); none more so than in advertising. Social media has meant that small producers can gain greater reach for much less capital outlay than within conventional media (Mannonen and Runonen, 2008). This has played a significant part in reviving the gin category. There has also been a rise in demand in recent years for online shopping for groceries and other foodstuffs such as distilled spirits which has further fueled gin sales (Archer, 2021).

4.1.4 Regulations The regulatory landscape between the different markets worldwide varies considerably, from botanicals being accepted and Generally Recognized as Safe (GRAS) to formulations needing prior approval (Alcohol and Tobacco Tax and Trade Bureau, 2007) before being allowed into the market. At its most fundamental, the regulations within Europe and the UK (currently) define gin as “a juniper-flavored spirit drink produced by flavoring ethyl alcohol of agricultural origin with juniper berries (Juniperus communis L.)” and regarding flavor “using flavoring substances or flavoring preparations or both shall be used for the production of gin so that the taste is predominantly that of juniper” (European Commission, 2019). The EU Regulations also define different styles of gin, as outlined in Table 4.1. The regulations above are both tight in their approach in one aspect, but also have some room for debate and creativity (European Commission, 2019). Other global regulations include:

r In the United States, gin is classified as spirit with “a main characteristic flavor derived from juniper berries produced by distillation or mixing of spirits with juniper berries and other aromatics or extracts derived from these materials” (Alcohol and Tobacco Tax and Trade Bureau, 2007). r According to the Russian regulation Gosudarstvennyy Standart (GOST) 34149, gin must have an aroma of juniper and spices and a taste that is soft, harmonious with a taste predominately of juniper without foreign after tastes (Rosstandtart, 2021). r The Australia New Zealand food standard code 2.7.5—Spirits describes in nonspecific terms the panoply of possible spirits which includes gin, a brief mention of everything being derived from food and then “so as to have the taste, aroma and other characteristics generally attributable to that particular spirit.”

4.1.5 Other styles of gin Other styles of gin that are not specified within the European regulations, but should be mentioned, are:

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77

TABLE 4.1 Gin-related excerpts from the European Regulation 2019/797. Gin Juniper flavored spirit drink. Produced by flavoring spirit of agricultural origin with juniper berries (Juniper communis. L). Minimum alcoholic strength by volume (ABV) 37.5%. Flavoring substances and/or preparations should be used so that the taste is predominantly juniper. The term gin can be supplemented by the word “dry” if no sweeteners are used beyond 0.1g of sweetening products per Liter of final product. Distilled gin Juniper flavored spirit drink produced exclusively by distilling ethyl alcohol of agricultural origin. The spirit used must be a minimum of 96% ABV. The combination of distillates of spirit of agricultural origin/flavoring preparations that keep the predominant flavor as juniper. Minimum ABV 37.5%. Must not be produced purely by essences. The term gin can be supplemented by the word “dry” if no sweeteners are used beyond 0.1g of sweetening products per Liter of final product. London dry Exclusively spirit of agricultural origin with a maximum methanol content of 5g per 100 L of 100% alcohol. Distillate must be 70% ABV. Additional spirit must also be of the same quality. No coloration. Must not be sweetened in excess of 0.1 g of sweetening product per Liter of final product expressed as final product. Must not have any additional ingredients besides those previously mentioned and water. Minimum ABV 37.5%. The term “London gin” may be supplemented by or incorporated with the term “dry”.

Old Tom Apocryphally named as a sign with an old tom cat which was used to signify gin availability to the illiterate. Characterized by the sensory attributes of sweetness, either by addition of sweeteners including sugar, but also the addition of licorice to the recipe. Sloe gin The most traditional non-juniper led gin with steeping of the fruit of the blackthorn bush or sloe (Prunus Spinosa), originally consumed as a hunting drink and made at small domestic scale. The tart, tannin rich fruit are steeped in a glass jar with gin and sugar. Recipes, ratios, and prime bush locations can be closely guarded family secrets. There is talk of piercing the berries with

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a silver needle, but most contemporary makers resort to the more modern, but less romantic, method of freezing the berries to destroy the internal cell structure.

Pink gin Part gin style, part color, part industry phenomenon, this gin style has had a meteoric rise within recent years, invigorating the category and creating waves within the market (Serjeantson, 2021). At best there is a subtle blend of berries which work with the gin, in a similar manner to sloe gin. At worst there is a total obliteration of all gin character which gives the drink any kind of subtlety and sophistication, with the addition of inverted sugar syrup and synthetic, nonspecific “pink” berry flavoring. Speciality gins As the gin sector grew dramatically within the last decade, there were an increasing number of gins entering the market. As is always the case, they were not all strong offerings, and many have come and gone. There was considerable backlash from the gin industry over so called “nongins,” that is the marketing of something completely out of specification of any of the regulations described earlier in this chapter, that are effectively misleading the consumer and diluting the name of gin. It is, however, worth bearing in mind that the likes of orange gin and lemon gin have been around for as long as gin has been on the market. This then becomes a labelling issue and so long as it is prefixed with the additional flavor, neither trading standards, nor the purists can complain too hard.

4.2 Raw materials The raw materials are a vital part of the matrix of any spirit; what you put into a still has a direct effect on the final product. Gin, as defined under EU law, in its most simple state, consists of only three ingredients: potable alcohol, water, and juniper. However, most gins include a range of botanicals (typically 6–9), with some containing in excess of 40.

4.2.1 Ethyl alcohol The main function of ethyl alcohol, often referred to as grain neutral spirit (GNS), beyond being the alcoholic content of the gin, is to form half of the matrix that the flavor constituents are dissolved in. The term grain neutral spirit can be confused with neutral spirit which is potentially misleading given that any feedstock of agricultural origin, e.g., molasses, can be used. Grain neutral spirit must be made from an exclusively grain based feedstock. The key point is that it cannot be the product of hydrocarbon extraction and cracking. The characteristics required are purity and strength which are intrinsically linked. They need to act as a canvas for the delivery of the flavor constituents without adding too many characteristics of their own. The purity of the spirit not only effects the flavor delivery directly due to stability, haze formation and off notes, but also indirectly due to increased solvent capacity. As mentioned above, the alcohol strength is required to be above 95% or 96% ABV minimum, depending on regulation. For further information on neutral spirit, please see Chapter 2: Vodka.

4.2 Raw materials

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4.2.2 Water Composition of the water added post-distillation is more of a concern than added predistillation, as any compounds likely to cause problems in the final product will be left behind in the still. The water added post-distillation for dilution to bottling strength must be heavily treated (potable and demineralized) to ensure shelf-life stability, prevention of haze and of unsightly flaky precipitates. The normal regime for using towns water (from the mains supply) is to run the water through an activated charcoal filtration system to remove chlorine, followed by demineralizing to a conductivity of around 5–10 microsiemens (Price, Veolia).

4.2.3 Botanicals and their flavors 4.2.3.1 Terpenes Before we explore specific botanicals in more detail, we need to discuss terpenes, the main group of flavor compounds present in gin. Considering that this is one of the most diverse classes of compounds possible within biology (Gershenzon and Dudareva, 2007), the following descriptions will only introduce the subject. For further studies, the literature from Eberhard Breitmaier, Terpenes Flavors Fragrance (Breitmaier, 2006) and Chemistry of Essential Oils of Flavor and Fragrances (Hüsnü et al., 2007) will give the reader an excellent start point. The term “terpene” originates from turpentine (lat. balsamum terebinthinae). The nomenclature of terpenes is structured around the number of 2-Methylbutane, isoprene units, present. Isoprene units contain five carbon atoms, (C5 )n , and according to Ruzicka (1953), terpenes are formed by head-to-tail rearrangement of two or more isoprene units (Fig. 4.1). This rule is called the “biogenetic isoprene rule”. The synthesis pathway via acetyl-CoA is the basic pathway, shown in Fig. 4.2, which is from the conversion of acetyl-CoA into mevalonic acid to the isopentenyl pyrophosphate, also isomerized to dimethylallyl pyrophosphate, the precursor to the isoprene unit (Bramley, 1997). A second pathway, the nonmevalonate or deoxyxylulose phosphate pathway, starts off with the condensation of glyceraldehyde phosphate and pyruvate affording 1-Deoxy-Dxylulose 5-phosphate (DOXP). Through a series of reactions Isopentenyl pyrophosphate (IPP) and Dimethylallyl pyrophosphate (DMAPP) are formed (Heldt and Piechulla, 2011). From this formation, monoterpenes for example, are formed from GPP–Geranyl pyrophosphate. They can broadly be divided into three subgroups: acyclic, cyclic (which is further divided FIGURE 4.1 Hemiterpenes (isopsrene or 2-methyl-1,3-butadiene) (Hüsnü et al., 2007).

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FIGURE 4.2 Outline of the conventional mevalonate-isoprenoid pathway in higher plants MVAP, MVAPP and HMGCoA denote mevalonate phosphate, melvonate pyrophosphate, hydroxymethylglutaryl CoA, respectively. IPP, DMAPP, GPP, FPP, and GGPP are the pyrophosphate esters of isopentenol, dimethyl alcohol, germinol, farnesol, geranyl geraniol, respectively (Luthra et al., 1999).

into mono, bi-, and tricyclic), and irregular monoterpenes. Further alterations can be found due to removal (reduction) of double bonds, as well as by addition of oxygen (oxidation) to form alcohols (-OH), ketones (-CO), aldehydes (-CHO), and esters (-OCO-) (Hüsnü et al., 2007; Zachariah et al., 2012). Through these pathways, a huge array of different terpenes is formed. The reasons why plants expend the energy required to make such compounds are as diverse, but include attracting pollinating insects, discouraging herbivores, and even actively encouraging forest fires in order to acquire an advantage over their competitors in the aftermath (Pichersky and Raguso, 2018). In principle, almost all plants are able to produce essential oils, defined as aromatic and volatile liquids obtained from plant material (Ríos, 2016). Terpenes form part of these essential oils (EOs), although some are only produced in trace amounts. In 1978, Clutton and Evans (Clutton and Evans, 1978) analyzed five different gins, three London Dry gins, one Plymouth gin, and one Geneva gin, and using Gas Chromatography Mass Spectrometry (GC-MS) directly compared the level of eight terpenes with their odor thresholds. This work was extended by (Vichi et al., 2005) where 70 compounds were identified and 66 compounds quantified in four London Dry gins, one Plymouth Gin and one Mahon gin (Spain). In a follow on study, this research team measured ten diterpenes in eight commercial gins of

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different origin and confirmed identification with juniper extracts (Vichi et al., 2008). Manool, manoyl oxide, and trans-totarol were identified in the highest concentration. The highest diterpene concentration was found in Mahon gin (wine base) and may give the possibility to use this method for authenticity tests on gin. Later, key flavors in gin were assessed by Mac Namara et al. (2007). For this, a mixture of 10 essential oils obtained from classic gin botanicals was used. Within these mixtures, a total of 101 terpenes were identified and used to create a compound library for identifying volatile compounds in gin including target ions based on relative abundance.

4.2.3.2 Overall botanical contribution It is important to be able to look at botanicals individually in order to fully understand their contribution to the whole. The information depicted in Fig. 4.3 and presented in Table 4.2 is based on Gas Chromatography-Mass Spectroscopy (GCMS) analysis of spirits produced from each of the core botanicals, distilled under industry replicable conditions by the steep infusion method at laboratory scale. Information on the compounds present in the conventional botanicals can be used as a starting point to consider other potential botanical sources of these compounds in order to add layers of complexity. The following sections give further details on key botanicals typically found in gin recipes. The most commonly used botanicals are shown in Table 4.2. A concise but by no means comprehensive list of volatiles found within gin can be found in Table 4.3 and where recorded there is a brief description of the aroma of the compound in isolation.

Lemon

Juniper

Juniper Orange Licorice

Cardamom Coriander

Juniper

Angelica

Orris

Cassia

FIGURE 4.3 The interconnected relationship of the volatile content of the botanicals based on the common compounds found within distillates (relating to Table 4.2) (M.S.V. Pauley, 2017).

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TABLE 4.2 Key flavor compounds found in common gin botanicals and their aromatic qualities (Vichi et al., 2007; Orav et al., 2010). Botanical

Compound detected

Aromatic qualities (good scents, company)

Juniper

α-pinene

Fresh, camphor, sweet, pine, earthy, woody

β-pinene

Dry, woody, resinous, pine, hay, green

α-cubebene

Herbal, waxy

β-phellandrene

Minty, terpenic

Coriander

Cassia

Angelica Root

Lemon

Orange

Cardamom

α-phellandrene

Citrus, herbal, terpene, green, woody, peppery

DL-limonene

Citrus, lemon, orange, fresh, sweet

γ -elemene

No aroma reference found

α-pinene

Fresh, camphor, sweet, pine, earthy, woody

α-phillendrene

Citrus, herbal, terpene, green, woody, peppery

β-linalool

Citrus, floral, sweet, bois de rose, woody, green, blueberry

Acetic acid, geranyl ester

Floral, rose, lavender, green, waxy

α-cubebene

Herbal, waxy

Humulene

Slight bitter, woody, oceanic

α-farnesene

Citrus, herbal, lavender, bergamot, myrrh, neroli, green

α-cubebene

Herbal, waxy

Humulene

Slight bitter, woody, oceanic

DL-limonene

Citrus, lemon, orange, fresh, sweet

Decanal

Sweet, aldehydic, waxy, orange peel, citrus, floral

Limonene

Citrus, lemon, orange, fresh, sweet

Nonanoic acid

Cheese, dairy, fatty, waxy

Octanoic acid

Fatty, waxy, rancid, oily, vegetable, cheesy

Decanal

Sweet, aldehydic, waxy, orange peel, citrus, floral

Dodecane

Gasoline

Heptadecane

No aroma reference found

Limonene

Citrus, lemon, orange, fresh, sweet

Nonanal

Waxy, aldehydic, citrus, fresh, green, lemon peel, cucumber, fatty

Tetradecane

Gasoline-like

Eucalyptol

Minty, spicey, cooling

α-terpineol acetate

Sweet, herbal, bergamot, lavender, floral

β-myrcene

Peppery, terpenic, spicy, balsamic, plastic

Limonene

Citrus, lemon, orange, fresh, sweet

Linalool

Citrus, floral, sweet, bois de rose, woody green, blueberry

Octadecane

Fuel-like (continued on next page)

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TABLE 4.2 Key flavor compounds found in common gin botanicals and their aromatic qualities (Vichi et al., 2007; Orav et al., 2010)—cont’d Botanical

Compound detected

Aromatic qualities (good scents, company)

Licorice

Caryophyllene

Sweet, woody, spicy, clove, dry

Dodecane

Gasoline

Furfuryl alcohol, tetrahydro-, acetate

Sweet, fruity, brown, rummy, ethereal, caramel

Limonene

Citrus, lemon, orange, fresh, sweet

Tetradecane

Gasoline-like

3-buten-2-one, 4-(2,5,6,6-tetramethyl-2cyclohexen-1-yl)-

Warm, woody, floral odor with balsamic and violets

Heptadecane

No aroma reference found

Limonene

Citrus, lemon, orange, fresh, sweet

Tetradecane

Gasoline-like

Orris

4.2.3.3 Juniper berries Juniper (Juniperus communis L.) an evergreen, perennial, long-lived (to 600 years or more) coniferous plant (Ložiene and Venskutonis, 2016). The plant is dioecious, aromatic, evergreen, and shrub like in nature (Köhler, 1883) (Fig. 4.4). They grow as scattered single individuals of the undergrowth in dry pinewoods or mixed forests or may form a stand. The plant is light demanding but also tolerates the shade (Ložiene and Venskutonis, 2016). Several species of the genus Juniperus grow throughout temperate Europe and Asia. Common juniper (Juniperus communis L.) is possibly the most widely distributed tree in the world (Orav et al., 2010). The botanicals are most commonly used in the form of berries in gin. It has needle-like leaves in whorls of three; the leaves are green, with a single white stomatal band on the inner surface (Foudil-Cherif and Yassaa, 2012). Growing region may have an impact on juniper composition, and consequent flavor/aroma attributes. The heat map in Table 4.4 gives an indication of the effect of latitude on juniper composition within the matrix of standard gin distillation (60% alcohol by volume maceration). The data indicates that juniper berries grown at higher latitudes contain higher levels of the compounds responsible for aromas such as green eucalyptus, minty, spicey, tropical fruit, and citrus as well as some of the baser earthy notes, whereas the lower latitudes favour a cooling, sweet pine with peppery spice aromatic profile based on the descriptors of the chemical in isolation (Pauley, 2023) 4.2.3.4 Coriander The second most important botanical, found in most gin recipes, is coriander seed. Actually fruits not seeds, these are obtained from the plant which starts flowering in June yielding round fruits consisting of two pericarps. The fruits are 2–8 mm in length from which the oil is obtained once they are dried (Zeb, 2016). The most abundant, and the main character impact compound in coriander seed oil is linalool, an oxygenated monoterpene. Coriander seeds have a sweet,

Group

Compound

Carbonyl

Acetaldehyde (bruised apples)

Acetone (pungent, solvent like)

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TABLE 4.3 Aroma compounds in gin (Clutton and Evans, 1978; Nykänen and Suomalainen, 1983; Vichi et al., 2005, 2008). Prominent aroma (in brackets aroma attributes) compounds, if available (Fenaroli and Burdock, 2016).

Diethyl acetal (ethereal)

2-decanal, -trans (aldehydic type) Hydrocarbon (excluding terpenes)

α-dimethylstyrene (citrusy-lemon-like)

Esters (of monocarboxylic acids)

Ethyl formate (pungent)

Ethyl laurate (waxy, soapy)

Ethyl acetate (ethereal fruity)

Ethyl myristate (sweet, waxy)

Butyl alcohol (a dry, burning taste)

Hexyl alcohol (herbaceous, woody)

Sec-butyl alcohol (alcohol)

Phenethyl alcohol (rose-like)

Methyl disulfide (onion)

Ethyl disulfide (meat and onion)

Ethyl sulfide (sulfurous)

α-pinene (woody, pine)

β-myrcene (terpy, herbaceous)

Camphene (terpene, camphorous)

α-thujene (woody, green)

α-terpinene (citrus, woody)

α-terpinolene (pine-like)

β-pinene (cooling, woody)

α-phellandrene (fresh, citrus)

β-phellandrene (mint, terpentine)

Sabinene (herb, terpy)

γ -terpinene (lemon, herbaceous)

Limonene (lemon, citrus)

δ-3-carene (turpentine-pungent)

p-cymene (chemical, woody)

Ocimene (green, tropical)

c-rose oxide (herbaceous)

Geranyl acetate (flowery)

c-sabinene hydrate

Verbenyl ethyl ether

Cuminal (pungent, cumin-like)

β-citronellol (floral, rose)

α-campholenal

Myrtenol (minty, medicinal)

Nerol (rosy, slightly citrus)

Linalool (green, apple, citrus, waxy)

Carveol (spearmint-like)

Geraniol (rose-like)

Bornyl acetate (camphorous)

Myrtenal (spicy, herbaceous)

Camphor (warm, minty)

Terpinen-4-ol (sweet citrus, green)

neryl acetate (sweet, floral)

Citronellal (intense lemon)

Terpenyl acetate (refreshing, herbaceous)

α-terpineol (pine-like)

Alcohol

Sulfur Terpene

Monoterpenes

Pentyl alcohol (fusel-like sweet)

(continued on next page)

4. Gin

Oxygenated Monoterpenes

Ethyl decanoate (waxy, soapy)

Sesquiterpenes

α-selinene (woody)

α-muurolene

α-humulene (wood, balsamic)

γ -muurolene (woody)

germacrene B (wood, spice, earth)

β-farnesene (Wood, citrus, sweet)

δ-and γ -cadinene (derived from the oil)

cadina-1,4-diene

α-calacorene (wood)

Germacrene D (woody, spice)

α-cubebene (herb, wax)

β-caryophyllene (spicy, pepper-like, woody)

α-copaene (wood, spice)

β-cubebene (citrus, fruit)

β-elemene (floral) Oxygenated Sesquiterpenes

Diterpenes

β-caryophyllene oxide (Herb, sweet, t-cadinol spice)

t-muurulol (herb, weak spice)

Torreyol

Spathulenol (earthy, herbal, fruity)

α-cadinol (herb, wood)

Elemol (green, wood)

Eudesmol (sweet, wood)

Manoyl oxide

cis-totarol

Abieta-8,11,13-trien-7-one

Manool

Trans-totarol

Trans-ferruginol

Abieta-8,13(15)-dien-18-ol

Dehydroabietal

4-epi-dehydroabietol

4.2 Raw materials

γ -elemene (green, wood, oil)

epi-manoyl oxide

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FIGURE 4.4 Botanical illustration of Juniperus communis L. showing vegetative matter and berries in all its forms (Köhler, 1883).

4.2 Raw materials

87

TABLE 4.4 Levels of volatiles in Juniper distillates from various regions (μg/L; Pauley, 2023).

fresh, herbal, spicy and turpentine-like taste at a concentration of 50 ppm. Small seeds tend to have a higher oil content than larger seeds, so tend to be used as a spice more often (Kumar et al., 1977). Grosso et al. (2008) describes the structure of coriander, pointing out that the oil is secreted in the central pericarp of the fruit. The solvent (water–ethanol) must overcome the resistance of mericarps (outer carpels of the fruit) during the distillation. Pretreatment, namely different levels of crushing, has been shown to have a significant impact on the oil extracted (Smallfield et al., 2001), with heavy crushing yielding the most.

4.2.3.5 Angelica root Angelica root (Angelica archangelica L.) is used in most gin, contributing savory, dry and incense flavors. Angelica root origin is France, Belgium, and Germany. Some compounds such as lactones, cyclic esters derived from lactic acid (C3 H6 O3 ), are present, usually in low concentrations, in angelica root essential oil. They are said to be important for their organoleptic properties, particularly for the musk like odors. Angelica roots main character impact compounds, responsible for the musk-like flavor, are lactones such as 15-pentadecanolide (Surburg and Panten, 2006). Although Angelica root is found in most gins, it can be replaced by other botanical additions with similar attributes but different aroma contribution, with examples being sarsaparilla, gentian root, and chinchona bark. 4.2.3.6 Citrus Citrus are part of the Rutaceae family, in which there are 160 genera including more than 1600 species divided into 7 subfamilies and 12 tribes (Talon et al., 2020). The genealogy of the major citrus cultivars is depicted in Fig. 4.5 and the full origin and evolution of citrus is discussed in Wu et al. (2018). In terms of production globally, oranges (especially sweet oranges) are the largest, but lemon, grapefruit and lime are major cultivars as well (Rouseff and Perez-Cacho, 2007). In gin, lemons could be considered as a key ingredient (Willkie et al., 1937). The most abundant terpene in the essential oil of citrus is limonene, with the exception of bergamot EO where linalool and linalyl acetate are found in larger quantities (Fang et al., 2004). The physical structure of citrus is a berry with a fleshy core, divided into sections containing the seeds and the juice vesicle. The leathery rind or peel is used in gin production. This comprises the flavedo, containing the oil glands in which the essential oils are stored, and the albedo or pith. Compounds with a “citrus” aroma are citral (neral and geranial), citral dimethyl

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FIGURE 4.5 Genealogy of major citrus types according to their genomic structures (Wu et al., 2018).

acetal, citronellal, α-sinesal, linalool, linalyl acetate and limonene (Legrum, 2015). Gaff et al. (2020) list influential factors on the volatile compounds of orange as: growing location, cultivars, ripening stage, storage condition and extraction methods during which changes can occur to the flavor compounds. Such changes can occur in the form of oxidization, hydration, cyclization, isomerization, and deacetylation, with the outcome being the formation of both desired, e.g., nootkatone, or undesired, p-cymene, amongst many other pathways. The citrus varieties most frequently used in gin production are summarized in terms of their essential oil character impact compounds and general aroma in Table 4.5. In gin distillation, citrus peel can be used in a fresh or dried state. In fresh peels the essential oil is lower than in oven dried peels (Kamal et al., 2011) which is at first counterintuitive. Furthermore, there is the aspect of change in aroma compositions. Research by Kamal et al. (2011) revealed that the drying process had an increasing effect on oxygenated monoterpenes.

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TABLE 4.5 Overview of common citrus botanicals and their character impact compound (McGorrin, 2002). Character impact compounds (CIC)

Botanicals

Odor

Lemon peel

Citrus limon L.

Citral (neral + geranial)

Fresh, tangy, juicy lemon with good lemon body notes (at 5–10 ppm)

Orange peel, Bitter

Citrus aurantium L.

Linalyl acetate and β-elemol

Sweet, citrusy, floral, tangerine, clementine, and orange-like with slightly astringent and terpy fruity nuances (at 10 ppm)

Orange peel, Sweet

Citrus sinensis L.

β-sinensal

Sweet, juicy orange, tangerine, aldehydic nuances (at 25 ppm)

Grapefruit peel

Citrus paradisii L.

Nootkatone

Dry, citrusy, mildly metallic and juicy aldehydic aftertaste (at 10 ppm)

Lime peel

Citrus aurantifolia L.

α-terpineol, citral (neral + geranial)

Lime, citrus, green and citral with a camphorous nuance (at 12.5 ppm)

When using fresh citrus, consideration needs to be made regarding solubility properties of extracts in water and ethanol and subsequent louching where there is a formation of cloudiness upon dilution of the product with water, most famously observed in Ouzo. Given that EOs are insoluble in water, fresh citrus can cause greater formation of haze (Puškaš et al., 2013). The ethanol concentration can also have an impact on extraction of flavor compounds. For example, limonene becomes soluble in a water-ethanol mix above a 0.5 molar fraction (∼78.6% ABV) (Cháfer et al., 2004), whereas linalool becomes soluble above a molar fraction of 0.2 (∼46% ABV) (Cháfer et al., 2005).

4.2.3.7 Licorice root Licorice (Glycyrrhiza glabra L.) is a plant with long history used for medicinal purposes, nowadays used in the tobacco industry and cosmetics (Wagner et al., 2016). Its contribution to gin is spice and sweetness. Glycyrrhizin, considered as the character impact component has taste characteristics of lingering sweet, with a brown, syrupy, woody aftertaste. However, it is a nonvolatile compound and has not been identified in distilled gin yet. Key aroma compounds γ -Nonalactone (coconut like), 4-hydroxy-2,5-dimethylfuran-3(2H)-one (caramel like), and 4-hydroxy-3-methoxybenzaldehyde (vanilla like) were identified via aroma extract dilution analysis (AEDA) (Wagner et al., 2016).

4.3 Gin production Gin production involves the infusion of compounds from plant materials (botanicals) into neutral spirit. There are a number of approaches to this, all based on the same theme: utilizing the solvent capacity of ethanol and water. Distillers and rectifiers, two principal distillation methods can be used: steep infusion or infusion by the action of vapor (Fig. 4.6). Distillation wholly relies on the fact that when the botanical and spirit mix is heated, the more volatile compounds are concentrated in the vapor phase. When this vapor is condensed, the distillate is stronger in concentration of these volatile compounds than the original mixture that was added to the still (Kiva et al., 2003).

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FIGURE 4.6 The four major methods of infusing flavor substances in the production of gin: steep infusion, vapor infusion, vacuum distillation, and compounding (Hodel, 2021). ࢞T = a change in temperature, ࢞t = change in time.

4.3.1 Steep infusion method Steep infusion, with or without maceration, is the most common method used to produce gin; botanicals and grain neutral spirit are first placed inside the still (Riu Aumatell, 2011). Typically, the botanicals are macerated for 12–24 hours with spirit at between 40% and 60% ABV, depending on the desired extraction profile. This ethanol-water mixture extracts the flavor compounds, such as terpenes, part of the essential oil (EO) of the botanical(s). When using this method, there are significant reactions between the botanicals inside the still (Willkie et al., 1937); these add to the flavor profile giving a more rounded sensory performance to the finished product. The extractives and the subsequent flavor profile of the distillate as it comes off the still will evolve over the progression of the run, mainly due to the progressive drop in % ABV and relative solubility of the compounds within ethanol initially and water in the latter stages of the run.

4.3.2 Multishot Multishot, also known as the fold method, involves effectively increasing the capacity and productivity of the still by producing a concentrated distillate by adding more botanicals to the steep. This can be a boon to the expanding distiller as it negates the need to buy new stills. However, the fold method makes cut points more crucial due to the increased concentration of volatiles and changes in their distribution within the distillation compared to a single shot distillation. Care must be taken when adjusting an existing recipe to ensure flavor matching and consistency. It is worth noting, that doubling the botanical ratio does not result in a twofold increase in extraction. Extractions on juniper berries via vapor infusion showed that the extraction behavior was not linear (Hodel et al., 2020). Also, towards the end of the run the mixture being distilled is thicker and more prone to hot spots and burn on. The multishot

4.3 Gin production

91 FIGURE 4.7 Vapor infusion basket with entrainment [1]; Hybrid or flex-infusion [2]; vapor infusion using a vapor chamber [3]. B – indicates placement of the botanicals (Hodel, 2021).

process in effect creates an essence so needs to be diluted with neutral spirit to the required flavor intensity before being adjusted to bottling strength.

4.3.3 Vapor infusion Vapor infusion methods require the botanicals to be placed within the still, but out of the liquid fraction, normally within the lyne arm before the condenser (Fig. 4.7). The hot and strong alcoholic vapor passes through the botanicals dissolving the terpenes and infusing the resultant distillate with volatile flavor compounds (Hodel et al., 2020). Vapor infusion has been used commercially for many years. Key differences between this process and steep infusion are the difference in temperature and in alcohol concentration of the vapor in contact with the botanicals (Hodel et al., 2019). Anecdotally, vapor infused gins are said to have a lighter flavor due to less thermal influence on the botanicals during the distillation process (Greer et al., 2008; Buglass et al., 2010; Aumatell, 2012). Different types of vapor infusion exist and have been defined as follows (Fig. 4.7): 1. Botanicals are placed above the water-ethanol mixture with low volatile extractives refluxing back into the boiler [still 1] (Hughes and Hartzog, 2018). 2. Botanicals are placed in the boiler as well as in an external basket, also called hybrid or flex infusion [still 2] (Brock, 2019). 3. Botanicals are placed in an external basket without refluxing and redistilling of low volatile extractives [still 3]. Preliminary studies suggest higher concentrations of humulene (earthy, woody, spicy herbal notes) are extracted using a hybrid or flex infusion still compared to vapor infusion in an external basket (Hughes and Hartzog, 2018; Brock, 2019). It is assumed that lower volatiles, if not recycled to the pot, mainly remain in the basket effluent and are only present in the final distillate in lower concentrations. Control mechanisms of the distillation are heat input, charge ABV, botanical location, reflux via plates, and reflux via partial condenser.

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When the process of flavor extraction from plant material is considered, extraction of essential oils is the first thing that comes to mind. However, the gin distillation process is described as a hydroalcoholic extraction process. Examples using ethanol as a solvent include research on juniper (El-Ghorab et al., 2008; Fierascu et al., 2018), orange (Gonçalves et al., 2015), and angelica (Kerrola and Kallio, 1994). Given that with vapor infusion the extraction only takes place during the distillation itself, the process is described as follows: the water-ethanol mixture in the base of the still is brought to boil and the vapor will rise to the top of the still into the extraction chamber or basket. The ratio of the concentration in the vapor depends on the still configuration. Eventually, the vapor passes through the plant material where the extraction takes place (Buglass, McKay and Gook Lee, 2010). Exact specifications of distillations are difficult to find, given that the recipe or botanical ratios, are proprietary. Information on gin recipe design was given in a very early work by Willkie et al. (1937) which was used for a quantitative comparison between steep and vapor infusion (Hodel et al., 2019). In this research, it was found that the extraction using vapor infusion was greater than steep infusion for 9 of 10 investigated terpenes. Information on initial alcohol strength of the charge (45%–65% ABV) are reported in literature (Murtagh, 1999; Aumatell, 2012). The extraction mechanics during vapor infusion are characterized by:

r r r r r

Entrance of the solvent into the plant matrix. Solubilization of compound/thermal oil exudation from the plant. Transport of compounds to the outside. Vapor–liquid equilibrium at the surface layer. Vaporization.

(Sovová and Aleksovski, 2006; Cerpa et al., 2008; Mandal et al., 2015; Valderrama and Ruiz, 2018). When using water and ethanol as solvents it is important to know that they form a nonideal mixture. Not only the interaction of the solvent and the solute, but also the solute amount is of interest. Research on vapor infusion has shown that the botanical ratio has the greatest influence on the extract. The influence of the ethanol concentration has the greatest effect on oxygenated monoterpenes (OMT), terpenes with a functional group such as linalool (green, apple, citrus, waxy) and terpinen-4-ol (sweet citrus, green); when the distillation unit is charged with an ethanol concentration of 45% ABV the extraction of OMT is greatest in the distillate (Hodel et al., 2020). Because of the polarity of these compounds, their affinity is greatest in a water rich environment. When the dipole moment and the electrostatic surface potential were calculated the OMT showed greatest dipole moment and interacted more energetically with water. The extraction kinetics of vapor distillation can be described with a normalized logistic function (Hodel, 2021): c a∗ = (4.1) c0 1 + e−k(x−xc ) With: c = terpene mass at time t [mg] a = amplitude or total terpene mass [mg] k = extraction  1 coefficient min x = time [min] xc = center or inflection point of the distillation [min] Where c0 [mg] is the total mass available in the plant material and a∗ = ca0 [dimensionless], which means that the equation is set in relationship to the total available terpene mass.

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If k, xc, and a are known, a distiller would be able to calculate the theoretic concentration of certain terpenes in the distillate. The process has to be parameterized using the distillation unit itself to provide sufficient accuracy for predictions. Furthermore, knowledge of the essential oil concentration is of importance. However, determination of essential oil concentration is very difficult as the total concentration cannot be extracted from the plant matrix. Von Rechenberg (Rechenberg, 1910) suggested the reason for this is their affinity to the nonvolatile lipids in the plant matrix. This was proven by (Koedam, 1979) by extending extractive distillations up to 24 hours, after which there were still hydrocarbons present in the plant material. During vapor infusion, lower volatile compounds will accumulate in the condensate and ultimately in the effluent in the basket. Thus, positioning of the basket, internal or external in the still, is of importance. Furthermore, whether this liquid is led back into the pot and redistilled or discarded at the end of the process will have an influence on the final distillate. However, even when using vapor infusion distillation with an external basket, over time lower volatile compounds will evaporate as distillation continues. The basket itself can be considered as a distillation stage. As such, separation of water and ethanol takes place, where the water fraction in the basket liquid phase is higher than in the vapor phase. Work on phase behavior of thymol, terpinen-4-ol and α-terpineol in oregano EO and hydro alcoholic solvents showed that a greater mass fraction of the solvent had a negative effect on the transfer of such oxygenated compounds into the ethanolic phase of the solvent during Liquid-Liquid Extraction (LLE) experiments (Capellini et al., 2015). Similar behavior was seen for linalool (Cháfer et al., 2005), geraniol (rose-like) (Li et al., 2012) and β-citronellol (floral, rose) (Zhang et al., 2012). In comparison, monoterpenes (MT) such as α-pinene and limonene (Li and Tamura, 2006), both key compounds in juniper and lemons, were extracted in much higher concentrations in the ethanol solvent phase when the solvent contained a higher ethanol concentration. With this, a gin distiller might gain further control over the extraction of pine and citrus flavors.

4.3.4 Vacuum distilling Gin may be distilled under reduced pressure using vacuum (Hsu and Robinson, 2017). The reduction in pressure reduces boiling points by reducing the forces of the air pressure physically pushing the molecules together. Research on the subject is limited, but Greer et al. (2008) investigated the impact on aroma of distillations under normal and reduced pressure. Most commercial applications use a standard rotary evaporator (Fig. 4.8) which utilizes a spinning flask to generate a thin film and therefore increased surface area available to generate vapor pressure under relative slight reduced pressure. Other bespoke vacuum stills rely on the action of the reduced pressure combined with a high-power cooling system and pump to generate very low internal pressure alone to generate vapor pressure (Greer et al., 2008). This method was compared to traditional methods to produce gin from juniper berries, coriander seed, angelica root and lemon peel. In the traditional method, the compounds with the largest increases, measured pre and post distillation, were α-pinene, α-phellandrene, Ecaryophyllene, β-myrcene, and valencene, all with a 10-magnitude increase. Using the novel, or vacuum distillation process, the aromatic compounds extracted in the largest concentrations were α-phellandrene, α-pinene, E-caryophyllene, and β-myrcene with a two fold increase. The extraction from the botanicals was therefore to be found lower when using vacuum distillation, with a sensory panel rating the resulting distillate as less pungent and more floral, with a less

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FIGURE 4.8 Classic rotary evaporator (Shutterstock).

spicy aroma (Greer et al., 2008). However, the effect this has on the terpene concentration in the distillate, a quantification over time, was not published. Later, the behavior of terpenes was studied using a small-scale distillation unit, controlling the reflux and rectification by introducing a packed column under different pressures (Hodel, 2021). Usage of a packed column mimicked the engagement of column plates, the charge was an artificial gin consisting of an ethanol-water mixture spiked with terpenes. Hence the behavior could be studied in form of an unobstructed distillation. Under atmospheric (Figs. 4.9 and 4.10) as well as reduced pressure (Figs. 4.11 and 4.12), the monoterpene group was found to be the most volatile resulting in the highest concentration in the first fraction and depleting towards the end. The oxygenated monoterpenes were measured to have increased concentration in the last fraction, with depleting ethanol concentration in the still, and can be described as the least volatile compounds. The sesquiterpenes were determined to be semi-volatile as they were measured throughout the distillation, although only in small concentrations. While the graphs show a change over time, the volatility of the compounds studied change with the ethanol concentration of liquid, and as seen in form of the solid line the ethanol concentration changes over time. Atmospheric pressure using a packed column, had a reducing effect on oxygenated monoterpenes and sesquiterpenes, which were measured to have a higher concentration in the distillate without a rectification.

4.3 Gin production

FIGURE 4.9 Atmospheric distillation without rectification (Hodel, 2021).

FIGURE 4.10 Atmospheric distillation with high rectification (Hodel, 2021).

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FIGURE 4.11 Vacuum distillation without rectification (Hodel, 2021).

FIGURE 4.12 Vacuum distillation with high rectification (Hodel, 2021).

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Under partial vacuum, high reflux and a high rectification resulted in highest concentrations of the oxygenated monoterpenes. Changing the reflux ratio showed the least impact on monoterpenes concentration in the distillate. However, behavior over time changed substantially (compare Fig. 4.10 to Fig. 4.11). The distillations in a partial vacuum resulted in generally higher concentrations of oxygenated monoterpenes and sesquiterpenes, but lower monoterpenes. For distillations at atmospheric pressure, with the constraint of distillate volume, a high degree of rectification and a high reflux ratio generated a spirit with elevated concentration of monoterpenes. Given the nature of these unsaturated terpenes, they are more prone to chemical alterations such as oxidation (Pino et al., 2007). Controlling these flavors while maximizing highly odorous oxygenated compounds should therefore be a priority of the gin distiller.

4.3.5 Distillation summary As a gin distiller a multitude of options for flavor extraction for gin production are available. Steep infusion is the most used, using a simple pot still with condenser. Here the options of distillation control are limited to heat input and heat removal, reflux cannot be controlled without additional water jackets or condensers. The extraction of flavor compounds takes place in the liquid either prior to distillation in a maceration stage or during the distillation. Using vapor infusion, the extraction takes place during the distillation only. Different hybrid models exist, and a combination of steep and vapor infusion has been realized commercially. Alteration of the pressure during the distillation has been described earlier and can be a powerful tool to control flavor components.

4.3.6 Compound gin Compounding takes advantage of the availability of flavorings, extracts or essences, either commercially produced or made in-house, which are blended with the spirit to achieve the desired flavor profile. This is defined by the regulation EU2019/787 as “flavoring substances or flavoring preparations or both shall be used for the production of gin so that the taste is predominantly that of juniper” (European Commission, 2019). However, when taking into consideration the cost of commercially produced flavoring agents, this process may not offer any financial advantages over producing a distilled gin, and the resulting product can have a relatively one-dimensional flavor profile when compared side by side.

4.4 Quality Fundamental research on the development of a gin flavor wheel was carried out by the Scotch Whisky Research Institute (SWRI) (Phelan et al., 2004). By evaluating 16 commercial gins a sensory vocabulary was constructed: pungent, spicy, herbal, citrus, floral, aniseed, juniper, fruity, sweet, sour, buttery, stale, oily, solventy, soapy and sulfury. The research group then applied the same approach to profile single botanical distillates: juniper berry, coriander seed, orange, and orris root (Phelan et al., 2004).

4.4.1 Flavor defects The main source of flavor defects is misplaced cut points. Although an adjustable parameter, depending again on recipe and intended flavor delivery, the foreshots cut point depends on many distillation parameters: still size, heat input, ambient temperature, and condenser

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design/volume, amongst others. Evaluation by sensory is still the best tool for a distiller. Once a recipe is defined, the process parameters (time, distillate ABV, heat input, volume) should be recorded and kept constant. Cut points to feints are best measured on ABV and will range from 65% ABV to 55% ABV, avoiding the danger of overextraction particularly of undesirable low volatility/water soluble compounds. These can lead to flavor delivery issues and off notes in the final product. Botanical storage is another area of potential off notes due to mould contamination resulting from inadequate drying or moisture ingress. There also needs to be awareness around storage conditions, with particular emphasis on stable temperatures to try and avoid condensation, too much headspace within half empty containers, containers with no lids, and exposure to UV radiation. Chlorinated pallets can be problematic and can result in formation of highly aromatic chlorine-based off notes that can transfer into the product. The role of copper within gin distillation is said to have an impact on the sensory performance. Even small amounts of copper in the botanical baskets or the condenser stage of the distillations act to remove sulfur compounds. These originate from the botanicals, and if not removed can present as a rotten egg like aroma which would suggest the presence of hydrogen sulfide and its derivatives.

4.5 Future As described in this chapter, gin is a cosmopolitan drink and has been buffeted by the tides of politics, legislation, and latterly the perils of popularity and market forces. There are many innovators within the industry, with some of the smaller companies able to take more risks than the larger concerns. Development might include the application of different techniques, whilst flavor chemists within the market are always striving to create exciting combinations to add to the repertoire of professional and enthusiast cocktail creators. It will be exciting to see what the future holds as, what has been one of the most rapidly growing spirit categories, continues to evolve and mature.

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C H A P T E R

5 Baijiu Jun Zhang a, Shuang Chen a, Yan Ping L. Qian b and Michael C. Qian c a

State Key Laboratory of Food Science & Technology, Key Laboratory of Industrial Biotechnology of Ministry of Education & School of Biotechnology, Jiangnan University, Wuxi, Jiangsu, China b Department of Crop and Soil Science, Oregon State University, Corvallis, OR, United States c Department of Food Science & Technology, Oregon State University, Corvallis, OR, United States

5.1 Introduction 5.1.1 The origin of Baijiu The long, rich history of Chinese alcoholic beverages has been well documented (McGovern et al., 2004; Zheng and Han, 2016; Huang, 2000). The production of alcoholic drinks in China can be dated back to ancient times. During the excavation of the Jiahu Site in Henan Province, archaeologists discovered organics absorbed by the pottery buried underground. After comprehensive compositional analysis, scientists identified the organic residue as alcoholic beverages brewed from rice, honey, grapes, and hawthorn, produced as early as the Shang and Western Zhou Dynasties (McGovern et al., 2004). In addition, the oracle bone inscriptions refer to three drinks called chang, li, and jiu dating back to the Shang dynasty (1200–1056 BCE). Alcohol distillation technology can be traced back to the Western Han Dynasty (202 BC to 8 AD), with a history of over 2000 years (Liu and Sun, 2018). According to historical records, a bronze still in a liquor warehouse was recovered in the tomb of Haihunhou from the Western Han Dynasty. Its construction principle was similar to the one used in traditional liquor retort distillation (Xiao et al., 2011). The alcohol distillation technology gave birth to Baijiu.

5.1.2 The value of Baijiu Baijiu is among the most famous seven distilled spirits globally, together with Whisky, Vodka, Brandy, Tequila, Rum, and Gin (Zheng and Han, 2016). As China’s national liquor, Baijiu deeply penetrates Chinese culture and history, including social, political, economic, and other aspects.

Distilled Spirits. DOI: https://doi.org/10.1016/B978-0-12-822443-4.00006-2

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FIGURE 5.1 Annual yield data of brewing industries in 2020 (billion liters).

FIGURE 5.2 Annual sales revenue of brewing industries in 2020 (billion yuan).

5.1.2.1 Baijiu and traditional culture Baijiu is not only a unique distilled liquor but also a cultural heritage. With thousands of years of development and integration, Baijiu has become an indispensable part of life for the Chinese. Baijiu cannot be separated from the social lives of Chinese people. The core of Baijiu culture is sharing and expressing. Baijiu is found in every significant ritualism, such as weddings, birthday parties, funerals, holidays, and other ceremonies and events, even as offerings to deities and ancestors. Another Baijiu’s ubiquity is how it is embedded in China’s business culture. Business deals are frequently made under the infusion of Baijiu. Baijiu has become a high-quality carrier to show deep historical precipitation and rich cultural connotations. 5.1.2.2 Baijiu and economy The Baijiu industry is a traditional industry with unique craftsmanships and holds an important position in China’s economy (Hong et al., 2021). With the development of new technology, the Baijiu industry has experienced rapid growth over the past three decades. As a result, Baijiu is now the world’s most popular liquor in volume consumed every year. According to data from the National Bureau of Statistics of China (https://data.stats.gov.cn/), the total output of the national alcoholic beverage reached 54.01 billion liters in 2020. Among them, the annual production of Baijiu was 7.41 billion liters, with sales revenue of 583.64 billion yuan and a net profit of 158.54 billion yuan (Figs. 5.1–5.3). The overwhelming net profits make it the best-selling spirit in the world. Also, the Baijiu industry directly increases the appreciation of agricultural and sideline products, promotes rural and regional economies, and the development of related industries. Overall, the Baijiu industry is the ‘Ł‘œeconomic leader‘Ł‘žand plays a crucial role in China’s economy. In modern times, Baijiu has been

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FIGURE 5.3 Annual profit of brewing industries in 2020 (billion yuan).

developing rapidly to meet the needs of the consumers and makes significant contributions to the country’s economic outputs.

5.2 Production of Baijiu 5.2.1 Baijiu and brewing techniques Baijiu is a distilled liquor made from solid-state fermentation of grains, typically sorghum, or a mixture of sorghum, wheat, corn, rice, and glutinous rice (Fan and Qian, 2006), with alcohol content ranging from 40% to 60% by volume (ABV). It is crafted with complex brewing techniques and a long production cycle, resulting in the wealthiest flavors of any spirits. Baijiu is made from three basic raw materials: water, grain (sorghum or mixed with other grains), and Jiuqu. All of these influence the quality and style of Baijiu. First, water and grain quality directly affect Baijiu’s quality and flavor. Secondly, the Jiuqu provides a unique microbial community, multi-scale enzyme systems, and flavor precursors for Baijiu brewing. Furthermore, the specific geographical environment of the brewing regions provides a natural selection and enrichment process of brewing microorganisms and the unique functional microbiota, which in turn catalyzes the development of their particular brewing techniques for the regions, which tailors its quality and style (Xin, 2019). As a result, Baijiu is the product of harmony between man and the natural environment.

5.2.2 The unique application of Jiuqu In Baijiu production, Jiuqu is a unique ingredient. Jiuqu is a starter with microbial and enzyme systems for fermentation. It also contains many trace elements, volatiles, and precursors which determine the aroma, taste, style, and quality of Baijiu. Jiuqu-making is a unique national heritage asset and represents the progress of an era. Jiuqu is a brick of crushed powder specially made from wheat, barley, and sometimes peas for the fermentation of Baijiu. It is an indispensable liquor starter and essential for saccharification, alcoholization, and flavor generation. Jiuqu consists of molds, yeast, microbial, and associated metabolites necessary for fermentation (Table 5.1) (Li et al., 2019; Zou, Zhao, and Luo, 2018). This complex is rich in functional microorganisms and microbial enzymes that determine the quality of Baijiu, especially the flavor. Jiuqu is mainly divided into Daqu, Xiaoqu, Fuqu, Mixed-qu, and others (Fig. 5.4) (Li et al., 2019).

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TABLE 5.1 Microbial diversity in Baijiu production: predominant genera in starter cultures and in fermentation (Zou et al., 2018). Bacteria

Bacillus, Streptomyces, Lysinibacillus, Staphylococcus, Rummeliibacillus, Brevibacillus, Brachybacterium, Weissella, Pediococcus, Lactobacillus, Acetobacter, Sporolactobacillus, Clostridium, Mycobacterium, Flavobacterium

Yeast

Wickerhamomyces, Saccharomycopsis, Meyerozyma, Candida, Pichia, Cryptococcus, Brettanomyces, Dekkera, Issatchenkia, Debaryomyces, Saccharomyces, Rhodotorula, Schizosaccharomyces, Kluyveromyces, Hansenula, Zygosaccharomyces

Mold

Lichtheimia, Aspergillus, Penicillium, Rhizomucor, Mucor, Rhizopus, Monascus, Emericella, Cladosporium, Gibberella

Archaea

Methanobacterium

FIGURE 5.4 Different types of Jiuqu used in Chinese Baijiu fermentation.

5.2.2.1 Daqu Daqu is primarily made of wheat, barley, and peas. Wheat and barley have a high starch content, and pea is rich in amino acids, vitamins, and other ingredients. The mixture is an excellent material for cultivating various microorganisms and subsequent production of enzymes (Shen, 1998). Daqu is not only a saccharifying agent for Baijiu production but also part of the brewing raw materials. It serves as a significant source of aroma compounds and precursors. Daqu is typically added to the production process in large quantities, which directly impacts the flavor and style of Baijiu (Wang, 2013). Most famous Baijiu, such as Wuliangye, Luzhou Laojiao, Moutai, Langjiu, and Fenjiu, are fermented with Daqu. 5.2.2.2 Xiaoqu Xiaoqu is a saccharification starter made of rice and glutinous rice. The rice is made into small balls or paste, and old Xiaoqu is used to inoculate the rice ball. The inoculated rice ball is used for solid-state inoculation under controlled conditions. The composition of microorganisms and active enzymes in Xiaoqu is not as complex as in Daqu, but they are critical in

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determining the quality and style of Xiaoqu Baijiu. In this ecosystem, the structure of the microbial community changes with the fermentation process (Wang et al., 2019). In addition, the microorganisms in it interact and influence each other to maintain the ecological balance of the fermentation process of Xiaoqu Baijiu to achieve quality products (Wang et al., 2014). Xiaoqu is used for semi-solid fermentation, and the dosage is low, generally 0.2%–0.5% of the grain, and varies in season (Li and Li, 2006). Xiaoqu Baijiu is brewed with rice-based grain or multiple grains, and the fermentation cycle is relatively short. Due to the fast fermentation cycle and a relatively small amount of Jiuqu used for fermentation, Xiaoqu Baijiu has a lighter aroma and taste characteristics than the strong-aroma type Baijiu. Xiaoqu Baijiu has an annual market share of approximately one-third of Baijiu’s total production. The south and southwest regions of China are the main producing areas of Xiaoqu Baijiu (Wang et al., 2014). Xiaoqu Baijiu produced in the Sichuan region is the most prominent (Li and Li, 2015; Zhu et al., 2017). Guilin Sanhua Baijiu and Guangxi Xiangshan Baijiu are representatives of high-quality Xiaoqu Baijiu with a mellow taste and elegant aroma. There is significant interest in increasing Xiaoqu Baijiu production.

5.2.2.3 Fuqu Fuqu is artificially cultivated by adding pure Aspergillus or other molds to steamed bran (solid material) (Shen, 1998). It has a short production cycle and does not form blocks, so Fuqu is also known as Sanqu. Fuqu has a low production cost and price and has been used partially to produce light-aroma, strong-aroma, soy sauce-aroma, and sesame-aroma type Baijiu. However, the aroma and taste of Baijiu produced with Fuqu are usually not as harmonious and elegant as Baijiu made from traditional Daqu. Therefore, more research and development are needed to incorporate Fuqu in Baijiu production. 5.2.2.4 Mixed-qu Mixed-qu is a blended product of Daqu and Xiaoqu. Baijiu distilled from Mixed-qu fermentation is called Mixed-qu Baijiu. The main microorganisms in Xiaoqu fermentation are yeast and saccharifying bacteria, while Daqu contains more aroma-producing microorganisms. Thus, Daqu Baijiu has a rich aroma but low yield. In contrast, Xiaoqu Baijiu has less aroma and a soft body with high alcohol production. The rich fragrance of Daqu and the mellow and sweet features of Xiaoqu is well integrated into Mixed-qu, which expands the types and numbers of microorganisms. Different functions of microorganisms in the Mixed-qu complement each other, giving a more harmonious product (Xie, 2018). Dongjiu, one example of a product made of Mixed-qu, carries characteristics of both styles of Xiaoqu and Daqu Baijiu, having a rich aroma and relatively high yield.

5.2.3 Solid-state fermentation progress The solid-state fermentation method refers to the fermentation mode of microorganisms on the solid substrate without or with a minimum amount of free water. The gas, liquid, and solid coexist in the concrete substrate; the porous solid substrate traps water and watersoluble substances (Zhou et al., 2014). The solid-state liquor technology is complicated. The fermentation mainly relies on the grain and the Jiuqu. The brewing technique of traditional solidstate fermentation liquor is an intangible cultural heritage and remains “mysterious” (Yang, 2021). Then, the liquor is distilled in a solid state and aged and blended to the identity.

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FIGURE 5.5 Process diagram for the production of soy sauce-aroma type Baijiu.

5.2.3.1 Production techniques and styles Based on the production techniques (solid-state and semi-solid state), types of starter, Jiuqu, and flavor characteristics (Lai, 2005), Baijiu can be mainly divided into the following types: soy sauce-aroma, strong-aroma, light-aroma, rice-aroma, te-aroma, fuyu-aroma, feng-aroma, laobaigan-aroma, sesame-aroma, herbal-aroma, and chi-aroma (Zheng and Han, 2016). The first four types were officially proposed and established at the third Baijiu evaluation conference in 1979. They are the most popular four types of Baijiu in China, while the rest are derived from the above types (Jia et al., 2020). Among them, the famous soy sauce-aroma type Baijiu is Moutai. Wuliangye represents the strong-aroma type Baijiu, the representative of light-aroma type Baijiu is Fenjiu, and the rice-aroma type Baijiu is Guilin Sanhua. Different Baijius have their native microbiota and flavor because of the geographical distribution of raw materials, Daqu (described previously), distinct production technology, and other factors. The production process flow diagrams of soy sauce-aroma Baijiu, strong-aroma type Baijiu, and light-aroma type Baijiu are shown in Figs. 5.5–5.7, respectively. Baijiu has a full-bodied aroma with a soft and elegant taste, which Chinese consumers much appreciate. The production process of solid-state fermentation is a unique production technique and is a valuable asset of Chinese Baijiu production (Zhou et al., 2014). While traditional craftsmanship is essential for manufacturing, and the Baijiu industry embraces modern technology to obtain reproducible and higher-quality Baijiu with consistency.

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FIGURE 5.6 Process diagram for the production of strong-aroma type Baijiu.

FIGURE 5.7 Process diagram for the production of light-aroma type Baijiu.

5.2.3.2 Fermentation and distillation Liquor making seems to be nothing more than microbial fermentation to produce alcoholic beverages with a particular concentration. But in practice, it is far from simple. The process of Fenjiu, a representative of the light-aroma type of Baijiu, is shown in Fig. 5.8. First, the

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grains are crushed, then hot water (over 85‘°C) is added to soak the crushed grains. Then, the grains are stacked for 24h, followed by steaming. The steamed grains are cooled and mixed with the Daqu powder. When the temperature of the mixture (grains and Daqu power) cooled down between 18‘°C and 20‘°C, the mixture is loaded into a porcelain jar and fermented under anaerobic conditions for about one month in solid-state (Zheng and Han, 2016), with the temperature not exceeding 33‘°C during fermentation (Ren, 2019). After fermentation, the grains are distilled with steam to distill alcohol and, more importantly, the aroma components (Li et al., 2012). Distillation done in the solid-state is an integral part of the Baijiu production process. The ‘Ł‘œZeng-tong‘Ł‘žused for solid-state distillation, as shown in Fig. 5.9, is composed of three main components: lid, barrel body, and reboiler. This equipment is widely used in Baijiu production by solid-state fermentation, which enables the distillation and concentration of distilled fractions with high alcohol content from fermented grains with low alcohol content in a single distillation. The fresh Baijiu needs to be further graded and then stored in pottery jars for 1–3 years, where the power of time is used to equalize both the taste and aroma. Due to the high alcohol content of the aged Baijiu, it needs to be diluted with water and then blended, and subsequently, a variety of Baijiu products with different flavor profiles are formulated with an ethanol content of 40%–55% by volume (Xu et al., 2010). Photos are kindly provided and permitted to reproduce by Shanxi Xinghuacun Fenjiu Group Co., Ltd. (Fenyang, Shanxi, China).

5.3 Quality: Flavor chemistry of Baijiu The flavor of Baijiu is formed from fermentation, distillation, and maturation. Besides ethanol, yeast fermentation produces a small amount of volatile and semi-volatile byproducts that determine the quality of Baijiu. Because flavor is a crucial factor influencing consumers‘Ł‘™preferences for Baijiu, it is always a hot topic from both scientific and manufacturing points of view. Higher alcohols, fatty acid esters, fatty acids, aldehydes, ketones, lactones, and acetals are the main volatiles identified in Baijiu. The concentrations of those compounds depend on the raw materials and technologies used in the production of different styles. These compounds are relatively high in Baijiu and are the backbone aroma contributors. Still, although present at trace levels, some other compounds can have substantial impacts on the flavor. For instance, geosmin has a low concentration but can impart an earthy and dirt note in Baijiu due to its low odor threshold of 0.11 μg/L (Fig. 5.10). In addition, 2-methyl-3-furanthiol is a vital sulfur-containing compound due to its extremely low olfactory threshold of 0.01μg/L, imparting sulfury and meaty odor. The concentration of these compounds in the baijiu depends on the raw materials and techniques used to produce different styles of Baijiu. The combination of different types and concentrations of aroma-contributing substances are responsible for the specific styles of Baijiu. Therefore, volatile compounds with low olfactory thresholds and unique aroma characteristics should be studied more in-depth because they could significantly affect the aroma of Baijiu.

5.3.1 Analysis of chemical composition 5.3.1.1 Extraction method Reliable analysis of volatile compounds in Baijiu is essential to understand their aroma impacts. However, the low concentration of these flavor compounds (Shen, 1998) and the high

(B)

(C)

(D)

5.3 Quality: Flavor chemistry of Baijiu

(A)

FIGURE 5.8 Process diagram for the production Fenjiu.

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5. Baijiu

FIGURE 5.9 Diagram of the distillation system (Zeng-tong) for Baijiu.

ethanol content make the analysis difficult. Therefore, it is necessary to select robust sample preparation to achieve effective extraction, concentration, separation, and quantitation of flavor substances. To date, liquid–liquid extraction (LLE), solvent-assisted flavor evaporation (SAFE), static headspace solid-phase micro-extraction (HS-SPME), and stir bar sorptive extraction (SBSE) are commonly used techniques for Baijiu volatile analysis. LLE is a conventional method for isolating volatiles based on “similar solubility” with the selected organic phase. Two wholly or partially immiscible liquid phases are thoroughly mixed to achieve redistribution of volatile compounds in the organic phase. The organic extract then passes through SAFE under a vacuum to remove nonvolatile compounds. Finally, the distillate is concentrated for further analysis. The LLE-SAFE process achieves the purpose of extraction, purification, and concentration of the volatiles. Therefore, the aroma extract by the LLE-SAFE process will more likely reflect the sample’s aroma profile, especially for complex food samples. Although it has many advantages, including minimum discrimination and minimum loss of heat-sensitive components, it is time-consuming, tedious, and cumbersome. Another major disadvantage is consuming a large amount of organic solvent for the extraction, which is not environment-friendly. In addition, some compounds have low recovery. HS-SPME technique relies on a solid phase fiber to extract volatiles from the samples and then desorb them directly onto the GC or GC-MS for analysis. It is a convenient technique and can be automated. However, the extraction efficiency is affected by the type of fiber coating, extraction time, extraction temperature, and matrix. In addition, volatile compounds have competition with the fiber. There are many fibers commercially available with different properties. The poly(acrylate)(PA) fiber generally has better extraction efficiency for polar volatile long-chain and high-boiling volatiles. Carboxen/polydimethylsiloxane (CAR/PDMS) fiber is suitable for extracting short-chain and low-boiling volatiles. Three-phase fiber (divinylbenzene/carboxen/polydimethylsiloxane, DVB/CAR/PDMS) can extract volatile components of various polarities and chain lengths (Câmara et al., 2007). Therefore, choosing the suitable extraction fiber significantly impacts the accuracy of quantitation results. Jiang et al. investigated the effects of salt (NaCl) concentration, extraction temperature, and extraction time on the extraction efficiency using a three-phase fiber (DVB/CAR/PDMS, 50/30 μm) (Jiang et al., 2016), and the result can be used as a guideline for HS-SPME parameter selection. Yan et al. compared HS-SPME and LLE to extract volatile components in Shedian laojiu. Results showed that aroma compounds detected by HS-SPME were less than those in the LLEtreated samples (Yan et al., 2015). Similarly, Niu et al. found more esters extracted by LLE than that by HS-SPME (Niu et al., 2020). In contrast, Gong et al. found that HS-SPME had higher

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FIGURE 5.10 The gradual deepening of the cognition of aroma compounds in Baijiu. a odor threshold determined in 46% ethanol aqueous solution; (Wang et al., 2014) b odor threshold determined in 46% ethanol aqueous solution;26 c odor threshold determined in 46% ethanol aqueous solution; (Niu et al., 2020) d odor threshold determined in 23% ethanol aqueous solution; (Song et al., 2019) e odor threshold determined in 53% ethanol aqueous solution (Du et al., 2011).

extraction efficiency for esters, while SAFE is better for extracting alcohols and fatty acids (Gong et al., 2016). These discrepancies are likely due to the sample volume used for the extraction and the concentration, so the comparison is meaningless. Also, the selection of the method depends on the objectives of the analysis. However, LLE-SAFE is a less discriminating method, with more or less total volatile extraction, whereas HS-SPME is a highly selective and competitive extraction technique. Therefore, for comprehensive extraction and nondiscriminated analysis of Baijiu, LLE-SAFE should be used. A more advanced technique, SBSE, can be used as an alternative to SPME with higher absorption capacity, better extraction recovery, lower detection limit, and better reproducibility. This method is particularly advantageous for extracting volatile components in alcoholic samples due to the low extraction efficiency for alcohol. The stir bar with different coatings can be selected according to the properties of the compounds. The most frequently used SBSE phases include polydimethylsiloxane (PDMS) and ethylene glycol (EG)/PDMS. SBSE technique can combine with advanced analytical instruments to achieve comprehensive and effective extraction of volatile compounds in the sample. Wang et al. demonstrated the high reproducibility and sensitivity of the SBSE-GC-MS technique for analyzing esters in Baijiu (Wang et al., 2008). In addition, Niu et al. applied the SBSE technique to analyze volatiles in five different flavored

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Baijiu samples with further optimization on the parameters of ethanol content, stirring speed, extraction time, and salt addition (Niu et al., 2015).

5.3.1.2 Instrumentation and detection methods The components in Baijiu can be analyzed by GC and HPLC (nonvolatile). Volatile analysis in Baijiu started as early as 1963 (Xiong, 2005). However the research needs to focus on flavorrelevant compounds, many at trace levels in the sample. The advancement of instrumentation and the continuous improvement of sample preparation have made trace volatile detection possible. More sophistical instruments such as gas chromatography-mass spectrometry (GC-MS), comprehensive two-dimensional gas chromatography/time of flight mass spectrometry (GC ‘—GC-TOFMS), liquid chromatography-mass spectrometry (HPLC-MS), and high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (HPLC-Q-TOF-MS) have been used in Baijiu analysis. Although GC-MS is efficient for volatile analysis of Baijiu, Ji et al. demonstrated that the GCxGC-TOFMS have much better separation efficiency and higher sensitivity than conventional GC-MS on trace volatile analysis (Ji et al., 2007). In addition, some vital nitrogenand sulfur-containing aroma compounds are present at extremely low concentrations, and a sole mass spectrometer detector can’t easily detect them. As a result, using element-specific detectors such as nitrogen phosphorus detector (NPD), flame photometric detector (FPD), and pulse flame photometric detector (PFPD) coupled with gas chromatography are essential. Wang et al. applied a gas chromatography-nitrogen-phosphorus detector (GC-NPD) to characterize the nitrogen-containing compounds in Guojing Baijiu and Sesame-flavor Baijiu (Ji et al., 2007). Lian et al. used gas chromatography- flame photometric detector (FPD) to detect the volatile sulfur compounds in Wuliangye (Lian et al., 1992). With a pulsed flame photometric detector (PFPD), volatile sulfur-containing compounds in Baijiu have been studied more thoroughly (Zhu et al., 2020). The application of these advanced instrumentations in Baijiu has dramatically improved the understanding of the aroma and flavor compounds and their formation during fermentation and aging.

5.3.2 Characteristics of flavor compounds Ethanol and water are the main components of Baijiu, which account for 98%–99% of the total volume. However, the quality and flavor of Baijiu are primarily determined by the trace amount (1%–2% of the total) of volatiles and semi-volatile compounds (Shen, 1998). Volatile compounds include alcohols, carboxylic acids and their esters, aldehydes, terpenic compounds, heterocyclic and aromatic compounds, and nitrogen- and sulfur-containing compounds. The volatile profiles of Chinese Baijiu are very complex and strongly affected by raw ingredients, the fermentation process, and geographically manufacture conditions. Although more than 2000 volatiles have been detected in Baijiu (Hong et al., 2020), not all of them contribute to Baijiu flavor. Fan and Qian first systematically investigated the aroma-active compounds in strong aroma type Baijiu in 2005 using GC-olfactometry (GC-O) technology and pioneered the flavor-oriented research on Baijiu (Fan and Qian, 2005). GC-O is a technique combining gas chromatography with human olfactory senses to screen aroma-active compounds in the sample. By this method, volatile compounds that have potential impacts on the overall aroma of Baijiu can be screened out based on aroma extract dilution analysis (AEDA). Early work by Fan and Qian identified more than 70 odor-active compounds

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in the strong aroma style of Baijiu, including Yanghe Daqu, Wuliangye, and Jiannanchun, by GC-O with AEDA analysis (Fan and Qian, 2006; Fan and Qian, 2005; Fan and Qian, 2006). Results elucidated that the aroma of strong aroma type Baijiu was mainly contributed by fatty acid esters, acetals, and short-chained fatty acids. Furthermore, several pyrazines were identified in Wuliangye and Jiannanchun, Moutai, impacting nutty, roasted character. In addition, more than 180 aroma-active compounds have been identified in Moutai, Langjiu, and other soy sauce aroma styles of Baijiu using fractionation and GC-O analysis (Fan et al., 2012). Results suggested that alkylpyrazines, including 2,3,5,6-tetramethylpyrazine and 2,3-dimethyl-5ethylpyrazine 2,3,5-trimethyl-6-ethylpyrazine, and 2,3,5-trimethylpyrazine were the important aroma-active compounds contributing the soy sauce and roasted notes in Moutai. Besides pyrazines, esters, and fatty acids, other aroma contributors were 4-methyl-2- methoxyphenol, ‘³decalactone, vanillin, ‘³-nonalactone, z-whiskylactone, 2,5-dimethyl-4-hydroxy-3(2H)-furanone, 4,5-dimethyl-3-hydroxy-2,5-dihydrofuran-2-one (sotolon), and geosmin. However, the GC-O results need to be interpreted with care because of the difference in individuals’ sense of smell. The aroma activity value (OAVs, ratio of concentration to odor threshold) based on quantitative data and the sensory threshold can be used to evaluate the potency of aroma-active compounds. Based on the OAV value, the contribution of odorant to the overall flavor of Baijiu can be assessed (Chen et al., 2013). The olfactory thresholds of many volatile compounds typically found in Baijiu have been determined in 46% aqueous ethanol solution (Fan and Xu, 2011). On the premise of accurate quantitative analysis, and odor threshold in simulated baijiu matrix, much progress has been achieved in understanding the flavor chemistry of Baijiu, especially on aroma compounds at much lower concentrations (Gao et al., 2014; Niu et al., 2020; Wang et al., 2020) Aroma recombination needs to be conducted to evaluate whether the aroma-active compounds were identified and quantified correctly. For example, Fan et al. reconstructed 31 odorants with OAVs ‘‰‘¥1 to simulate the typical aroma of chi-aroma type Baijiu (Fan et al., 2015). Similarly, Gao et al. successfully mimic a light-aroma type Baijiu by recombining 27 odorants with OAVs ‘‰‘¥1 (Gao et al., 2014). It demonstrated the accuracy of the aroma-active compounds identified by the technology mentioned above. Off-flavor research in Baijiu is also fundamental. Off-flavor compounds have detrimental impacts on product quality. Du et al. (2011) identified ‘Ł‘œgeosmin‘Ł‘žas the compound that caused the off-flavor of Baijiu, giving an earthy odor (Du et al., 2011). Similarly, through addition and omission tests combined with sensory analysis, Wang et al. (2020) identified a group of sulfur-containing compounds which contributed to the off-odor in Moutai Baijiu, imparting a pickle-like odor. They are 2-Methyl-3-furanthiol, methional, methyl 2-methyl3-furyl disulfide, dimethyl trisulfide, 2-furfurylthiol, methanethiol, dimethyl disulfide, and bis(2-methyl-3-furyl)disulfide. These findings provide important technical guidance for production quality control. However, it needs to keep in mind that the same compound could be a ‘Ł‘œpositive‘Ł‘žaroma contributor at low concentration and ‘Ł‘œoff-flavor‘Ł‘žhappens when the level exceeds a critical concentration. Reliable quantitative analysis and sensory evaluation should be employed for flavor study. Some of the aroma-active compounds identified in different flavors of Baijiu were summarized in Table 5.2 (Wang et al., 2014; Gao et al., 2014; Niu et al., 2020; Song et al., 2019; Du et al., 2011; Zhu et al., 2020; Wang et al., 2020; Niu et al., 2017; Zhao et al., 2018; Zhu, 2020; Ma et al., 2020; Yan et al., 2020; Song et al., 2020; Dong et al., 2019; Wang, 2014; Sha, 2017; Chen et al., 2017; Zhang et al., 2019; Yan et al., 2021)

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TABLE 5.2 Information on the aroma compounds of Baijiu with different flavors. No.

Compounds

Aroma descriptor

Concentration range (μg/L)

Olfactory threshold (μg/L)

OAVs

Alcoholic, fermented

10,900–5,820,000

224,000a