Cool-Climate White Wine Oenology E-Book

Volker Schneider

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 333

Veröffentlichungsjahr: 2024

Beschreibung

Cool-Climate White Wine Oenology is dedicated exclusively to the technology and science of white still wines and sparkling base wines, as they are produced by the rapidly growing British wine industry and in countries with a similar climate. It has a strong focus on sensory issues and guides the reader through the entire process of white winemaking - from the crush pad to bottling – clearly defining which measures to take and which to avoid. Whilst this book does not neglect the scientific fundamentals of oenology, it also gives numerous practical hints and technical details of hands-on winery work and provides valuable insights into the inherently cross-disciplinary nature of white winemaking and a holistic view of one of the most fascinating fields of contemporary oenology.

Leseprobe

First published in 2024 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2024

© Volker Schneider and Mark Tracey 2024

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data

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

ISBN 978 0 7198 4371 6

The right of Volker Schneider and Mark Tracey to be identified as authors of this work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Frontispiece: bulk storage area of a contemporary winery.

Cover design by Sergey Tsvetkov

CONTENTS

Preface

1Assessing Fruit Ripeness

1.1 Beyond Sugar Levels and Acidity: Physiological Ripeness

1.2 Sensory Assessments of Fruit Maturity

2Pre-Fermentation Strategies

2.1 Must Acidification and the Issue of Safe pH

2.2 Reductive vs Oxidative Grape Processing

2.3 Skin Contact

2.4 Pressing

2.5 Juice Treatments

3Fermentation Strategies

3.1 Optimising Fermentation Conditions

3.2 Spontaneous Fermentations

4Acidity Corrections

4.1 Biological Deacidification by Malolactic Fermentation

4.2 Spontaneous Acidity Losses by Potassium Bitartrate Precipitation

4.3 Chemical Deacidification

5Use and Effect of Reducing Agents During Storage

5.1 Sulphur Dioxide

5.2 Ascorbic Acid

5.3 Hydrolysable Tannins

5.4 Inactive Dry Yeast Preparations

5.5 Post-Fermentation Yeast Lees

6Practical Use of Yeast Lees

6.1 Avoiding Counterproductive Interventions on Fruity Wines

6.2 Barrel Ageing, Sur-Lie and Bâtonnage

6.3 Wines without Added Sulphites

7Limiting Oxygen Uptake

7.1 Sensory Impact of Oxygen Uptake in Filtered Wines

7.2 Sources of Oxygen Uptake

7.3 Importance and Measures of Gentle Wine Treatment

8Preparing Wine for Bottling

8.1 Bentonite Fining for Protein Stabilisation

8.2 Final Corrections of Taste, Flaws and Faults

8.3 Crystal Stabilisation

8.4 Adjusting Free Sulphur Dioxide Before Bottling

Bibliography

Index

PREFACE

This book is dedicated exclusively to the oenology of white wines, with a particular focus on those from cool-climate growing areas and the sparkling wines obtained from them. It is the authors’ view that many of the latest developments in white wine oenology are often poorly appreciated by practising winemakers. In particular, their integration with current techniques to optimally vinify cool-climate fruit, such as that of the rapidly developing British wine industry, is frequently little understood. Accordingly, we seek to address this issue in a science-based but approachable manner.

Whilst the oenological concepts of red winemaking hardly apply to white winemaking, those of cool-climate white wines diverge even more from common oenological principles. Apart from their long-lasting aftertaste, freshness, and lively but often too high acidity, they are strongly associated with their aromatic properties. However, these aromas are fragile and easily lost by inappropriate grape and juice processing, unsuitable cellar operations and storage conditions, or due to poor wine stabilisation with regard to post-bottling shelf life. Hence there is need for detailed discussion of topical aspects such as grape maturity assessments, grape processing, juice treatments, acidity corrections, fermentation strategies, sur-lie treatments, stabilisation procedures, and the complex role of oxygen and reducing agents. Discussion extends to often confusing stylistic options such as oak barrels and alternatives, clay amphorae, and spontaneous fermentation. All issues are explained with care, traced back to their scientific fundamentals, and illustrated by extensive original data obtained from more than 40 years of the authors’ experience in commercial winemaking conditions, quality control and research in various countries. Numerous practical hints, technical details of hands-on winery work and solutions to typical engineering issues complete the picture.

However, winemaking is more than process technology. After decades of a tendency to technocracy and even over-processing, a growing number of winemakers embrace a trend towards minimal or non-interventionist winemaking in order to respect consumer expectations and traditions. Recent research has allowed us to understand why some ancient techniques, evolved through experience, can be beneficial. It has provided knowledge, insights, and carefully selected strategies that can even improve the fine heritage of traditional winemaking. Examples such as the utilisation of oxygen in must, working with yeast lees after fermentation and the challenge of winemaking without added sulphites are discussed in detail.

Ultimately, this book seeks to present a valuable insight into the inherently cross-disciplinary nature of cool-climate white winemaking, unifying knowledge scattered across chemistry, microbiology and technology. It is written for professional winemakers, hobby winemakers with some proficiency in school-level chemistry, grape growers considering moving their whole winemaking process in-house, consultants and oenology students. The listing of comprehensive bibliographical references allows for deepening up-to-date expertise on specific subject areas. The authors hope that the readers will find Cool-Climate White Wine Oenology both edifying and enjoyable, and that it will be considered a valuable resource for years to come.

Volker Schneider and Mark Tracey

CHAPTER 1  

ASSESSING FRUIT RIPENESS

In cool-climate growing regions, wine growers traditionally aimed to harvest their grapes when sugar levels were as high as possible and acidity reasonably low. It could be assumed that at this point all other grape constituents, specifically the aroma compounds (and in red grapes, also the tannins), had reached their optimal qualitative expression, also referred to as physiological ripeness. However, in our times of global climate change, the process of physiological ripeness has become increasingly uncoupled from increasing sugar levels and lagging behind them. Wines made from physiologically underripefruit display an aroma reminiscent of freshly cut grass, green pepper, or no aroma at all, regardless of grape sugar. This is the reason that trivial sugar measurements have lost much of their former importance, to be replaced instead by an assessment of physiological ripeness. After all, sugar and acidity levels can be adjusted in the cellar, but deficient aromatics cannot. However, the chemical complexity of aroma makes its analytical measurement practically impossible. Accordingly, this chapter explains how aromatic ripeness of grapes can be assessed using smell, taste, visual appearance and tactile sensations.

Ripe grapes are the most important prerequisite for serious wine quality.

1.1Beyond Sugar Levels and Acidity: Physiological Ripeness

At the beginning of any winemaking process are the grapes. Their ripeness and health determine the quality of the wine at least as much as the oenological processes involved in winemaking. Therefore, no book on oenology can begin without considering the grapes and their evaluation before harvest. This results in harvest decisions, but also affects oenological decision-making in the subsequent steps of must and wine processing.

Sugar level, titratable acidity (TA) and pH are standard measurements every grape grower and winemaker is familiar with. They stand in the foreground during the monitoring of grape ripeness because they are easy to assess by simple technical means. As ripeness progresses, sugar increases and TA decreases, whilst pH is inversely related to TA. However, wine is more than an aqueous solution of alcohol and acids. Ripeness in the sense of grape sugar alone does not guarantee a pleasurable drinking experience. Nonetheless, there is still a widespread erroneous belief that quality can be determined solely by grape sugar content measured as specific gravity (SG) or Oechsle, possibly in combination with a moderate TA. Most winemakers claim that they do not depend on such figures, but it’s a rare one who does not rely on SG, TA and pH readings when it comes to determining whether the grapes are ripe to pick.

Only recently has yeast-assimilable nitrogen (YAN) been increasingly measured in musts. Whilst not directly related to ripeness, it is crucial for a smooth fermentation. This will be discussed in more detail in chapter 3.

The Concept of Aromatic Ripeness

Grape sugar content exclusively determines alcoholic ripeness, that is, the potential alcohol content. However, beyond that, one can also identify a physiological ripeness, comprising aromatic ripeness and, in the case of red grapes, phenolic ripeness. It is not directly associated with alcoholic ripeness, at least not in accordance with the contemporary understanding of ripeness, and even less as global climate change progresses. Therefore, one can find completely unimpressive and one-dimensional wines obtained from high-gravity juices from physiologically unripe grapes.

The concept of aromatic ripeness is often ignored. In reality, long after the increase of sugar content has come to a standstill, the synthesis of highly valuable aromatic compounds in the berries continues. These are the compounds that, at comparable grape sugar and acidity levels, allow us to differentiate between white wines, especially between a cheap table wine and a complex vintage wine expressing its identity in terms of origin and variety. Otherwise wine description would be reduced to a monotonous repetition of the five basic tastes – sweet, sour, bitter, salty and umami – and possibly the tactile sensation of astringency.

Since aromatic ripeness and crop yield are interdependent, overcropping can be a reason for deficient aromatic ripeness. However, not only high harvest yields, but also extraordinary weather events may explain why aromatic ripeness does not run proportionally to alcoholic ripeness. Under humid climate conditions, grey rot often brings the development of desirable aroma compounds to a complete standstill. Conversely, as global climate change progresses, even cool-climate growing areas can be affected by deficient aromatic ripeness due to drought. In extreme cases, this can cause a wine made from 1.0105 SG grapes to remind us of one with an aroma profile of 1.0060 SG fruit.

Deficiency in aromatic ripeness may present in three ways:

• A shortage or complete absence of any aroma.

• The appearance of an aroma defect called ‘atypical ageing’ at a relatively early stage of white wine development, sometimes quite soon after fermentation. The olfactory feature of those wines is reminiscent of mothballs, naphthalene, soap, acacia blossom, or similar. Its underlying cause is a hormonal stress in the vines that might be induced by premature harvest, overcropping, or drought (Schneider 2014).

• Vegetal-green aromas, the distinguishing mark of unripe grapes or any other fruits and deriving from a group of compounds designated as methoxypyrazines, colloquially referred to as ‘MPs’. They are most often responsible for deficient aromatic ripeness under cool-climate conditions.

Overcropped vineyard with more grapes than leaves. The desirable leaf-to-fruit ratio depends on a plethora of viticultural variables, but less than ten leaves per grape cluster, as shown in this image, barely allows for physiological ripeness of the grapes.

Vegetal-Green Aroma Caused by Methoxypyrazines (MPs)

The vegetal-green or herbaceous fraction of wine aroma caused by MPs is reminiscent of cut green grass and green capsicum. Everybody is familiar with this kind of smell from their daily lives, but optimistic expectancy or the emotional investment people might have in their own wines can prevent them from identifying it in them. Some also euphemistically describe it as ‘vibrant green fruits’. It can be so intense that it eventually dominates the overall aroma such that one can no longer discern the desired, pleasurable aroma attributes such as ripe fruits, flowers, or minerals.

In some wine, such as those obtained from Sauvignon, MPs are accepted to contribute to varietal flavour, provided that their concentration does not exceed a certain limit and their contribution to total aroma is in balance with other aromatic compounds (Allen and Lacey 1993). If there is not such a balance, they dominate the flavour by their green-vegetative characteristics. Besides smell, they also adversely affect in-mouth sensations by sensory synergisms, feigning more acidity than the wine actually contains. In cool-climate growing areas, such a flavour feature can become a serious issue in bad years or after premature harvest

MPs are stored in the grapes before véraison, and their synthesis is accelerated under humid growing conditions. After crossing a concentration peak, they decrease continuously during ripening. This decrease is due to the impact of sunlight and correlates with the breakdown of acidity. All viticultural factors contributing directly or indirectly to a better exposure of the grapes to sunlight, including leaf removal, accelerate the decrease. Regardless of this photo-degradation induced by sun exposure, high temperatures during ripening act in the same way. However, high yield and humid climate act inversely. Leaf removal and cluster thinning are more effective means for decreasing MPs than a low number of buds during pruning. Fungal infection of unripe grapes yields higher concentrations not only because it mandates an early harvest, but also by promoting MP extraction from the prematurely destroyed skin tissue (Kotseridis et al. 1999).

Skins, seeds and stems contain more MPs than the respective juice fraction. As they are highly soluble, they are easily carried over from the grape tissues into the must, where they reach their maximum concentration after one day of skin contact. The presence of leaves and harsh mechanical grape processing such as excessive pumping and pressing give rise to a further enhancement, while removal of the last pressing fractions may lower their concentration. Furthermore, in freshly pressed juices, a certain amount of MPs are bound to solids. Thus, a proper juice clarification (seesection 2.5) may help reduce them. However, there are no oenological measures able to completely avoid the appearance of vegetal-green flavour if this is an intrinsic feature of fruit quality.

Chemically, MPs are quite stable molecules. Common must and wine treatments related to finings, oxidation or reduction during vinification and storage hardly affect their concentration. In particular, the lack of any reactions with adsorbing materials is responsible for their stability against fining agents and filtration media. Their only reaction with practical importance is their photo-degradation discussed previously – that is, their breakdown under the influence of light. This reaction can also take place slowly after bottling in white glass. However, standard storage conditions in the dark do not facilitate any reduction of herbaceous flavour.

While there are no oenological means of efficiently reducing MP levels, yeast-derived aroma compounds can mitigate their vegetal-green flavour to a certain extent in very young wines. However, these secondary metabolites of yeasts have a relatively short life span. After approximately one year, and under improper cellar operations and storage conditions significantly faster, the yeast’s impact on overall aroma has disappeared to a great extent (seesection 3.1). Therefore, viticultural tools for exploiting the full aroma potential of ripe grapes and lowering their MP levels during ripening are of utmost importance. The most important of these tools is to postpone the time of picking as far as possible. Under cool-climate conditions, extending hang time rarely leads to what, in hot-climate areas, is pejoratively called an ‘overripe’ type of wine. Furthermore, whereas sugar levels and acidity can be easily corrected in the winery, a lack of aromatic ripeness cannot. Hence, it makes little sense attempting to preserve natural acidity as it is sought for sparkling wines when this is achieved at the expense of aromatic ripeness.

1.2Sensory Assessments of Fruit Maturity

Since viewing sugar levels as the sole quality criterion has become unsatisfactory and obsolete, there have been significant efforts to replace it by a direct measurement of grape aroma potential. One analytical approach led to the determination of the so-called glycosyl-glucose. It is based on the assumption that the grape-derived aroma compounds, initially odourless and far too diverse for their individual measurement, are predominantly bound to glucose, from which they are gradually released during the winemaking process to become odour-active (Williams et al. 1995). Another method of determining aromatic ripeness is based on the fact that the aroma of most grape cultivars consists overwhelmingly of terpenols, which can be distilled off and measured in the distillate (Dimitriadis and Williams 1984).

Although results of such measurements correlate to some degree with sensorially perceived quality, unfortunately their routine use is far too cumbersome under commercial winemaking conditions. Therefore, the sensory evaluation of physiological ripeness remains of outstanding importance. Such an evaluation uses the human senses of touch, taste, smell and sight, as detailed below.

Ripe grape berries display a yellow-green and slightly transparent skin.

Visual Assessment of Grape Quality

• Lightly transparent, yellowish-green-coloured skins with golden shades indicate ripeness of white grapes; green skins reveal its lack.

• Behind the skins of the intact berries one can see the seeds.

• When perfect ripeness is reached, the berries can be easily removed from the stems, which are partially lignified and display a brown colour.

• Rotten grapes are easy to recognise and should be discarded at harvest. However, this becomes a problem when rot spreads strongly in rainy years. Musts from grapes with more than 10 per cent rotten berries require specific treatment, which is discussed in section 2.5.

The skin of ripe berries clearly reveals the seeds inside.

Squeezing the Berries

• Ripe berries remain deformed after mild squeezing with the fingers, but unripe berries are elastic and turn back to their initial shape.

• When the berries are completely crushed, brown and hard seeds are easy to detach from the pulp if the grapes are ripe.

• If only a little juice can be squeezed out of a gelatinous pulp with adhering seeds, the grapes are underripe.

• Gelatinous adherence of the pulp to the skin or seeds goes along with a lack of ripeness.

• In unripe fruit, the seeds are green, soft or mealy; they also have a bitter flavour.

The seeds of ripe berries display a brown colour and can be easily detached from the pulp.

Grapes lacking physiological ripeness can be easily identified by their green and opaque skins.

Smelling and Chewing the Berries

• It is easy to distinguish unripe berries from ripe ones only by smell. To be ripe, the pulp should be free of herbaceous notes and viscosity.

• In most aromatic cultivars (Bacchus, Ortega, Muscat varieties), the varietal aroma is already clearly recognisable in the smell of the squeezed berries, but hardly so in Sauvignon, in which it is only released by yeast during fermentation.

• The skins should be crumbly after chewing and not tough.

Fruit Sampling

The simple sensory assessments described above can yield very useful indicators of grape ripeness, but only if the sample tested is really appropriate. Conclusions about grape ripening status are often drawn from too small and unrepresentative grape samples. In practice, varieties and even blocks of the same variety are likely to have quite different ripening patterns. Hence, a systematic fruit sampling strategy is crucial to overcome the variability of ripeness within a vineyard block. It requires blind picking berries from numerous separate grape clusters, from different parts of the clusters, and from different parts of the canopy each time one walks through the rows. Generally, more than one sampling will need to be performed in each vineyard when the anticipated harvest date comes closer, in particular when there are weather changes affecting fruit quality expected.

CHAPTER 2  

PRE-FERMENTATION STRATEGIES

Pre-fermentation operations performed between the grapes’ crushing and the start of fermentation have a widely underestimated impact on wine quality and its sensory stability during storage. A skin contact period of up to one day is frequently used to enhance aromatics by their extraction from the skins, provided that the grapes are perfectly ripe. More important than the technical modalities of pressing is the issue of the generally recommended addition of sulphur dioxide to must. The oxidation and browning of must resulting from omitting it is not related to the oxidation of wine, and even mitigates it by lowering detrimental phenols, thus improving the wine’s shelf life and reducing its astringency.Aroma losses frequently attributed to it only occur in a few specific grape varieties. Protein stabilisation by bentonite fining and any acidity corrections deemed necessary are already useful at this stage. Another important measure to achieve flawless wines with pristine aroma is juice clarification. Choice of clarification procedure is not decisive, but rather the level of clarification obtained, evaluated as residual turbidity. The use of pectolytic enzymes is strongly recommended for this purpose.

Closed-cage membrane press awaiting its next load of grapes.

2.1 Must Acidification and the Issue of Safe pH

From a historical perspective, until the end of the twentieth century, must acidification had never played a major role in cool-climate growing areas. The acidity was usually high enough and often too high, so that deacidification was more important. High acidity was also accompanied by low pH, although this inverse correlation is weak. As is generally known in the wine industry, a low pH contributes to microbial safety. Hence, no thought had ever been given to microbial hazards caused by high pH levels. However, this situation has changed in the meantime, and the pH has become a hotly debated topic of conversation even in cool-climate areas. There are two reasons for that.

The first reason can be found in the development of the New World wine industry in the second half of the twentieth century. Most of their wine growing areas sprouted in hot regions that yielded low acidity and high pH figures. Therefore, acidification became a necessity to achieve a balanced taste and, at the same time, a decrease in pH to improve microbial safety. Since absolute safety was considered paramount, much importance was and still is attached to the lowering of pH to values considered safe through the addition of tartaric acid. As these countries quickly became opinion-leading in the global wine industry, the fear of supposedly too high pH levels spread to Old World wine-producing countries as well.

The second reason is global climate change. It has led to the fact that even in cool-climate growing areas, hot and dry vintages occur more often, providing musts with low acidity and actually high pH figures. That is why most of the countries concerned have now legalised must and wine acidification. This raises the question of how far the pH should be lowered and the acidity increased.

Interpreting pH Correctly

With a few exceptions, pH in musts ranges from 3.0 to 4.0. Some very conservative schools of thought continue advocating lowering pH to 3.5 for safety reasons and to add as much tartaric acid as necessary to achieve this goal regardless of the sensory outcome. As a consequence, many winemakers are terrified by higher pH figures, because they mean more risk of adverse microbial activity since SO2 is less effective in the higher pH wine. This is absolutely true. The molecular gaseous fraction of free SO2, which alone is responsible for microbial protection, decreases logarithmically with increasing pH (Figure 5.1).

Other winemakers have learnt to handle high pH musts and wines by using modern techniques such as cooling, filtration and sterile bottling that were not available some decades ago. These winemakers feel quite comfortable vinifying and bottling particularly high-end wines with pH in the range from 3.5 to 3.8, stating that wines acidified according to pH taste thin, harsh and tough. This is also true. Many of the great wines of the world would be considered undrinkable by contemporary quality standards if their pH was lowered to 3.5. Unpleasant wines that have been distorted by overacidification just for pH concerns are easy to find.

Factors Affecting pH

It is frequently believed that low titratable acidity (TA) automatically leads to high pH and vice versa. However, this relationship is not that clear, as shown in Figure 1.1. Indeed, pH is also affected by potassium. It is the most common alkali mineral cation in wine and neutralises acids. Just as acids drive pH down, potassium drives it up (Figure 4.4). Thus, the pH actually measured is the result of an interaction between acids and potassium. Musts with high TA from bad rainy years do not necessarily display low pH figures, because they also tend to have high potassium levels. However, the reverse can also become true when, in the course of global climate change, hot and dry growing conditions yield musts with little potassium (section 4.2).

Figure 1.1: Relationship between titratable acidity (TA) and pH. Data in the oval circle represent white wines.

When Lowering pH Through Acidification is Really Necessary

Generally, a TA in the range of 7–9g/L and a pH of 3.2–3.6 is preferred in white musts. Somewhat higher acidity levels can be useful for the production of sparkling base wines. After the precipitation of potassium bitartrate during and after fermentation, wines generally display a lower TA and also a lower pH than the musts they have been obtained from (section 4.2). This counterintuitive behaviour, that despite less TA the pH is also lower, is partially explained by the decrease of potassium that accompanies this process. The pH decrease always occurs unless the initial must pH is above 4.1, which is practically never the case. Older theories, according to which the initial pH only decreases when it is below 3.65 and increases when it is above, are based on aqueous solutions that do not take into account the impact of alcohol content and ionic strength of wines (Boulton et al. 1996).

The pH drop due to potassium bitartrate precipitation relativises the significance of high pH figures in must. Only when must is actually high in pH (>3.8) or very low in acidity (for example, less than 6g/L TA), should these numbers be corrected by acidification at this early stage. For that purpose, tartaric acid is used, usually in amounts of 1 or 2g/L. It is the organic acid that gives the greatest reduction in pH. An addition of 1.0g/L tartaric acid leads to a reduction of pH by 0.15–0.20, depending on how much of it remains in solution and on the buffering capacity of the individual must. It is important to note in this context that about 80 per cent of sensorially perceived sourness is determined by TA, whilst pH only contributes approximately 20 per cent. Therefore, TA must be given at least the same importance as pH (Schneider and Troxell 2022).

Special Features of Acidification with Tartaric Acid

Tartaric acid differs from the other acids contained in wine in that it can precipitate as insoluble salts. This results in the following peculiarities of its application:

• The addition of 1.0g/L tartaric acid only temporarily increases TA by 1.0g/L, because a significant part of it precipitates together with potassium as a potassium bitartrate after some days or weeks.

• If the added tartaric acid were to precipitate completely as potassium bitartrate, an addition of 1.0g/L would only result in a permanent increase in TA of 0.5g/L. In practice, this value is usually around 0.6g/L.

• Potassium in wine contributes to sensory sensations that are described as volume, weight and body on the palate. Its inevitable decrease by addition of tartaric acid makes the wine thinner. Details are outlined in section 4.2.

These properties indicate that any acidification of must, if really necessary, should be approached with caution. Under cool-climate conditions, questions about acidity and pH management arise primarily in connection with deacidification. Whether this is performed on the must or in the wine depends on numerous factors. This issue will be discussed in detail in Chapter 4. Furthermore, it is not admissible to state in a simplified way, as so often happens in the lay press, that low pH or high TA improve the shelf life of wines. On the contrary, shelf life and aroma stability are primarily determined by variables such a phenolic composition and must treatment (section 2.2), oxygen uptake post-fermentation (Chapter 7) and storage temperature.

2.2 Reductive vs Oxidative Grape Processing

When white grapes are crushed without previous SO2 additions, the juice they release undergoes rapid browning and displays a smell reminiscent of fresh bread. Many winemakers are startled by this appearance and believe it will remain in their later wine. Therefore, they immediately add SO2 to the grapes or, at the latest, to the juice.

Grapes and wine contain polyphenolic compounds that easily oxidise to brown pigments when the juice picks up atmospheric oxygen. This oxidation is an enzymatic one caused by grape-derived enzymes called polyphenoloxidase (PPO) or, more specifically, tyrosinase. It results in a profound quantitative and qualitative change in the phenolic constitution of the juice. The process ends up with the oxidation and polymerisation of flavonoid phenols to larger molecular aggregates, which are insoluble in the aqueous environment of juice and precipitate as brown, solid particles.

Juices without SO2 added rapidly undergo browning during and after pressing.

Flavonoid phenols are also designated as flavonoids, flavanols or catechins. Together with their much larger counterpart in white wines, the nonflavonoid phenols, they constitute the total phenol content. While grape-derived nonflavonoids do not play a major sensory role, flavonoid levels describe very precisely the content of ‘bad’ phenols or tannins that the wine industry aims at minimising in white wines. Thanks to a relatively simple spectrophotometric method, they can be specifically measured in routine analysis and expressed as mg/L catechin equivalents, or CE (Zironi et al. 1992, Schneider 2019).

Figure 2.1: Precipitation of flavonoid phenols during oxidation of four white grape musts as a function of the amount of oxygen consumed.

Back to the crush pad. Musts can keep up to 8mg/L oxygen in solution. To the extent that this dissolved oxygen is consumed by phenols before, during and after pressing, the must can absorb further amounts of it, which are also consumed. At some point, after sufficient oxygen has been taken up and consumed by the phenols, flavonoid phenols are almost all precipitated. This is illustrated in Figure 2.1 on the basis of four musts.

When solid brown flavonoid particles are removed from the juice by filtration, the juice left behind displays the normal green-yellow colour of standard white wines. In practice, the particles are removed during juice clarification via settling or otherwise. The smell of fresh bread disappears at the latest on the second day of fermentation due to the reductive strength of the yeast (Schneider 1998).

Interestingly, there is no direct correlation between the amount of oxygen consumed and the extent of flavonoid phenol precipitation. One of the reasons is variable levels of glutathione, a peptide that acts as a reducing agent (section 5.4) and interferes in the reaction (Singleton et al. 1985). However, it can be stated that in about 90 per cent of all juices, the oxygen absorbed during and after pressing (Cheynier et al. 1993, Day et al. 2019) is sufficient to precipitate the great majority of flavonoids in a few hours if the process is not stopped beforehand by the addition of SO2.

Sensory Properties of Flavonoid Phenols

Since the primary sensory results of must oxidation derive from the depletion of flavonoids, a closer look at their sensory properties is helpful to understand fully the impact of oxidative or reductive grape processing on the resulting wine.

Colour

Flavonoids are the only phenols in white wine able to produce significant browning under conditions of oxygen uptake. Their concentration correlates with the browning rate and intensity (Schneider 1998, Lee and Jaworski 1988, Salacha et al. 2008). Figure 2.2 depicts the course of browning over time in young filtered white wines kept under air without any SO2 additions. Under these conditions, wines containing less than 5mg/L of flavonoids remain almost stable in colour. Browning was measured spectrophotometrically as the absorbance at 420nm (A 420).

Figure 2.2: Time course of browning (A 420) in young filtered white wines exposed to air without SO2 additions as a function of their content of flavonoid phenols (F).

Figure 2.3: Relationship between flavonoid content and maximum browning intensity in white wines.

The higher the flavonoid content, the more intense is the resulting browning, as clearly shown in Figure 2.3. This browning potential is the direct visible evidence of a wine’s propensity to undergo drastic chemical changes upon oxygen uptake. The underlying reaction of browning is the polymerisation of monomeric flavonoids to polymeric pigments. Before analytical methods for flavonoid measurements were available, the sensory stability of the wine was estimated visually, based on the browning intensity in the absence of free SO2.

The same applies when juices from red grapes intended for the elaboration of rosé or Blanc de noir wines are oxidised. After juice clarification, their slightly red colour reappears, although some anthocyanins responsible for that colour will inevitably have been destroyed by oxidation. However, these losses are generally not strong enough to completely remove red colour as is required for Blanc de noir production. Instead, the use of charcoal, bentonite or a specific enzyme will be required for that purpose (section 2.5).

Taste

With regard to taste, flavonoids are by far the most important drivers of astringency, bitterness, and what is called ‘phenolic taste’. When they undergo polymerisation during wine ageing, their astringency and colour increase (Lea et al. 1979, Delcour et al. 1984, Robichaud and Noble 1990). Hence, astringency of white wines also depends on their age. The increasing astringency of white wines displaying relatively high flavonoid levels is a phenomenon widely known by practitioners. When it occurs, it is usually noticed long before any visible browning can be observed. Table 1gives an overview of the evaluation of flavonoid phenol concentrations in white wines. Of course, the hedonic appraisal of astringency depends on that one is used to and on the cultural context.

Table 1: Sensory assessment and technological evaluation of flavonoid concentrations in white wines

Concentration (mg/L catechin)Interpretation1Lowest value ever measured3–5White wines with long shelf life~ 10Astringency caused by flavonoids can be perceived15–25White wines obtained by consistently reductive grape and juice processing≥20Accentuated astringency, which is always perceived as such

Odour

With regard to odour, flavonoids are non-volatile and odourless. Nevertheless, they affect aroma perceived by smell, long-term aroma stability, and white wine shelf life in general when they interact with oxygen. The odour changes occurring thereby are designated as oxidative or typical ageing. It is reminiscent of dry herbs, boiled potatoes, canned mushrooms, corn, boiled vegetables, honey, toasted bread, nuts, among others. These aroma attributes appear long before any taste and colour changes can be observed. They also occur in the presence of free SO2, but are greatly accelerated when free SO2 is no longer present. Any increase in colour indicates that profound flavour modifications have already taken place.

Ultimately, there is a close relationship between the content of flavonoid phenols and oxidative ageing as perceived by smell when white wines are aged under conditions of mild oxygen uptake as it can occur through certain bottle closures (section 8.4). Figure 2.4 shows this effect on two white varietal wines supplemented with flavonoid phenols extracted from grape seeds and the same amount of a pure flavonoid, (+)-catechin. Similar results can be expected when commercial grape tannins are added to white wines. However, these effects are largely absent if the wine is stored under oxygen exclusion.

When this effect of odourless flavonoids on aroma stability was documented for the first time (Schneider 1989), it was hypothesised that flavonoids act as a sort of catalyst in the formation of oxidative ageing perceived by smell to an extent not known from other grape-derived phenols. It would take another quarter of a century before this phenomenon was substantiated by analytical data: for three chemical markers of oxidative ageing – methional, phenylacetaldehyde and sotolon – an increased formation was shown in the presence of elevated contents of flavonoids under conditions of nanooxygenation, prevailing when bottles are sealed with oxygen-permeable closures (section 8.4). Oxidation products of flavonoids and acetaldehyde were hypothesised to be responsible for that formation. Out of 54 white wines bottled with cork closures, those displaying less than 3mg/L flavonoids showed no noticeable formation of sotolon, methional and phenylacetaldehyde during prolonged bottle ageing. The winery and winemaking techniques had a strong effect (Pons et al. 2015). Figure 2.5 gives an example of how this process of typical ageing affects both smell and taste in a way most consumers reject. White wine ageing is rarely perceived as positive.

Figure 2.5: Example of accelerated typical ageing of an unoaked Chardonnay sealed with cork during twelve months of bottle storage at 18°C.

It is important to emphasise that 2-aminoacetophenone, which is often listed in the context of white wine ageing, does not belong to the aforementioned group of oxidation markers. It is the impact molecule of a distinct kind of ageing denominated as ‘atypical ageing’, which in reality is an aroma defect reminiscent of mothballs and furniture varnish emerging sometimes in rather young wines a few weeks or months post-fermentation. It is induced by viticultural stress factors as the ultimate cause. Although oxygen is involved in its formation, it is only required in trace amounts that are far from sufficient to trigger oxidative ageing (Schneider 2014). Viticultural factors also are at the root of the so-called ‘petrol flavour’, which occurs primarily in aged Riesling wines regardless of oxygen exposure. These differences underline the importance of sensory discrimination between different types of ageing, precursors, and reaction mechanisms.

The Difference Between Typical Ageing and ‘Premox’

Obviously, flavonoids are not the only driver of detrimental white wine ageing. Oxygen exposure, storage temperature and free SO2 levels play an additional role. As odourless non-volatile phenols, flavonoids act as a catalyst accelerating the degradation of fruity varietal aroma only in the presence of oxygen when wine picks it up under inappropriate cellar storage conditions (section 7.2) or through certain bottle closures (section 8.4). The chemical reactions involved also occur in the presence of free SO2 (section 5.1). Their sensory outcome is a progressive drift of the fruity or floral aroma attributes of young wines towards those previously discussed (Figure 2.5). This is what is designated as oxidative or typical ageing, but not yet as distinctive oxidation. When wine is stored under absolute oxygen exclusion, as happens in rare cases, flavonoids only act on the palate as the main driver of astringency and also contribute to bitterness.

When oxygen uptake causes the complete disappearance of free SO2 by oxidation, this adverse development is accelerated. Concomitantly, acetaldehyde is released from its SO2-bound form and dominates the aroma profile with its typical smell of sherry and bruised apples (section 5.1). This equates to a complete oxidative breakdown and is called ‘premature oxidation’ or ‘premox’ when it occurs in a relatively early stage of wine ageing. The cause is that free SO2 before bottling was not adjusted to the oxygen exposure of the wine (section 8.4).

Reductive Grape Processing Preserves Flavonoid Phenols

Returning from the sensory aspects of flavonoids to the basic outcome of must oxidation:

• Must oxidation leads to a depletion of flavonoid phenols, which are nothing but tannins with their well-known astringent and bitter flavour that is quite unpopular in white wines. Thus, sensory problems of that kind, as well as wine finings required for their rectification, are largely avoided.

• Flavonoids that are removed by must oxidation are no longer available for oxidation and browning in wine. The wine’s susceptibility to ageing greatly diminishes.

Figure 2.6: Effect of free SO2 on the oxygen consumption rate of must.

However, traditional oenological teaching erroneously associates juice oxidation with wine oxidation. Accordingly, it advocates impeding juice oxidation by the addition of sulphur dioxide, thus preventing it from browning. The basic role of SO2 in this context is to irreversibly inactivate polyphenol oxidase (PPO), which is a grape-derived enzyme that transfers oxygen to phenols. Consequently, dissolved oxygen (DO) is no longer depleted by PPO activity. For that purpose, the presence of only a small amount of less than 10mg/L free SO2 is necessary, as shown in figure 2.6.

However, these tiny amounts of free SO2 are misleading, because in order to obtain them, it is necessary to add at least twice the amount, since in musts within the standard SG range, about 55 per cent of the added SO2 is loosely bound to glucose. Moreover, a much more important decrease of SO2