Wine E-Book

Jokie Bakker

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 780

Veröffentlichungsjahr: 2011

Beschreibung

Wine Flavour Chemistry brings together a vast wealth of information describing components of wine, their underlying chemistry and their possible role in the taste, smell and overall perception. It includes both table wines and fortified wines, such as Sherry, Port and the newly added Madeira, as well as other special wines. This fully revised and updated edition includes new information also on retsina wines, rosés, organic and reduced alcohol wines, and has been expanded with coverage of the latest research. Both EU and non-EU countries are referred to, making this book a truly global reference for academics and enologists worldwide. Wine Flavour Chemistry is essential reading for all those involved in commercial wine making, whether in production, trade or research. The book is of great use and interest to all enologists, and to food and beverage scientists and technologists working in commerce and academia. Upper level students and teachers on enology courses will need to read this book: wherever food and beverage science, technology and chemistry are taught, libraries should have multiple copies of this important book.

Leseprobe

Contents

Preface to the Second Edition

Preface to the First Edition

Chapter 1 Introduction

1.1 Scope of the book

1.2 Historical background

1.3 Wine flavour

1.4 Wine colour

1.5 Vinification

1.6 Physiological effects

Chapter 2 Grape Varieties and Growing Regions

2.1 Wine grapes

2.2 Vine plant characteristics

2.3 Soil, climate and ripeness

2.4 Grape growing regions of the world

2.5 Chemical composition of grapes, must and finished wines

2.6 Quality control and classification of wines

Chapter 3 Basic Taste and Stimulant Components

3.1 Introduction

3.2 Basic taste perception

3.3 Ethyl alcohol

3.4 Acidity

3.5 Sweetness

3.6 Bitterness, astringency and mouthfeel

3.7 Colouring matter

3.8 Other constituents

3.9 Changes in maturation

Chapter 4 Volatile Components

4.1 General

4.2 Volatile compounds detected in wines

4.3 Contents and sensory evaluation data

4.4 Changes during maturation

4.5 Aroma detection and quantification

4.6 Chemical structure and physical properties

Chapter 5 Wine Tasting Procedures and Overall Wine Flavour

5.1 Wine tasting

5.2 Wine tasting procedure

5.3 Factors influencing sensory perception

5.4 Balance of taste sensations in wine

5.5 Wine aromas

5.6 Wine and food flavour

5.7 Aroma indices and statistical methods

Chapter 6 Sherry, Port and Madeira

6.1 Introduction

6.2 Sherry

6.3 Port wine

6.4 Madeira

Chapter 7 Formation Pathways in Vinification

7.1 Introduction

7.2 Process variables in vinification

7.3 Production of ethyl alcohol

7.4 Production of individual groups of compounds

7.5 Noble Rot

Appendix I

I.1 Chemical formulae nomenclature

I.2 Stereochemistry

I.3 Chemistry of the oxidation of organic compounds

I.4 Estimation of partition coefficients of volatile compounds in air/water

I.5 Grape varieties and cultivars

Appendix II

II.1 Units

II.2 Data sources

Index

Advertisements

This edition first published 2012 © 2004, 2012 by Blackwell Publishing Ltd.

Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientific, Technical and Medical business to form Wiley-Blackwell.

Registered OfficeJohn Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

Editorial Offices9600 Garsington Road, Oxford, OX4 2DQ, UKThe Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK2121 State Avenue, Ames, Iowa 50014-8300, USA

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell.

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

Library of Congress Cataloging-in-Publication Data

Bakker, Jokie.Wine flavour chemistry / Jokie Bakker, Ronald J. Clarke.p. cm.Clarke’s name appears first on the earlier edition.Includes bibliographical references and index.

ISBN 978-1-4443-3042-7 (hardcover : alk. paper) 1. Wine–Flavor and odor.2. Wine–Chemistry. I. Clarke, R. J. (Ronald James) II. Title.TP548.5.F55C53 2004663′.2–dc23

2011014370

A catalogue record for this book is available from the British Library.

This book is published in the following electronic formats: ePDF [9781444345995]; Wiley Online Library [9781444346022]; ePub [9781444346008]; Mobi [9781444346015]

Preface to the Second Edition

Wine is primarily consumed for pleasure, and despite some attributed health benefits, it does not form an essential part of our diet. Therefore the sensory properties of wines are considered very important and the appreciation of its flavour arguably gives the wine drinker most pleasure. The wine is bought for its appeal in the bottle, for the knowledge the wine drinker has about the sensory properties and the anticipated enjoyment of consuming the wine. After evaluating the colour of the wine in the glass, most wine consumers will smell the wine, and judge its qualities. The release of flavours from wine continues when drinking the wine, and gives further flavour sensations in addition to the perception of many other taste and mouthfeel compounds that should be present in balanced amounts in the wine. There is a very wide range of well made wines available, so if a wine does not deliver the flavour the wine drinker desires and appreciates, a different choice can be made for the next occasion. Since wine flavour plays such a crucial role in wine, this makes a book devoted to Wine Flavour Chemistry particularly relevant.

The technological advances in viticulture, wine-making and the resulting improved wine flavours have been based upon the scientific exploration of vines, grapes and wines, their constituents, their chemistry and all aspects of the wine-making process by scientists in many disciplines in research establishments world-wide. The understanding of flavour chemistry and its perception is determined by numerous scientific disciplines, ranging from chemistry and laws of physics to human physiology. Since the first edition of Wine Flavour Chemistry more research has become available on wine flavours, with new compounds still being identified. In addition, scientists place much emphasis on determining the potential sensory contributions of flavour compounds in wine, making flavour information as relevant as possible. Advantageous in all research dealing with flavours is the increased scientific understanding of the perception of volatiles and the award of the Nobel Prize for Physiology and Medicine in 2004 to Richard Buck & Linda Axel for their pioneering research on the genetics of the perception of odour (ref. in Chapter 4) has given great impetuous to this research field.

Although aspects of wine-making may always remain an art, such as the numerous choices to be made in the wine production process in order to optimize the wine flavour, science has definitely got a very sound and well deserved foothold in the wine making industry. Many highly trained and skilled wine makers work in the wine industry, helping to adapt advances in wine science in order to successfully influence wine making and ultimately wine flavour. All wine drinkers benefit from the well made wines with a wide range of wine flavours available nowadays. This updated book, Wine Flavour Chemistry may attract many different readers interested in wine, ranging from wine consumers, students, academics and people working in the wine industry. Information has been gathered from scientific research, review papers and books to update this comprehensive overview of the subject.

I wish to express my grateful thanks to all the scientists who have kindly shared their research information, which was of invaluable help in preparing this updated book.

Jokie Bakker MSc (Wageningen), PhD (Bristol)

Preface to the First Edition

This volume on wine flavour chemistry has been in gestation for many years; an original draft was started some ten years ago. A number of events led to our renewed interest in getting this book published. First, in the UK, wine has become a drink enjoyed by many consumers at numerous occasions, whereas previously, wine tended to be a drink shrouded by mystique and enjoyed mainly by more knowledgeable people. In contrast, in countries where wine production has been established for a long time, a long-standing culture of wine consumption, mainly with meals, has been established. Since there are now many people in the UK as well as in other areas of the world interested in consuming wines there is also an increased quest for knowledge about wine, making a book focused on the flavour and its chemistry particularly pertinent. Second, during the last two decades, there has been an enormous development in knowledge about viticulture and the technology of wine making worldwide. This has resulted in a much-improved wine quality. Third, many ‘new’ wine regions have been established. These have not been inhibited by cultural pre-conceptions about wine production, and have experimented in many different ways, pushing the boundaries of both viticulture and wine making. Fourth, financial investments in vineyards and wineries, hand in hand with generally vastly improved wine production skills, have given an array of wine flavours from grapes, which wine-makers in the past could barely have believed possible. The cultivation and production of single variety/cultivar wines have given consumers an insight into the many flavours possible in wines. Of course, the technological advances in viticulture and wine making have been based upon the scientific exploration of grapes and wines, their constituents and their chemistry, by scientists in many disciplines worldwide. Advances in analytical laboratory instruments have proven to be a great help. For example, mass spectrometers have, over the last ten years, become much more sensitive, much smaller, much cheaper and easier to use. This has resulted in an explosion of new data regarding the volatile compounds of wine. Numerous other advances in analytical techniques have aided the quest for knowledge about wine flavour, colour and taste. Interestingly, alongside these technological advances has been the development of sensory science. There is now an array of scientifically based sensory analytical techniques allowing scientists to determine our perception of wine, including the measurements of differences between wines and descriptions of sensory properties.

Two recently published scientific books on wines summarize much of the wine information available. In 1994, Jackson in Canada published his excellent and wide-ranging book, Wine Science, with a second edition in 2000. This was followed in 2000 by the equally excellent Handbook of Enology from Ribéreau-Gayon and his colleagues in France. This new book on the chemistry of wine flavours draws together aspects of wine making pertinent to wine flavour, and tries to link chemistry, flavour composition and sensory properties. Our volume draws much information from these antecedent books for which due acknowledgement is readily made. We make similar grateful acknowledgement to several highly perceptive and entertaining, but not overtly scientific, books on wines by British wine writers and journalists. Of course, information from many scientific papers is also used to give a comprehensive overview of the subject. This book, Wine Flavour Chemistry, differs from all the foregoing in that it is uniquely devoted to the subject of the flavour of the wine. It is based on the chemistry of the compounds, both volatile and non-volatile, together with the application of the techniques of modern sensory analysis.

The quantities of volatile compounds in head-space air, that is, the air above a glass of wine, are determined by partition coefficients, which express the ratio between the amount of those compounds in the liquid and in the air above, both amounts often only in parts per million or much less. The quantities present have to be above threshold levels of detection and recognition in order for them to be perceived effectively by the olfactory organs of the nose. There are also threshold levels for non-volatile compounds present in much larger quantities in wines (several grams per litre), and detected only by the taste buds of the tongue, which detect the basic tastes of acidity, sweetness, bitterness and saltiness. Signals from these two highly sensitive organs (the olfactory epithelium and taste buds) are transported through nerve fibres to the brain. Certain other elements of sensory analysis, such as colour, appearance and tactile sensations in the mouth, including astringency, contribute to the overall flavour assessment. Perhaps it is the naming of all the different flavour sensations in all the different wines that is of the greatest intellectual and aesthetic interest, together with their association by scientists with particular chemical substances or groups of substances.

This volume, therefore, attempts to bring together in a readable and accessible form the most recent research from this rapidly developing field. It is aimed to be of interest to consumers with an inquisitive mind about wine, and to all those involved with the production of and trade in wines with an interest in the chemical and technical aspects of wine flavour.

A considerable amount of threshold flavour/odour and other flavour compound information has been recently compiled by Flament in his comprehensive book, Coffee Flavor Chemistry (2002). Many wine odour compounds, now known to number around 400, are also to be found among the much larger number of volatile compounds (about 800) in green and roasted coffee brews. In wines, alkyl esters from the fermentation of the must and unchanged terpenes from the grapes are particularly characteristic, though they may undergo change during subsequent ageing; in coffee, many distinctive compounds develop from the roasting process.

Reference to coffee flavour, indeed that of other beverages, should not be surprising since one of the authors (RJC), whilst an enthusiastic wine bibber, spent some forty years in the scientific study and industrial manufacture of coffee, and is the author or co-editor of several books on that subject. The other author of this book (JB) brings a wealth of knowledge and experience directly from her research into wine, especially within fortified wines, with numerous scientific research papers published on this subject.

We wish to express our grateful thanks to all those who have helped us with this book, in particular Professor Clifford of the University of Surrey, UK, and Professor André Charrier, ENSA M, France.

Ronald J. Clarke MA(Oxon), PhD(Hon), CEng, FIChemE, FIFSTJokie Bakker MSc(Wageningen), PhD(Bristol)

Chapter 1

Introduction

1.1 Scope of the book

The primary meaning of the word ‘wine’ is the product of the aqueous fermentation by yeasts of the sugars in the juice of grapes. The fermented juices of many other fruits are sometimes also called wines, though they do not enjoy the same popularity or prestige as the grape wines. Fermented liquids from materials containing starch or cereals are usually called ‘beers’. The term wine is incorrectly used, for example, in rice wines, since the sugars in rice are stored as starch. Since fermenting yeasts can only convert sugars into alcohol, raw materials containing starch need to be processed so that, first, sugars are generated, for example, by hydrolytic cleavage of the starch. Uniquely, grapes contain tartaric acid, which has preservative qualities, and which, in addition to the presence of fermentable sugars, gives wines both a relatively high acid and alcohol content.

This book is solely concerned with wine from grapes. The focus is on the chemistry and flavour of table wines, which are normally consumed with meals. These wines have an alcohol content of 9–15% v/v (percent by volume), typically 11.5–14% v/v for red wine. Many red wines from hot wine regions exceed this percentage since the grapes are picked more mature with higher sugar levels. Wines consumed before a meal (aperitifs) are usually ‘dry’ (low sugar content) and often fortified to raise the alcohol content to about 20% v/v. Wines consumed after a meal tend to be sweet, for example, made by fortification with alcohol before the yeasts have converted all the sugar into alcohol, giving fortified wines such as Port as made in the Douro region in northern Portugal. Such fortified sweet wine styles are also made in other regions, and will be referred to as Port style. Fortification of dry wine followed by a special maturation process gives Sherry, as made in the Jerez region in Spain. Some other wine regions also use variants of this production to make Sherry style wines. Sherry can either be kept dry to be served before a meal or sweetened to be served after a meal. The wine-making process of the classic wines Port, Sherry and the less popular Madeira will be described separately.

There is a very wide range of types of table wines, from sweet to dry and from still to sparkling, including its most famous example, Champagne. Table wines can be red, rosé or white, the colours depending on the choice of grapes and the wine-making processes used. The wines can be sweet or dry, although red wines tend to be always dry, while white wines are produced from dry to very sweet, with a range of different sweetness levels in between. A most remarkable sweet white wine is made from grapes infected by Noble Rot that is caused by a mould (Botrytis cinerea). The term ‘wine’ will be restricted to the main species of the vine plant, Vitis vinifera, which covers about 98% of the total wine production from grapes.

1.2 Historical background

There is much historical information on wine, for example, Johnson’s (1989) excellent writing and McGovern (2003). It is generally considered that vines originated from the Caucasus area of Russia, between the Baltic and the Caspian Seas. After the Stone Age, some 6000 years ago, settled agricultural practices developed in the ‘Fertile Crescent’ of Mesopotamia and Egypt. Wild vines, botanically known as Vitis vinifera sylvestris, became domesticated and strictly speaking became the so-called Vitis vinifera L. sativa.

The Vitis genus contains many similar species, with other names such as Vitis labrusca (see the next paragraph). From those very early origins in Mesopotamia and Egypt, vines and wine-making methods were exported to the Greek- and Latin-speaking world of the Mediterranean. After the decline of these civilizations, wine production in Europe was not established until late medieval times. Wines were shipped in barrels and even countries with little or no wine production could enjoy drinking wine. Fortified wines such as Sherries (the Sack of Shakespeare’s Falstaff) developed as a result of the Arabian invention of distillation, which gave the required skills to prepare fortifying spirit that could be added to wines. In the late nineteenth century wine production was fully established in France. France, Italy and Spain are still the three largest wine producers.

Speculation remains, however, about the flavour of these early wines compared with the wines we know today. Grape juice is fairly easily fermented by ubiquitous yeasts, and the fermented product can be reasonably stable due to its relatively high alcohol and acid content. However, there are longer-term storage problems and it is likely that wine spoilage was a frequent problem. This was due to a lack of understanding of the actual processes involved in vinification and their effective application. For example, it was not until the days of Louis Pasteur in the 1860s that the role of yeasts in wine fermentation, in addition to the role of some lactic acid bacteria in wine spoilage, was uncovered. The scientific achievements of Pasteur regarding the discovery of the microbiological processes involved in wine-making laid the foundations for the modern wine industry.

As late as the eighteenth century, wines were mostly sweet, although even from these more recent times we have little information regarding the sensory properties of the wines. Roman wines are thought to have been more like syrups. Wines were stored and transported in amphorae, which were long earthenware vessels fitted with stoppers, often made of waxy materials. The Romans used glass decanters to bring wine to the table but glass was too fragile for storing wine. An especial boost came with the invention of glass bottles in the early seventeenth century that were sufficiently strong to allow the transport and storage of wine. Once corks started to be generally used to seal wine bottles, it was a relatively small step to mature the wine in bottles, which had to lie at a fairly even temperature to prevent leaking. Wines have long been imported to the UK, where there was an appreciative market for so-called fine wines. For example, there was a marked interest in red wines from Bordeaux in the early twentieth century, to accompany the lengthy Edwardian dinners.

The native vine plant is confined to certain latitudes of the world and its domesticated version similarly requires favourable growing conditions. In particular, vines thrive in a climate with the right combination of sun and rain, although varieties/cultivars have been adapted to suit various climatic conditions. The type of soil is important, with adequate drainage being a prerequisite for successful vine cultivation. The areas of growth include North and South America, outside the tropics and excluding the very temperate zones. The commercial production of wines in many regions outside Europe did not really develop until the late nineteenth century. White settlers in Australia and New Zealand were interested in wine-making but only after World War II did the wine industry really develop. The spread of vine and wine is probably also closely linked to social and cultural aspects of communities.

Viticulture in Europe and elsewhere, like the production of other domesticated plants cultivated for food and drink, has been closely associated with the activities of plant breeders. Hence, over the centuries many varieties/cultivars of the species Vitis vinifera have been selected, e.g. Vitis vinifera Pinot Noir, and are responsible for the various wines that are available in the market place. An important part of the history of wine is the disease caused by Phylloxera, a root louse pest accidentally imported from America that struck nearly all vines in Europe in the 1870s, devastating many vineyards by killing the vines and thereby ruining the wine industry. It was not until the discovery that grafting local European vines onto American imported root-stocks conferred resistance to the disease that the wine industry in Europe started to recover. Ironically, Phylloxera eventually attacked vines in California around 1980, damaging many vineyards.

1.3 Wine flavour

The smell and taste of a wine are directly associated with the chemistry of the entire wine-making process. The word flavour usually indicates the combination of smell (or odour) and taste. However, when assessing the sensory properties of wine, the word ‘tasting’ is used to indicate that the flavour of the wine is being judged. The flavour of wine originates from (1) the grapes, (2) the treatment of the must (grape juice) and its fermentation and (3) the maturation process of the wine. The chemistry of the flavour compounds derived from these three sources will be discussed in some detail for both non-volatile (Chapter 3) and volatile (Chapter 4) compounds.

Wine writers in numerous books and articles, many in the English language, have dealt with the subject of wine flavour. Some texts are aimed at the marketing aspects of wine and emphasize the opinions of expert wine tasters. Other texts are more critical, such as Barr (1988). Of course, there are also numerous texts in French and German, dealing with all aspects of wine. The number of technical texts which directly relate the flavour of the wine to its chemistry is much more limited, though there are some chapters in books on food and beverage flavour in general (see Bibliography). Many scientific papers describe only individual aspects of wine flavour and its chemistry. None of these texts are complete; they omit to raise many questions and fail to answer many others. An exception is the comprehensive scientific book of Jackson (1994, revised for the second and third edition in 2000 and 2008 respectively), which discusses in detail the three interrelated topics of wine science: grapevine growth, wine production and wine sensory analysis. Ribéreau et al. (2006) have published a similar work in two volumes (in English).

The term ‘wine tasting’ is often used and suggests ignorance of the essential nature of wine flavour, which is a combination of (a) the five taste sensations (sweet, salt, sour, bitter, umami) from non-volatile substances perceived on the tongue and (b) the aroma (or smell) sensation from volatile substances perceived by the olfactory organs behind the nose. Volatile substances reach the olfactory organs by two routes, sometimes referred to as the nasal and retronasal routes. Nasal means that volatile compounds will reach the olfactory organ through the nostrils of the nose during the period of ‘nosing’ the wine from the glass. Nosing is the traditional sniffing of the air space above the glass of wine, before any sample is placed in the mouth. Once in the mouth, the wine is warmed up, moved around in the mouth and there is the option of noisily sucking air through the mouth. All these actions help the volatile compounds to escape from the wine and to travel retronasally via the back of the mouth to the olfactory organ. Volatile compounds detected during nosing are often described separately, and may or may not be similar or identical to those detected on the palate. Wine tasting will be discussed in Chapter 5.

There is no consensus in the use of terms like bouquet, aroma, etc. and different wine writers may use them with different meanings. The term ‘aroma’ is most commonly used to describe the smell of the wine derived from the grapes, while the term ‘bouquet’ tends to refer to the smell of the wine formed as part of the development during maturation.

The quality and quantity of colour as well as the clarity of the wine are assessed entirely by eye, usually before the tasting. Next, our sense of smell and taste are used to assess the flavour of the wine. The depth of intensity and the multicomponent detection of flavour notes in wines (usually described in terms of flavour notes from other fruit/vegetable/mineral/animal sources) that are used to describe wine attributes by many expert wine tasters is surprising to the non-expert wine-drinker, and at times stretches credulity. In addition to flavour recognition and description, there are also the perceptions of mouthfeel, temperature, bubbles, etc., which all are registered and assessed by our senses. Over and above the enjoyment of the wine flavour, wine is also drunk for its stimulant properties, derived from up to 15% v/v ethyl alcohol, formed by the fermentation of sugars in the must by fermenting yeasts.

The flavour of wine is determined by the grape variety (or varieties), in combination with the growing conditions, such as climate, agronomic factors during growth and harvest, and these are reflected in the composition and organic chemistry of the must. Perhaps equally important is the process of vinification used; in particular, must treatment, temperature, yeast strain, use of fermentation aids, filtration and other processes used, together with any maturation (ageing) process. The relative importance of these factors is a moot point, but they are all determined by chemical causes. Interestingly, for example, French wines are essentially characterized by the region in which they are produced, as referred to in the Appellation d’Origine Contrôlée (AC), usually without mention of the grape varieties used (although some French wines nowadays list the grape(s) used on the label). Although the grape varieties are defined in the AC, the proportions used may vary from year to year. As from 1 May 2009 the term ‘Appellation d’Origine Contrôlée AOC or AC’ has, with resistance from some producers, been progressively replaced with the new European standard, Appellation d’Origine Protégée (AOP). In contrast wine makers in many other countries, especially in the ‘New World’, make a feature of characterizing their wines by the grape variety used. In short, currently the French emphasize the ‘terroir’, while in many relatively new wine-making countries the emphasis is on grape variety. A particular grape variety (e.g. Chardonnay, Cabernet Sauvignon) can, evidently, produce a rather different wine flavour as a result of the method of vinification and maturation, even though, usually, the characteristics of the wine flavour for the grape variety remain recognizable. A given variety grown in a certain region is also claimed to give a different wine than when grown in another region, even though the process of vinification is essentially the same. This subject is further explored in Chapter 5.

Therefore the flavour of the wine derived from the grape has to be considered in terms of its complex chemical composition, which is detailed in subsequent chapters. Current wine-making practice is outlined in Section 1.5 to give a better understanding of the background in which chemical changes related to flavour occur. Formation pathways of flavour compounds during vinification are discussed in Chapter 7. Some physiological aspects of wines related to wine chemistry will be described briefly.

1.4 Wine colour

Wines are primarily distinguished by their colour and fall into three groups (1) white wines, which include most sparkling wines, (2) red wines, including most fortified Port style wines and (3) rosé wines, essentially an intermediate between white and red wines. A wine’s colour is determined by the choice of grape and the vinification process. White grapes, which usually have pale yellow skins, give white wines, while black grapes, which have blue, red or even black skins, depending on the amount of colouring matter in the skins, give mostly red wines. Red grapes can give a range of wine colours, from deep red to rosé, depending on the wine-making process, and by careful handling they can even yield white wines, for example the ‘blanc de noirs’ in Champagne production. Since most of the colour is in the grapes’ skin, the choice of the vinification technique for red grapes allows a lesser or greater extraction of colour into the wine.

Within each colour, however, there will be differences between wines, which are easily perceived in the wine glass. The clarity may also differ, due to very small amounts of very finely suspended insoluble matter (not desired), although nowadays most wines are clarified before reaching the consumer. The changes in colour that occur during ageing, whether the wine is stored in-bottle or in-cask, are determined by chemical composition; the colour of red wines depends on the content and composition of anthocyanins. Colour and the chemistry of the changes in colour during maturation will be discussed briefly in Chapter 3.

1.5 Vinification

Traditionally good wines were made in regions where the conditions were frequently just right to give healthy, ripe grapes, with somewhat cooler weather during vinification. Although undoubtedly much knowledge was collected over many years, there was limited control over the process and wine-making was, to an extent, considered to be an art. With the advance of our scientific knowledge of many aspects of wine-making and the improvements of technology used in wine-making, in particular the use of refrigeration at various stages, there is now much control over the process. Nonetheless, the wine maker still faces many choices that determine the properties of the wine, and so the art still remains in making the best possible wine that is typical for the grapes and the region. The use of modern technology has also enabled good quality wines to be made in many more regions, including ones once thought to be too warm.

A basic understanding of the wine-making process (‘vinification’) is necessary in any study of wine chemistry and wine flavour. Grapes are the key ingredients, and they should be healthy, mature and in good condition. The choice of grape variety will influence the wine flavour and colour and to an extent depends on the region. Grape varieties and some of the main growing regions are discussed in Chapter 2. In essence the grapes are picked, crushed to form a ‘must’ (grape juice) and fermented by yeasts to convert the sugars present into alcohol. There are three stages in wine-making, all of which can influence the flavour and colour of the wine:

(1) Pre-fermentation, during which various pre-fermentation treatments of the grapes or must can be given (such as sulfur dioxide addition, sugar or acid adjustments, nitrogen contents and possible addition, clarification of must, contact time with the skins or ‘maceration’ and cooling).

(2) Fermentation of the must, during which several factors have to be managed (such as choice of fermenting yeast, fermentation temperature, maceration time and pressing conditions).

(3) Post-fermentation, during which several different treatments are available. Some are probably essential (such as racking to remove the spent yeast or ‘lees’), while others are optional, depending on the desired characteristics of the wine (such as filtration, cold stabilization). Wines can be made to drink when young, or after maturation (or ageing) in different types of vessels (such as old vats, new oak barrels or bottles).

Several general rules of modern wine-making have emerged. The production of both red and white quality wines requires attention to the following, as emphasized in many wine-making publications:

(1) Grapes should be picked at optimum ripeness, in sound and healthy condition, at as low a temperature as possible (in very warm conditions, they should be cooled) and transported to the winery with minimal delay for immediate processing. The must for white wines should be cooled both before and during fermentation.

(2) Strict adherence to cleanliness of all wine-making equipment; to prevent the growth of spoilage organisms on the grapes, in the must or in the wine at all stages of fermentation and maturation. A particular risk are Acetobacter bacteria, which convert alcohol into acetic acid, hence spoiling the wine into vinegar.

(3) Non-oxidizing atmospheres should be used, especially in the early stages of vinification of white wines, for example by blanketing the must or wine with inert gas and/or by addition(s) of sulfur dioxide.

(4) The temperature of the fermentation should be controlled. Heat produced during fermentation (an exothermic process) in stainless steel tanks should be removed by efficient cooling and refrigeration. This is usually done by cooling the outside of the tank or sometimes by pumping the must through an external heat exchange unit. Fermentation in barrels may lose heat through the relatively larger surface area; however, barrels are difficult to cool, though ice is sometimes used.

The success of wines produced in regions such as Australia, California and South Africa has been attributed to careful attention in controlling the vinification process, especially to the factors listed above. Of course, great care is also given to planning the vineyard (site selection, considering soil, climate and choice of the most appropriate grape) and vineyard management (pruning, fertilization, preventing disease and picking at grape maturity). Modern scientific methods of wine-making analysis and control have also been adopted. Nevertheless, traditional wine makers in Europe have not been slow to adopt modern practices and the overall quality of wine now available to consumers has improved significantly.

The equipment used for fermentation and details of that used for both pre- and post-fermentation stages have been well described in detail (Jackson, 2008; Ribéreau-Gayon et al., 2006), while Robinson (1995) has given a good account for the general reader. The actual equipment used differs between wineries, depending on local conditions and the style of wine that is made.

Spoilage of wine is what all wine-makers want to avoid. This risk is present at all stages of wine-making, and can be both microbiological and chemical in nature. Under the wrong conditions, Acetobacter bacteria can change wine into vinegar in a very short time. Yeasts also pose a risk, as reviewed by Loureiro & Malfeito-Ferreira (2003), pointing out that understanding the ecosystems of wineries is crucial in prevention of spoilage. Excess air is also a known enemy, possibly causing chemical spoilage by oxidation of the wine, and enhancing the risk of microbial spoilage.

1.5.1 Vinification process

The basic wine-making process described here, highlighting those parts that greatly influence the resulting end product, covers many components that are common to both red and white wines. Recent textbooks by Jackson (2008) and by Ribéreau-Gayon et al. (2006) offer the reader a more in-depth treatment. The formation of specific flavour compounds during vinification (Chapter 7, devoted to reaction pathways) and volatile compounds (Chapter 4) are discussed elsewhere. A recent review on red wine-making reviews steps of wine-making affecting colour (Sacchi et al., 2005), and interestingly cold soaks and the addition of sulfur dioxide tended to have an effect on the wine only over a short time, after some maturation the differences became minimal. Other parameters, such as yeast selection and carbonic maceration showed that the grape varieties affected the results.

Flow charts for both the production of red and white wines (Figs. 1.1 and 1.2 respectively) give a helpful overview of the various wine-making steps. Figures 1.1 and 1.2 reflect the headings below, as appropriate. Specific information for the production of red (Section 1.5.2) and white (Section 1.5.3) wines is included below. These brief descriptions focus on the differences from the general vinification process and, therefore, not all captions will be used in Sections 1.5.2 and 1.5.3.

Figure 1.1 Typical processing sequence in vinification of red wines. Some steps are optional (see text). Oak contact can occur during fermentation and/or storage. Prolonged storage in-barrel or tank will mature wines.

The production of specialized wines is described separately, briefly emphasizing the part of the process that makes these wines different. As stressed above, some wine-making steps are essential, but the wine maker can make numerous decisions that will determine the overall style of the wine.

Pre-fermentation

Grapes/harvest

The first important step is the grape harvest. The grapes are picked at commercial maturity, which is usually determined by both their acid and the sugar content. The maturity of the grape will also affect the aroma and phenolic composition typical for the variety, an aspect of increasing importance since they affect the varietal character and quality of the wine. As grapes mature, their acid content decreases and their sugar content increases. In hotter climates, the mature grapes tend to be high in sugar and low, sometimes even too low, in acid. In cooler climates the reverse happens: the grapes may struggle to reach full maturity and remain low, sometimes even too low, in sugar content and high, sometimes too high, in acid content.

Figure 1.2 Typical processing sequence in vinification of white wines. Some steps are optional (see text). Oak contact can occur during fermentation and/or storage. Prolonged storage in-barrel or tank will mature wines.

The grapes can be picked by hand or by mechanical harvester. Hand picking allows access in even the most inaccessible vineyards (steep slopes, very soft soils), does not restrict the pruning style of the vines and some selection in the vineyard can be made (rejection of immature fruit and overripe, mouldy fruit). However, hand picking has a high labour cost. Machine picking is fast, costs much less in labour and can be done at night, which is attractive in hot regions to keep the fruit as cool as possible. Fast picking may help to ensure that the grapes can all be picked at optimum maturity, especially important in hot climates where a short delay in picking may give overripe and shrivelled grapes. However, mechanical picking has specific requirements on vineyard lay-out, access and vine pruning and training, etc. No selective picking of individual grape bunches is possible and it may not be desirable to pick grape varieties with very thin skins.

Transport/handling/cooling

Grapes should be handled with care to avoid damage that would encourage the growth of undesirable micro-organisms and lead to oxidative browning. Hence, hand-picked grapes are generally placed in shallow containers to prevent them being pressed under their own weight. The fruit must be kept cool and transported to the winery for processing with minimal delay. When using a mechanical harvester, it is important to avoid damaging the fruit.

De-stemming/crushing

Next, the grapes are usually de-stemmed and crushed. De-stemming involves the removal of stems, grape stalks and leaves, thereby minimizing the extraction of phenols and other compounds considered undesirable in the wines. Some of these phenols may impart bitterness and astringency to wine (Chapter 3), whilst some of the other compounds can give off-flavours or a haze in the wine.

The grapes are crushed, usually immediately after de-stemming, as gently as possible since excess pressure may damage the seeds, which will lead to the extraction of an excess of compounds (phenols) that, due to their bitter and astringent properties, would tend to give a harsh wine. With crushing the grapes are broken open, thus releasing the juice from the grapes. This facilitates the onset of fermentation, since the yeast has easy access to the sugar-containing juice. Most modern wine-making equipment carries out both stages, and it is generally thought that de-stemming before crushing gives the least extraction of undesirable compounds in the wine. In the past the stems were, notably, not removed, and their presence in the fermentation of red wine facilitated the pressing procedure. Modern equipment for pressing the wine does not require the presence of stems. Some traditional wine makers may still choose to leave the stems in during the fermentation stage, although any vine leaves should always be removed.

Additions/adjustments

Additions of sulfur dioxide, sugar, acid, nitrogen and enzymes can be made but also acid adjustments can be made. Dependent on the soundness of the grapes, sulfur dioxide may be added at crushing. Sulfur dioxide has several functions; it protects the must against both browning reactions and, to an extent, unwanted micro-organisms. It suppresses the growth of wild yeasts and delays the onset of the fermentation. It is probably most effective in its use to protect against undesirable spoilage bacteria, especially if the pH of the must is low. However, it can also inhibit the malo-lactic fermentation, so its use should be avoided if an early malo-lactic fermentation is desired. It also damages the grape skin, and therefore enhances extraction of compounds from the skins.

Just prior to fermentation several adjustments may be made to the must, depending on local conditions and regulations. Grapes from cool regions containing insufficient sugar to ensure an adequate final alcohol content in the finished wine may receive an addition of dry sugar (sucrose, in crystal form). This process is called chaptalization, and was first introduced in France by M. Chaptal in 1801. Usually no more sugar than the equivalent of 2% v/v alcohol may be added. In addition, if the must contains excess acid, which tends to be a risk in cool climates, it may need to be de-acidified, often by the use of ‘Acidex®’, a form of calcium carbonate. In the same way, must prepared from grapes from hot regions containing insufficient acid may need to be acidified, usually by an addition of tartaric acid, otherwise the wines may taste flat and may have a high pH, which is undesirable for both chemical and microbiological stability.

When it is suspected that insufficient nitrogen is available to complete the yeast fermentation, an addition of diammonium phosphate can be made to the must to ensure an adequate concentration of nitrogen for yeast growth and maintenance. In particular highly clarified juice for white wine fermentation or must made from Botrytis infected grapes may not have a sufficient nitrogen concentration. There are two reasons for this addition. Firstly to avoid the fermentation getting stuck before the process is finished and secondly to avoid the production of hydrogen sulfide by yeasts, which is produced when the yeasts have prematurely depleted nitrogen in the fermenting must, as reviewed by Bell & Henschke (2005) and Ugliano et al. (2007). However, Jackson (2008) considers the addition of diammonium phosphate unnecessary and suggests that it does not prevent the formation of hydrogen sulfide. Possibly other yeast nutrients may also be inadequate in the fermenting must, not just nitrogen, for more detailed discussion see Ugliano et al. (2007). They suggest that cleaner, fruitier style wines may be obtained if wine-makers ensure that there is between 250 and 300 mg L−1 nitrogen available for yeast fermentation. During the last decade much more information regarding the interaction of yeast and nutrients in the must and the effect on the formation of flavour has become apparent and is further discussed in Chapter 7.

Additions of enzymes, including pectolytic enzymes to enhance colour extraction can also be made, however, the review by Sacchi et al. (2005) concluded that pectinases do not seem to increase anthocyanins in wines but only increase the extraction of other phenolic compounds in wines. Presumably, anthocyanins were rapidly lost by the naturally occurring maturation reactions these compounds participate in, as suggested by the increased polymeric pigment formation observed in pectinase treated wines.

Additions of enzymes can also be made to stimulate the hydrolyses of glycosides from terpenes, since only the non glycosilated terpenes contribute to the volatile varietal aroma characteristics of the must, or more importantly the wine. The review by Maicas & Mateo (2005) gives an in depth overview of the current enzymes available, which gives wine makers potential tools to enhance the varietal aroma. The authors also warned against some potential adverse effects, such as the production of off-flavour as a result of vinyl phenols, resulting from the presence of undesirable cinnamate decarboxylase activity. Overall, with further studies on both the definition of desirable varietal wine aroma and precisely defined action of added enzymes, the use of these glucolytic enzymes give scope in the future to manipulate the wine aroma.

Enzymes can also be added as processing aids, such as enhancing the ease of pressing or aiding juice clarification.

Maceration

This is the process of steeping or soaking the skins and seeds of the grapes in the grape juice that was released from the grapes during crushing. It forms an important part of red wine-making, since during maceration the phenols, including the coloured ones (anthocyanins) and the flavour compounds from the grape skins, the seeds and the residual grape stems are dissolved into the juice, or into the fermenting must. The efficiency of the maceration depends on the temperature, the length of time and the amount of agitation the macerating mash receives. A recent review of Sacchi et al. (2005) lists the various techniques commonly used and their effects on the extraction of colour of red wines.

The maceration of a red wine typically starts before the onset of fermentation, and continues during part or the entire fermentation, and in some instances even after fermentation. The extraction of compounds will change in the presence of an increasing concentration of ethanol. How much maceration the wine should get is decided by the wine maker. Generally, longer macerations will lead to darker, more phenolic wines, requiring longer maturation before the wine is ready to drink. Red wines made to be drunk young tend to have short maceration times and are run off the skins before the fermentation has finished. Most of the anthocyanins are extracted during the first five days, whilst prolonged extraction increases the phenolic compounds in wines. Astringency of red wines is mostly attributable to phenols, hence longer extraction of red grapes tends to give more astringent wines. Many of the flavour compounds are in the skins of the grapes; hence maceration also increases the extraction of flavour compounds into the wine, although little information is available on red wines.

Temperature of maceration can also be varied, and before the fermentation starts some wine-makers elect to do cool (about 15°C) to cold (4°C) macerations; temperature effects on colour and flavour have been reported (see Jackson, 2008), although increased risks of spoilage due to the adaptation the indigenous flora has also been reported.

As well as the temperature and duration of the maceration, during fermentation the agitation is also usually controlled. Without agitation, solids (grape skins, seeds, etc.) would float to the top with the stream of carbon dioxide produced during the fermentation and form a thick surface layer, known as the ‘cap’. Obviously there would be no extraction from such a cap, and the risk of spoilage organisms growing in the cap (which is warm once the fermentation gets on the way from the heat released from the fermenting yeasts) is extremely high. Therefore it is imperative to keep the cap wet by some way of ‘working’ the must. Probably the most commonly used method is known as ‘pumping over’, in which juice is drawn from underneath the cap and quite literally pumped over it. Often such a process can be done semi-automatically every few hours, the time required depending on the size of the fermentation vessel, temperature, etc.

Of the several other methods used to keep the cap wet, some are quite simple, although labour intensive, while others rely on more sophisticated fermentation equipment. If the fermentation is performed in a shallow open vat, the cap can physically be punched down with the aid of simple punching sticks. If the fermentation takes place in a tank, a physical restraint (a metal grill or wooden beams) can be fitted just under the expected level of the must when the tank is full, to keep the cap submerged. There are also fermentation vats designed specially to keep the cap wet, such as autovinification vats (using pressure from the carbon dioxide released during fermentation to pump over the must) and roto-fermenters (using rotation of the tank, usually the tank being fitted in brackets on its side). A combination of colour extraction, must cooling and aeration can also be achieved by a technique of rack and return, a widely used method also known as délestage. After the start of the fermentation and the formation of the cap, the juice is drained into a holding tank, sprayed into a second tank and then pumped back into the fermentation tank. At the drainage stage seeds can be removed, desirable if grapes are immature to avoid excess extraction of extractable phenols.

Another maceration technique discussed was the use of freezing to enhance extraction (Sacchi et al., 2005). They reported that freezing tended to enhance the extraction of both anthocyanins and phenols, presumably because freezing damages both the skin of the berries, allowing easier extraction of the coloured anthocyanins and the seeds of the fruit, thus aiding the extraction of phenols. The authors also suggested that the use of dry ice has the added advantage of protecting the berries from oxygen. However, although such studies add greatly to our understanding of the processes in wine-making, currently it would seem that freezing is too expensive to have commercial applications.

Fermentation

Fermentation process

The next major step is the fermentation, in which the fermentable sugars (glucose and fructose) present in the grape juice (including any added sugar) are converted by yeasts into ethanol (ethyl alcohol) and carbon dioxide, with the generation of heat, the excess of which needs to be removed. The fermentation temperature is an important variable. The fermentation also produces many of the aromatic characteristics of the finished wine. The fermentation is usually carried out in large, closed stainless steel tanks (capacity 2000 L up to 2000 hL), which are temperature controlled so as to lower the fermentation temperature as appropriate. Open fermentation vats are also used, although not for modern white wines. The fermentation time increases with decreasing temperature. The effect of fermentation temperature on the formation of volatile flavour compounds, in particular esters, is discussed in Chapter 7.

Yeasts are unicellular micro-organisms that are classified taxonomically as ‘fungi’. Yeasts have several commercial applications, and they are used also for beer brewing, baking and biomass production. Yeasts used in wine-making generally belong to the Saccharomyces genus, the most important species of which, cerevisiae, has some unique characteristics – perhaps one of the most useful ones being its tolerance to ethanol (up to 15% v/v), a very toxic compound for most other micro-organisms. Naturally occurring fermenting yeasts will start the fermentation at 18°C, though, increasingly, cultured yeasts are used. Some wine makers rely on fermenting yeasts that naturally occur in the winery, to start the fermentation and do not add any yeasts, allowing the natural yeasts to ferment the must, in order to achieve wines with a character believed to be more typical for the region. Generally, a strain of S. cerevisiae is added to the must, ensuring that the fermentation will start without much delay. The fermentation in wine-making may include several other types of yeast, some indigenous ones, although most yeasts will only grow at very low levels of ethanol (Chapter 7), and they tend to be more sensitive to sulfur dioxide. The fermenting yeast, inoculated or from the winery, will in most cases take over from all other organisms that may have become established during the early part of the fermentation and ferment the must to ‘dryness’ (low residual sugar).

Once dried fermenting yeasts became commercially available, most wine-makers deciding to inoculate the must did so mostly to avoid problems such as slow or even stuck fermentations and exert greater control over the fermentation, whilst minimizing the potential role of indigenous yeasts. An inoculum of active fermenting yeasts generally guarantees a rapid fermentation, also reducing the risk of spoilage. However wine-making knowledge has grown substantially over the last decades regarding the role of the yeasts in wine fermentation and its effect on wine aroma and quality. There are recent reviews discussing and summarizing the role of yeasts in wine fermentation, wine aroma and wine quality giving details of current knowledge (Fleet, 2008; Swiegers et al., 2008; Ugliano & Henschke, 2009; Bisson & Karpel, 2010). The current view is that the fermentation is more complex than previously assumed, with indigenous wild yeasts contributing significantly to the fermentation and the resulting wine quality, whereas it was assumed that these organisms died soon after the fermenting S. cerevisiae took over.

Further studies on flavour modulation by yeasts and the use of a mixture of yeasts to inoculate fermentation, may give in future yeasts even better tailored to fermentation and flavour production than currently available. The selection of yeast strains with suitable characteristics for wine-making, the use of mixed strains allowing a succession of fermentations to take place, and making these commercially available will in the long run give the wine-maker many more opportunities to select the yeast according to the desired wine style and quality, and create the wine most suited to the current consumer preferences. This further future exploitation of yeasts requires detailed information regarding the attributes valued in wines, so accurate selection criteria for the fermenting yeasts can be developed. Aspects regarding the effect of yeasts on wine aroma are discussed in more detail in Chapter 7.

Draining/pressing

The juice is taken from the grape skins when sufficient maceration has occurred (the fermentation may or may not have finished; to be decided by the wine maker) either by running off the juice without exerting any pressure on the grape skins (free run juice) or by pressing further juice (pressed juice) out of the remaining grape mash, usually referred to as pomace. The pressed juice tends to contain more tannins than the free run juice. The wine maker can choose either to keep pressed juice separate from the free run juice or to mix some or all of the pressed juice with the free run juice. For lighter wines, made to drink early, the pressed juice is kept separate, and often used for distillation. White wines are pressed before the fermentation (Section 1.5.3), while red wines are pressed after (or possibly during) the fermentation stage (Section 1.5.2).

There are various designs of presses available, and there is some variation in the properties of the pressed juice depending on the press, most important variables seem to be an excess of suspended solids in the juice, requiring extra fining to remove, and increasing amounts of anthocyanins and phenols in higher press run fractions. It is up to the wine-maker how the pressed wine fractions should be used.

Malo-lactic fermentation

A type of second fermentation, usually referred to as the malo-lactic fermentation, often occurs in wine, in which lactic acid bacteria convert the harsh-tasting malic acid into the softer tasting lactic acid, producing a small amount of carbon dioxide gas and raising the pH value of the wine. This fermentation can take place either approximately concurrently with the yeast fermentation or in the young wine. It seems still a matter of debate what the optimum time for this process is. Malo-lactic fermentation tends to be encouraged in wines with an excess acidity and almost without exception is carried out for red wines. However, this acid conversion is not desirable in wines already high in pH or low in acidity. Wines can be inoculated with selected micro-organisms, or the wine maker may rely on the indigenous flora of lactic acid bacteria for the malo-lactic fermentation. Inoculation with malo-lactic organisms does not automatically mean this fermentation will occur, since this process does not appear to be controlled easily. Wines kept at 20°C or above are likely to undergo the malo-lactic fermentation, while high levels of sulfur dioxide and high acidity tend to inhibit the process. Malo-lactic fermentation also changes the flavour in the wine due to the volatile compounds produced, which may or may not be desirable, depending on the style of wine the wine maker is aiming to produce. A comprehensive review by Bartowsky & Henschke (2004) describes current knowledge on malo-lactic fermentation, and in particular its contribution to the buttery flavour in wine. Bottle ageing of red wines is also reported to affect the aroma compounds formed during malo-lactic fermentation (Boido et al., 2009).

Post-fermentation

Racking/clarification

Young wines are stored and matured in small (225 L) or large wooden barrels, or in wooden, stainless steel or concrete vats. Whatever the mode of storage, a few weeks after the fermentation has ceased the wine will, usually, clarify and the yeasts be deposited at the bottom of the container. The wine is then usually taken off the lees (i.e. the precipitate of mostly yeast cells) and placed in a clean container. Thus, racking involves drawing off the wine from the barrel or tank to just above the level of the sediment or lees and transferring the wine to clean barrels or vats. This process may need to be repeated, depending on how long the wine is left to mature. Wines should be racked as required, more frequently when the wines are very young, and possibly annually when the wines are a few years old.

Oxygen

Aeration of the wine due to racking is thought to be crucial to the development of the flavour and colour of red wine but is generally detrimental to that of white wine. The higher phenol content in red wines, in particular the flavonoids, use oxygen for complex chemical reactions, leading to a softening of the wine. In contrast, white wines have much lower concentrations of phenols, mostly based on tartrate esters of hydroxycinnamic acids, prone to oxidation and forming hydrogen peroxide in the process, which can oxidize other compounds in wine, such as ethanol being oxidized into acetaldehyde. These oxidation processes were first reported by Wildenradt & Singleton (1974). Hence care should be taken when pumping white wine around at various stages post fermentation to protect the wine from the uptake of oxygen, often achieved by using a blanket of inert gas and adding some sulfur dioxide if required. In contrast, red wine can be deliberately aerated, often by pumping over the wine or storage in wooden barrels which allow the slow ingress of oxygen, to enhance the reactions involving phenolic compounds, which lead to softening the wine and colour changes and has impact on the formation of the typical red wine aroma. Another technique used to allow a controlled amount of oxygen uptake is referred to as micro-oxygenation. This is used when the extraction of oak flavours into the wine is to be avoided and hence, inert vessels for wine storage are preferred. A specially designed cooperage can be used to allow controlled oxygen uptake of red wine, or silicone tube diffusers can be used to supply oxygen below the rate of consumption. Various instruments are also available to supply oxygen at selected rates via microporous diffusers.

An excess of oxygen in wines can lead to oxidative browning, a process avoided by ensuring minimal contact with oxygen, in particular in the case of white wines, and/or the use of sulfur dioxide as an antioxidant to control browning. Enzymic browning occurs primarily in must, whereas non-enzymic browning generally occurs in wines. A recent review (Li et al., 2008) describes the pathways involved in browning, and discusses the central role of iron and copper in initiating non-enzymic browning, suggesting that control of these metals may help to avoid browning.

Oxygen in must, during fermentation and during storage impacts on red wine aroma and quality, in addition to the above mentioned effect on phenolic compounds (Kilmartin, 2009; Toit et al.