Understanding Wine Chemistry

Wine chemistry inspires and challenges with its complexity, and while this is intriguing, it can also be a barrier to further understanding. The topic is demystified in Understanding Wine Chemistry, Special Mention awardee in the 2018 OIV awards, which explains the important chemistry of wine at the level of university education, and provides an accessible reference text for scientists and scientifically trained winemakers alike.

Understanding Wine Chemistry:

• Summarizes the compounds found in wine, their basic chemical properties and their contribution to wine stability and sensory properties
• Focuses on chemical and biochemical reaction mechanisms that are critical to wine production processes such as fermentation, aging, physiochemical separations and additions
• Includes case studies showing how chemistry can be harnessed to enhance wine color, aroma, flavor, balance, stability and quality.

This descriptive text provides an overview of wine components and explains the key chemical reactions they undergo, such as those controlling the transformation of grape components, those that arise during fermentation, and the evolution of wine flavor and color. The book aims to guide the reader, who perhaps only has a basic knowledge of chemistry, to rationally explain or predict the outcomes of chemical reactions that contribute to the diversity observed among wines. This will help students, winemakers and other interested individuals to anticipate the effects of wine treatments and processes, or interpret experimental results based on an understanding of the major chemical reactions that can occur in wine.

Professor Andrew L. Waterhouse, Department of Viticulture & Enology, University of California, Davis, USA.
Andrew Waterhouse received his PhD in organic chemistry from UC Berkeley, and has been a wine chemist at the University of California, Davis since 1991. He teaches wine analysis, graduate level wine chemistry, and an online introductory wine course, and is Chair of the Viticulture and Enology graduate studies program. Former graduate students and postdocs are academics, industry scientists and winemakers. His research lab has reported key wine oxidation reactions and has developed new methods to analyse wine components including those using LC-MS with isotope filtering, as well as NMR techniques. The research has focused on wine phenolics, oak compounds and oxidation products. In addition his lab has also been addressing the metabolic products of phenolics. He publishes in numerous international journals in the fields of chemistry and agriculture, and serves as a chief editor at the Journal of the Science of Food and Agriculture. See: waterhouse.ucdavis.edu. 

Dr Gavin Sacks, Department of Food Science, Cornell University, USA.
Gavin Sacks received his PhD in analytical chemistry from Cornell University, and following post-doctoral studies in nutritional sciences and biogeochemistry he began as a faculty in food science at Cornell in 2007. He has served as Director of Undergraduate Studies for Cornell’s interdepartmental Viticulture and Enology undergraduate major, in which he also teaches courses in wine analysis and in wine flavor chemistry. His research interests include the development of both low-cost and state-of-the-art approaches to analysis of odorants and other organoleptically important compounds; and applying these tools to understanding the role of plant genetics, cultural practices, and post-harvest processing on sensory attributes of foods and beverages, particularly of wine. 

Dr David Jeffery, School of Agriculture, Food and Wine, University of Adelaide, Australia.
David Jeffery received his PhD in synthetic organic chemistry from Flinders University, and has been involved with wine chemistry for over a decade, initially as a researcher at The Australian Wine Research Institute before transitioning to The University of Adelaide in 2010. He teaches wine chemistry to undergraduate and Master level students, delivering topics associated with stabilization, clarification, distillation, wine aroma, polyphenols, and analytical methods. He also helped to develop and deliver a free online wine education course called Wine 101x, offered on the EdX platform. David’s research areas extend to on many aspects of wine chemistry, with special interests in polyphenols and aroma compounds and their precursors. He utilizes his expertise in synthetic organic chemistry and natural product isolation, purification and characterization, along with experience in developing and applying analytical methods, to improve understanding of grape and wine chemistry, particularly in relation to chemical composition and quality.

Foreword xiii

Preface xv

Introduction xvii

The chemical diversity of wine xvii

What is wine? xvii

Chemical reactions in wine xx

Chemistry as a historical record xxi

The chemical senses and wine flavor xxi

References xxiv

Part A Wine Components and Their Reactions 1

1 Water and Ethanol 3

1.1 Introduction 3

1.2 Chemical and physical properties of water 3

1.3 Properties of ethanol and ethanol–water mixtures 4

1.4 Typical ethanol concentrations in wines 6

1.5 Sensory effects of ethanol 7

References 9

2 Carbohydrates 11

2.1 Introduction 11

2.2 Nomenclature, representation, and occurrence of sugars 11

2.3 Physical, chemical, and sensory properties of sugars 14

2.4 Polysaccharides 17

References 18

3 Acids 19

3.1 Introduction 19

3.2 Organic acids in wine 19

3.3 Organic acids, pH, and wine acidity 21

3.4 Acid adjustments 28

3.5 General roles of organic acids and pH in wine reactions 31

3.6 Sensory effects of acids 31

References 32

4 Minerals 34

4.1 Introduction 34

4.2 Origins of metals in wine 35

4.3 Reactions involving metals 35

4.4 Sensory effects of metals 37

4.5 Metals and wine authenticity 37

References 38

5 Amines, Amino Acids, and Proteins 40

5.1 Introduction 40

5.2 Chemistry of amines 40

5.3 Amino acids and related major nitrogenous compounds in wines 41

5.4 Nitrogenous compounds with health effects 44

5.5 Odor-active amines 45

References 48

6 Higher Alcohols 51

6.1 Introduction 51

6.2 Properties of higher alcohols 51

6.3 Origins and concentrations of higher alcohols 52

6.4 Six-carbon (C6) alcohols 53

6.5 Methanol 54

References 55

7 Esters 57

7.1 Introduction 57

7.2 Chemistry of esters 57

7.3 Esters in grapes 60

7.4 Esters formed during winemaking and storage 60

7.5 Sensory effects 65

References 66

8 Isoprenoids 68

8.1 Introduction 68

8.2 General chemical and sensory properties of isoprenoids 68

8.3 Monoterpenoids 69

8.4 Sesquiterpenoids 73

8.5 C13-Norisoprenoids 74

References 76

9 Aldehydes, Ketones, and Related Compounds 79

9.1 Introduction 79

9.2 Acetaldehyde 80

9.3 Short and medium chain aldehydes 80

9.4 Complex carbonyls 84

9.5 Carbonyl reactivity 85

References 86

10 Thiols and Related Sulfur Compounds 88

10.1 Introduction 88

10.2 Varietal sulfur aroma compounds – polyfunctional thiols 91

10.3 Fermentative sulfur aroma compounds 93

10.4 Other sulfur-containing aroma compounds 94

Chemical principles: nucleophiles and electrophiles 95

References 97

11 Introduction to Phenolics 99

11.1 Introduction 99

11.2 Non-flavonoids 102

11.3 Flavonoids 103

References 104

12 Volatile Phenols 105

12.1 Introduction 105

12.2 Structure and chemical properties 105

12.3 Concentrations in wine and sensory effects 107

12.4 Origins in wine and effects on volatile phenol profile 107

References 110

13 Non-flavonoid Phenolics 112

13.1 Introduction 112

13.2 Hydroxycinnamates 112

13.3 Hydroxybenzoic acids 113

13.4 Stilbenes 114

References 115

14 Flavan-3-ols and Condensed Tannin 117

14.1 Introduction 117

14.2 Monomeric catechins 117

14.3 Oligomeric proanthocyanidins and polymeric condensed tannins 119

14.4 Sensory effects 122

Chemical principles: electrophilic aromatic substitution 123

References 125

15 Flavonols 127

15.1 Introduction 127

15.2 Concentrations of flavonols 127

15.3 Effects of growing conditions and winemaking 129

References 129

16 Anthocyanins 131

16.1 Introduction 131

16.2 Structures and forms 131

16.3 Non-covalent interactions: co-pigmentation 133

16.4 Bisulfite bleaching 134

16.5 Wine pigments 135

References 138

17 Sulfur Dioxide 140

17.1 Introduction and terminology 140

17.2 Acid–base chemistry of SO2 141

17.3 Sulfonate adducts, “bound SO2,” and antioxidant effects 143

17.4 Typical sources and concentrations of SO2 in wine 146

17.5 Measurement of molecular, free, and total SO2 146

17.6 Sensory effects 147

References 148

18 Taints, Off-flavors, and Mycotoxins 149

18.1 Introduction 149

18.2 Common wine taints 150

18.3 Off-flavors in wine 155

References 156

Part B Chemistry of Wine Production Processes 159

19 Outline of Wine Production 161

19.1 Introduction 161

19.2 Basic workflow 161

19.3 Processes involved 164

19.4 Tanks and fermenters 166

19.5 Beyond fermentation 169

19.6 Specialty wines 169

References 170

20 Grape Must Composition Overview 172

20.1 Sampling 172

20.2 Sugars 172

20.3 Acids 173

20.4 Phenolics 174

20.5 Nitrogen species 175

20.6 Lipids and waxes 175

20.7 Minerals and vitamins 176

20.8 Isoprenoids 176

20.9 Insoluble materials 176

References 177

21 Maceration and Extraction of Grape Components 179

21.1 Introduction 179

21.2 Pre-fermentative treatments 183

21.3 Maceration treatments during fermentation 186

21.4 Post-fermentation maceration 189

References 190

22 The Biochemistry of Wine Fermentations 194

22.1 Glycolysis 195

22.1.1 Introduction 195

22.1.2 Glycolysis and alcoholic fermentation 196

22.1.3 Glyceropyruvic fermentation 198

22.1.4 Succinic acid and other Krebs cycle intermediates 200

22.1.5 Consequences of glycolysis on wine chemistry 202

References 204

22.2 Fatty Acid Metabolism 205

22.2.1 Introduction 205

22.2.2 Long-chain fatty acid metabolism 205

22.2.3 Mid-chain fatty acids (MCFAs) and ethyl esters 209

22.2.4 Increasing MCFA and their ethyl esters in winemaking 211

References 212

22.3 Amino Acid Metabolism 214

22.3.1 Introduction 214

22.3.2 Nitrogen uptake and catabolite repression 214

22.3.3 Amino acid anabolism, catabolism, and carbon skeletons 216

22.3.4 Higher alcohol formation 216

22.3.5 Acetate ester formation 218

22.3.6 YAN in the winery – requirements, approaches, and consequences 219

References 221

22.4 Sulfur Metabolism 223

22.4.1 Introduction 223

22.4.2 Sulfide production and assimilation 223

22.4.3 Nitrogen sources and H2S formation 226

22.4.4 Timing of formation and residual H2S 227

References 228

22.5 Bacterial Fermentation Products 230

22.5.1 Introduction 230

22.5.2 Lactic acid bacteria 230

22.5.3 Spoilage of wine by bacteria 235

References 237

23 Grape-Derived Aroma Precursors 239

23.1 Glycosidic Precursors to Wine Odorants 240

23.1.1 Introduction 240

23.1.2 Formation of glycosidic aroma precursors in grape berries 242

23.1.3 Glycosidic aroma precursors – extraction 245

23.1.4 Hydrolysis of glycosidic aroma precursors – mechanisms 246

23.1.5 Hydrolysis of glycosides under fermentation and aging conditions 249

References 252

23.2 S-Conjugates 256

23.2.1 Introduction 256

23.2.2 Formation of S-conjugate precursors in berries and juice 257

23.2.3 Conversion of S-conjugate precursors during fermentation 259

23.2.4 Mass balance and alternative pathways to volatile thiol formation 262

References 263

23.3 Conversion of Variety Specific Components, Other 265

23.3.1 Introduction 265

23.3.2 Polyunsaturated fatty acid precursors of C6 compounds 265

23.3.3 Hydroxycinnamic acids, Brettanomyces, and volatile phenols 269

23.3.4 S-methylmethionine and dimethyl sulfide 273

References 275

24 Wine Oxidation 278

24.1 Introduction 278

24.2 Redox reactions 278

24.3 The central tenets of wine oxidation 280

24.4 The central tenets of quinone reactions 283

24.5 The central tenets of the Fenton reaction and byproducts 288

References 291

25 Topics Related to Aging 294

25.1 Introduction 294

25.2 Reactions involving red wine pigments 294

25.3 Hydrolytic and pH-dependent reactions 298

25.4 Activation energy and temperature effects on aging 303

25.5 Effects of oak storage 304

25.6 Sensory effects of different aging conditions 309

Closure performance 312

References 313

26 The Chemistry of Post-fermentation Processing 318

26.1 Cold Stabilization 319

26.1.1 Introduction 319

26.1.2 KHT crystal properties and solubility 320

26.1.3 Critical factors for KHT precipitation 322

26.1.4 Testing for KHT stability 325

26.1.5 Treatments for preventing KHT precipitation 327

26.1.6 CaT and related precipitates 330

References 330

26.2 Fining 332

26.2.1 Introduction 332

26.2.2 Tannin fining with proteins 333

26.2.3 Protein fining with bentonite 338

26.2.4 Miscellaneous fining and related treatments 341

References 343

26.3 Particle Filtration and Reverse Osmosis 346

26.3.1 Introduction 346

26.3.2 Definitions, principles, and characteristics of winery filtration 347

26.3.3 Filtration and fouling 350

26.3.4 Reverse osmosis 354

26.3.5 Sensory effects of filtration 355

References 357

26.4 Distillation 359

26.4.1 Introduction 359

26.4.2 Vapor–liquid equilibria 360

26.4.3 Batch and continuous distillation 365

26.4.4 Spirit composition and cask maturation 373

References 375

27 Additives and Processing Aids 377

27.1 Introduction 377

27.2 Regulations and terminology 377

27.3 Additives and processing aids: functions and comparison across regions 379

References 382

Part C Case Studies: Recent Advances in Wine Chemistry 383

28 Authentication 385

28.1 Introduction 385

28.2 Fraud – categories and detection approaches 385

28.3 Stable isotope ratio analysis to detect glycerol adulteration 387

28.4 Future challenges in wine authentication 389

References 390

29 Optimizing White Wine Aromas 392

29.1 Introduction 392

29.2 Enhancement of varietal thiols 392

29.3 Cofermentation and spontaneous fermentation 394

References 395

30 Appearance of Reduced Aromas during Bottle Storage 397

30.1 Introduction 397

30.2 Potential latent sources of compounds responsible for reduced aromas 397

References 399

31 Grape Genetics, Chemistry, and Breeding 400

31.1 Introduction 400

31.2 Breeding new varieties 400

31.3 Genetics and selection 402

References 403

32 Analytical Innovations and Applications 404

32.1 Introduction 404

32.2 Typical approaches to wine analysis 405

32.3 Multivariate data analysis and chemometrics 408

32.4 Chemometrics in practice – rapid methods for wine analysis 409

32.5 Targeted and untargeted metabolomics of wine 413

References 414

33 New Approaches to Tannin Characterization 417

33.1 Introduction 417

33.2 The challenge of astringency subclasses 418

References 418

Index 420

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