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- Herausgeber: tredition
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
WHITE WINE ENOLOGY could also be titled as SCIENCE AND PRACTICE OF WHITE WINEMAKING. It is characterized by two particular features: Firstly, it is authored by a practitioner who has worked his whole life in decision-making technical positions in the wine industry. Secondly, it is dedicated exclusively to the enology of white wines, and focuses its attention on flavor preservation rather than strategies that seek to obtain short-lived quality benefits. In doing so, it addresses one of the most important issues of white winemaking, which is the limited shelf life and flavor stability of most such wines, and shows how different white wine enology is from red. Due to the complex chemical process of white wine aging that goes well beyond oxidative aging, this book has a strong focus on sensory analysis throughout. It specifies the flavor-active compounds responsible for aging-related faults, identifies the chemical mechanisms of their formation, and clearly defines the decisive measures to take in order to mitigate them and improve flavor stability. While this book includes a deep treatment of the scientific fundaments of the different kinds of white wine aging, it also examines typical engineering issues common to commercial winemaking. Thus, it provides numerous practical hints and technical details of hands-on winery work and a valuable insight into the inherently cross-disciplinary nature of fine white winemaking. Extensive personal experience was decisive for this purpose. Volker Schneider was founder of the international consulting firm Schneider-Oenologie, which specialized in innovative winery operations, product development, quality control, and research. He has authored more than 450 technical articles and a range of scientific papers on these topics. He was also lecturer of enological chemistry at Geisenheim University.
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Veröffentlichungsjahr: 2024
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White Wine Enology
OPTIMIZING SHELF LIFE AND FLAVOR STABILITY OF WHITE WINES
- 2nd Edition -
Volker Schneider
tredition, Ahrensburg, Germany
White Wine Enology
2nd edition 2024
Copyright © 2024 Volker Schneider
Printing and distribution by order of the author:
tredition GmbH, Heinz-Beusen-Stieg 5, 22926 Ahrensburg, Germany
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronical or mechanical, including photocopying, or by any other information storage and retrieval system, without permission in writing from the copyright holder, except in the case of brief quotations embodied in critical articles and reviews.
The author is responsible for the content. Publication and distribution are carried out on behalf of the author, to be contacted at: tredition GmbH, Impressumservice, Heinz-Beusen-Stieg 5, 22926 Ahrensburg, Germany.
ISBN: 978-3-384-01615-7
Although the author has taken all reasonable care to ensure the accuracy of the information contained in this book, neither he nor the publisher can accept liability for any consequences arising from the information contained herein, or from use thereof. It is the responsibility of every practitioner to evaluate the appropriateness of a particular opinion in the context of actual technical situations and to consult appropriate information sources, especially for new or unfamiliar procedures.
Table of Contents
Preface…………………………………………………………
7
1.
Introduction………………………………………............
11
2.
Typical and oxidative aging…………………………………..
13
2.1.
Chemical pathways, reaction products, and sensory results…...
13
2.1.1.
Non-oxidative aging reactions…………………………………….....
13
2.1.2.
Oxidative aging………………………………………………………
16
2.2.
Influence of the phenolic composition………………………....
24
2.2.1.
Oxidation of phenols in wine………………………………………...
24
2.2.2.
Total phenols and the importance of flavonoid phenols………..........
29
2.2.3.
Measurement of flavonoid phenols………………………………….
43
2.3.
Influence of grape processing…………………………………..
48
2.3.1.
Skin contact…………..………………………………………………
48
2.3.2.
Pressing………………………………………………………………
52
2.4.
Influence of juice processing………………………….………..
63
2.4.1.
Effects of sulfur dioxide and oxygen in juice………………………..
63
2.4.2.
Active must oxidation………………………………………………..
74
2.4.3.
Passive must oxidation………………………………………………
78
2.4.4.
Basic compositional effects of must oxidation………………………
83
2.4.5.
Sensory effects of must oxidation on wine…………………………..
84
2.4.6.
Reductive vs. oxidative must processing in a nutshell………………
91
2.4.7.
Effect of fining agents on phenolic compounds……………………..
92
2.4.8.
Importance, practice and evaluation of juice clarification…………...
99
2.4.9.
Tools for juice clarification by cold settling…………………………
105
2.5.
Effect of reducing agents in wine………………………………
121
2.5.1.
Effect of sulfur dioxide on oxygen-related reactions………………..
121
2.5.2.
Wines without added sulfur dioxide……………………….………...
137
2.5.3.
Effect of ascorbic acid……………………………………………….
139
2.5.4.
Effect of ellagitannins………………………………………………..
145
2.5.5.
Effect of sulfur-containing amino acids and inactive dry yeasts…….
148
2.5.6.
Oxygen consumption by post-fermentation yeast lees………………
157
2.5.7.
Working with yeast lees in practice………………………………….
164
2.6.
Extent and impact of post-fermentation oxygen uptake………..
171
2.6.1.
Oxygen uptake through container materials…………………………
171
2.6.2.
Oxygen uptake though the wine surface and headspace inertization..
175
2.6.3.
Oxygen uptake upon wine treatments………………………………..
182
2.6.4.
Measures of gentle white wine treatment…………............................
185
2.6.5.
Sensory assessment of oxygen uptake in filtered white wines………
195
2.6.6.
Measurement of dissolved oxygen…………………………………..
201
2.6.7.
Oxygen uptake during and after bottling and its impact on aging…...
204
2.7.
Effect of storage temperature…………………………………..
216
3.
Reductive aging………………………………………………..
225
3.1.
Volatile sulfur compounds eliciting off-flavor…………………
225
3.1.1.
Definitions, causes, and key compounds…………………………….
225
3.1.2.
Differentiation between reductive taints and empyreumatic odors….
231
3.2.
The role and reactivity of precursors…………………………...
235
3.2.1.
Formation of precursors by yeast metabolism……………………….
235
3.2.2.
Conversion of precursors to odor-active compounds………………..
237
3.3.
Identification and removal of reduction flavor…………………
245
3.3.1.
Practical identification of reduction flavor…………………………..
245
3.3.2.
Removal of reduction flavor in practice……………………………..
246
3.3.3.
The importance of depth filtration after copper fining………………
257
3.3.4.
Removal of excess copper…………………………….……………..
258
3.4.
Effect of copper additions pre-bottling…………………………
263
3.4.1.
Assessing wines' proneness to post-bottling reductive aging………..
263
3.4.2.
Copper-management pre-bottling……………………………………
268
3.4.3.
Measuring total and free copper……………………………...……...
272
3.4.4.
Stability and toxicity of copper………………………………………
276
3.5.
Effect of bottle closures on reductive aging……………………
281
3.5.1.
Importance of the oxygen permeation through bottle closures………
281
3.5.2.
Screw caps using liners with varying OTR………………………….
285
3.5.3.
Screw cap liners scavenging reduction flavor……………………….
289
3.5.4.
Reductive aging of canned wines…………………………...……….
294
4.
Atypical aging…………………………………………………
297
4.1.
Sensory identification and compounds involved……………….
297
4.2.
Limited significance of microbial formation…………………...
301
4.3.
Chemical formation of 2-aminoacetophenone…………………
302
4.4.
Viticultural causes and countermeasures……………………….
305
4.5.
Enological measures against atypical aging……………………
309
4.6.
Assessing wines' proneness to atypical aging…………...……..
313
5.
Petrol flavor…………………………………………………...
315
5.1.
Sensory characteristics and causes……………………………..
315
5.2.
Viticultural countermeasures…………………………………...
318
5.3.
Enological countermeasures……………………………………
319
6.
Epilog…………………………………………………………..
323
7.
Literature……………………………………………………...
325
Index…………………………………………………………...
353
Preface to the 1st Edition
The enological literature tends to put emphasis on red wines; comprehensive treatises on the enology of white wines are scarce. When such treatises appear, they tend to focus upon maximizing aroma and quality in the widest sense. This book takes a different approach. It deals primarily with the preservation of quality. Therefore, the reader should not be disappointed when he finds other topics such as acidity management and fermentation strategies are only marginally dealt with. And as a highly specialized book, it is dedicated exclusively to fruity white wines regardless of whether they are dry or off-dry. Ultimately, it shows that the enology of white wines is quite different from that of red wines in many aspects.
Fruity white wines are usually unoaked or have undergone only limited barrel aging, which does not detract much from the original freshness and varietal character of young wines. They are the opposite of wines that are characterized by deliberate aging in oak barrels and the typical flavors they impart. They constitute the largest share of the global white wine market. However, their rapid and often premature aging in one or another way is a problem almost all wineries are concerned with. This applies despite the fact that some of these wines might undergo what is considered graceful aging. In an innovative approach to address these concerns, this book deals with flavor preservation rather than with vinification strategies aiming at obtaining short-lived quality benefits. In doing so, it addresses one of the key issues of white wine enology, which is the limited shelf life and poor flavor stability of most of these wines. However, it does not address the hedonic question of 'good' or 'bad', which is well known to be subjective and one which countless opinion leaders feel called upon to answer.
By definition, wine aging is not related to common wine faults of microbial origin, which might accidentally arise during aging or be detected only after some time of storage. In contrast, it is based on chemical reactions occurring at variable rates. As will be shown in chapter 4, these rates and their sensory outcome can be affected by global climate change, which introduces new challenges past generations did not experience.
As a chemical process, white wine aging comprises much more than the commonly known oxidative aging, frequently referred to as premox. Thus, discussing adverse aging means, firstly, differentiating the various kinds of aging according to sensory criteria, specifying the flavor-active compounds responsible for them, and identifying the chemical mechanisms of their formation. That's why this book has a strong sensory focus. In a second step, there are clear definitions of enological measures to be taken or avoided in order to mitigate the aging reactions and, hence, to improve flavor stability. These measures start as early as on the crush pad and continue throughout all phases of juice processing, wine stabilization, bottling, and storage.
In response to these complex challenges, this book covers one of the key areas of enology hardly ever embraced in a single volume. It aims at providing a valuable insight into the inherently cross-disciplinary nature of fine white wine making, unifying knowledge scattered across chemistry, technology, and microbiology. All issues it raises are traced back to their scientific fundamentals and illustrated by original data, most of them obtained under commercial winemaking conditions. The listing of comprehensive bibliographical references allows for deepening up-to-date expertise on specific subject areas.
Nevertheless, this book is not intended to be a merely scholarly one written like so many others in this field from a purely academic perspective, but rather reflects the industry background of the author. It is aimed at professional winemakers, lecturers of enology and consultants without experience in practical winemaking, lab staff, students willing to look behind theory, and amateur winemakers with some scientific background. In light of their frequent questions, it also suggests solutions to typical engineering issues in winery operations. Thus, numerous practical hints and technical details of hands-on winery work round up the picture and provide a holistic view of one of the most fascinating fields of contemporary winemaking.
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 toward minimal or non-interventionist winemaking in an attempt to respect consumer expectations and traditions. Recent research has allowed us to understand why some traditional techniques, evolved through experience, can be beneficial. It has provided knowledge, insights, and carefully selected techniques that can even improve the fine heritage of traditional winemaking. Examples such as the utilization of oxygen in must or working with yeast lees after fermentation are discussed in detail.
As a guidance for readers and in order to make reading easier, each major chapter designated by a two-digit number is preceded by a short introduction summarizing the contents. Cross-references within the text provide additional guidance.
The author has many friends and colleagues to thank for being willing to critically review and comment the manuscript of this book before publication. He accepts liability for any errors it might contain. Their disclosure will be taken into account in a future edition.
The author
Preface to the 2nd Edition
In the five years following the publication of the first edition of this book by Board & Bench Publishing, there was a lot of feedback from interested readers, which in turn led to many stimulating discussions. Simultaneously, many new insights have emerged, both from the scientific side and on the basis of practical experience from a wide range of wine growing countries and climatic regions. All this constituted a significant expansion upon the book's content, thus justifying a new edition.
Whilst the original approach of combining science with practice has been preserved, there have been significant additions to the content on must processing, the issue of reduction flavor and protection from oxygen uptake after fermentation. Additional real-world examples have been included.
It is important to note that this book was written without any support or involvement from the wine supply industry. It is imperative, however, to thank Mark Tracey, England, for the fruitful discussions without which the book would not have reached this level of completeness.
The author
1. Introduction
The quality and market value of white table wines strongly depend on their distinctive flavor, which may be varietal, origin-related, or fermentation-derived to a variable degree. Much effort and enological knowledge is dedicated to its production, but much less attention is paid to its preservation. However, white wines are sensitive products. They are subject to a far more dynamic aging than red wines. In the majority of cases, the sensory outcome of this aging process is opposed to current understanding of quality; white wines aged in a positive way are rare exceptions. The limited shelf life of white table wines after bottling is a global problem.
All wines change during storage. These changes are driven by intrinsic and extrinsic factors. A gradual decay of fruity aroma attributes, particularly of those produced by the fermentation metabolism of yeast, is common to all kinds of wine and considered unavoidable. This process is referred to as maturation. In a second phase, distinctive aging flavors appear in white wines, usually after bottling. Their occurrence adversely affects the sensory profile and quality patterns initially intended by the winemaker.
In order to take targeted measures for optimizing white wine shelf life and flavor stability, it is indispensable to differentiate the various kinds of aging flavor according to their sensory characteristics and chemical pathways. For this purpose, there is a need of specific terms applied in descriptive sensory analysis. Their precise use depends on sensory training, experience, and linguistic expertise of the tasters involved. Unfortunately and much to the disadvantage of wine quality control, sensory terms are frequently misused, abused, or exchanged among themselves. The term "oxidized" is a good example of linguistic confusion. It gives no information about whether there is the typical smell of free acetaldehyde involved in the aroma pattern as it might only occur in the absence of free sulfur dioxide, or whether the smell is elicited by other oxidation products generated despite the presence of free sulfur dioxide. Misunderstandings caused by imprecise language use often lead to erroneous decisions when it comes to choose enological countermeasures for preventing or remedying premature aging flavors.
According to prevailing sensory criteria, there are four different kinds of white wine aging:
Typical aging
, driven by temperature and/or oxygen, gives rise to a wine commonly called maderized, evolved, or simply oxidized. It is reminiscent of cooked vegetables, boiled potatoes, wet soil, black tea, honey, nuts, hay, and straw. Additionally, the odor of free acetaldehyde reminding one of bruised apples may appear and mask these olfactory descriptors when free sulfur dioxide has been decreased to nil by oxygen uptake. An intensification of color and an increase of astringency on the palate may occur simultaneously, but must not do so.
Reductive aging
leading to the formation of volatile sulfur compounds, particularly thiols and hydrogen sulfide, whose stinky smell is reminiscent of burnt rubber, cooked cabbage, rotten eggs, and garlic. Its appearance is fostered when wines prone to produce it are stored under anoxic conditions or bottled using bottle closures with a low oxygen ingress rate. Therefore, it is also known as post-bottling reduction flavor.
Atypical aging
resulting essentially from the conversion of a phytohormone called indole-3-acetic acid into 2-aminoacetophenone and some by-products reminiscent of mothballs, soap, floor polish, acacia blossom, and laundry in wines produced from stressed or underripe fruit.
Petrol flavor
calling to mind gasoline, kerosene, and dry apricots. It is related to an acidic hydrolysis of grape-derived precursors found in wines obtained from a very limited number of grape varieties, Riesling in particular. It is not affected by reactions of oxidation or reduction.
Typical aging due to oxidation with its familiar sensory pattern has always been known and is still the most common form of aging of white wines. Therefore, it is also considered as aging per se. The other variants of aging are considered abnormal or faulty deviations from typical white wine evolution. The frequently encountered confusion between typical and atypical aging is a particularly serious problem in enology with far reaching consequences.
As a matter of principle, any white wine is subject to one or another kind of aging. The only question is which specific form of aging will occur, and how fast it will do so. It is also possible that two forms of aging appear concomitantly. Combinations of aging flavors that are frequently observed include:
typical aging + atypical aging,
petrol flavor + typical aging,
atypical aging + reductive aging,
reductive aging + petrol flavor.
The sensory identification of these combinations of concomitantly occurring types of aging places high demands on sensory training and enological expertise.
2. Typical and oxidative aging
2.1. Chemical pathways, reaction products, and sensory results
Introduction: This chapter commences by describing the flavor-active compounds generated during what is considered typical aging, the odor profile they elicit, their precursors, and the most important reaction pathways responsible for their formation. It distinguishes between products always formed under oxygen-free conditions and those relying on oxygen uptake. To understand these issues, we will make a small excursion into the broad field of organic chemistry responsible for what we smell and taste in white wines. Footnote explanations facilitate understanding for those who do not deal with chemical terminology on a daily basis. Those who want to turn immediately to the more practical aspects can start reading with section 2.3. However, when too much school chemistry has fallen into oblivion and questions about it arise, then it is advisable to return to the beginning of this book.
2.1.1. Non-oxidative aging reactions
Under standard winery conditions, wine picks up oxygen before, during, and after bottling. The amounts picked up are highly variable and hardly ever checked.
Based on a given amount of oxygen picked up by the wine, oxidation of white wines leads to a substantially different outcome than that of red wines. It is standard knowledge that a certain amount of oxygen is required for maturation of red wines. Oxygen uptake, however, rarely improves the sensory quality of white wines that are produced to display fruity, floral, vegetative, or mineral aromas considered prerequisite for the sensory expression of their cultivar or origin.
From a chemical point of view, typical wine aging is a complex process that is not only driven by oxidation. Before going into details about how typical aging can be mitigated by enological measures, an up-to-date overview of the underlying reactions and compounds is useful.
Non-oxidative aging reactions in any wine
Whilst a considerable part of typical aging is driven by oxygen-related reactions in most wines, it is obvious that there are also non-oxidative reactions taking place simultaneously. They occur in any wine regardless of its oxygen exposure, albeit their significance is often underestimated. For a better differentiation and understanding of oxidative aging's nature as the very traditional problem in white winemaking, they are covered at first.
Every winemaker is familiar with the rather unspecific decay of fruity aroma attributes of any kind of wine during the very first weeks and months after alcoholic fermentation. It is basically related to a gradual loss of fermentation-derived aromatics, which are not specific to the grape variety the wine originated from but rather to the yeast strain it was fermented with. The hydrolytic breakdown1 of acetic acid esters with higher alcohols resulting from yeast fermentation metabolism plays a major role in this process (Rapp and Mandery 1986, Garofolo and Piracci 1994). Depending on storage temperature, this reaction comes to a complete halt after some months or years, when hydrolysis achieves an equilibrium between esters and their corresponding alcohols. It is unavoidable in very young wines, but not necessarily considered as a kind of aging detrimental to quality.
A striking example of non-oxidative losses of fruity varietal aroma by hydrolysis can be found in Sauvignon blanc wines stored in thoroughly topped stainless steel tanks or screw capped bottles, in which oxidative losses are negligible. These wines, when obtained from ripe fruit, contain various polyfunctional thiols2, which are responsible for their varietal aroma of tropical fruits. One of these thiols is 3-mercaptohexan-1-ol acetate (3-MHA), an ester with considerable sensory impact in young Sauvignon wines and responsible for their passion fruit and grapefruit aroma. This ester undergoes hydrolysis and declines steadily in concentration, originating 3-mercaptohexanol (3-MH) and acetic acid (Herbst-Johnson et al. 2011). As 3-MH has an approximately 15-fold higher perception threshold than 3-MHA (Coetzee and du Toit 2012, 2015), this hydrolysis results in a lower aroma intensity and a different aroma profile within a few months post-fermentation.
However, there is more. In an early work, several lactones produced by multifarious reaction mechanisms have been identified as partially responsible for an off-odor of white wines aged under anoxic3aging conditions (Muller et al. 1973). At a later stage, the formation of odor-active compounds by reactions between amino acids on one hand and dicarbonyl compounds (as diacetyl) or ketones (as acetoin) on the other hand was proven. Thus, in the presence of amino acids such as methionine, leucine, isoleucine and phenylalanine, the production of higher aldehydes, pyrazines, thiazoles, thiazolidines, and oxyzoles by the Maillard and Strecker reactions was observed at relatively low temperature and wine pH. These strong-smelling compounds display odors of corn, roasted hazelnuts, popcorn, sulfur, and ripe fruits (Marchand et al. 2000, Pripis-Nicolau et al. 2000). As might be expected, the synthesis of these compounds strongly increases with temperature (Section 2.7).
It will be shown subsequently that the aroma attributes referred to above are not very different from those produced under conditions of oxidative aging. Therefore, it can be fairly difficult to distinguish by sensory means whether typical aging has been caused by oxidation, by mere thermal load under anoxic conditions, or by both. Clearly, oxidation accelerates typical aging as perceived by smell, but exclusion of oxygen does not totally prevent it.
The aforementioned reactions of non-oxidative aging occur in any wine since the compounds responsible for it are not specific to a particular type of wine. In some wines, however, their detrimental sensory expression is not perceived because it is masked and superimposed by other kinds of white wine aging that are not considered as 'typical'. They comprise
The relative share of non-oxidative reactions has gained in importance since almost air-tight metal-lined screw caps (Section 3.5) were adopted in many countries such as Australia, New Zealand, and Central European ones for sealing bottled wines. These seals are able to create an anoxic environment excluding oxidative aging reactions, but cannot avoid a rapid decay of fruity varietal aromatics and the appearance of a roasted or nut-like aging aroma when the storage temperature is inadmissibly high. In particular, they foster the appearance of reduction flavor even at low temperatures.
Sensory studies concerning the impact of temperature on the rate of typical aging and petrol flavor are available (Sections 2.7 and 5.3). These studies and their practical implications deserve much more attention than they presently do. Further work in this field should try to assess the relative proportions of oxygen-dependent and non-oxidative reactions in the overall rate of typical aging.
2.1.2. Oxidative aging
Traditionally, oxygen pickup has been poorly controlled in the wine industry. As a consequence, typical aging is closely associated with oxidation. The sensory characteristics of oxidative aging are widespread and have always been observed by any winemaker. They include aroma descriptors such as cooked vegetables, boiled potatoes, black tea, damp garden soil, nuts, honey, and hay. Frequently, an increase in astringency as well as a color increase toward a darker yellow or even brown can also be noted. These sensory changes are also dependent on the flavonoid phenol content (Section 2.2.2).
For a long time, there was only limited knowledge about the chemical reactions and reaction products responsible for oxidative spoilage. Only after the wider dissemination of more sophisticated analytical tools over the last quarter of a century, knowledge about typical aging has become more extensive.
The role of malodorous carbonyls produced upon oxidation
Carbonyl compounds4 play a central role in the flavor of oxidative aging. Under storage conditions allowing for oxygen uptake, the coupled oxidation of vicinal dihydroxy-phenols leads to the formation of acetaldehyde and higher aldehydes, which substantially contribute to the aroma of Sherry wines (Wildenradt and Singleton 1974) and, at lower concentrations, also to that of other wines. Whilst the enzymatic generation of acetaldehyde during alcoholic fermentation is common to all wines, its non-enzymatic formation by oxidation can severely compromise the quality of white wines under certain conditions (Baro and Quiros Carrasco 1977). In the further course of this section, it will be shown that these conditions only exist when no free sulfur dioxide is present. Finally, for Riesling wines, it has already been shown a long time ago that under conditions of aerobic storage a large array of compounds is produced that is not observed when storage takes place under anoxic conditions. These odor-active compounds include benzaldehyde, furfural, and acetaldehyde (Simpson 1978).
When wine was stored in wooden barrels allowing for oxygen uptake, an increase of saturated and unsaturated carbonyl compounds as well as methyl ketones was observed. Under these conditions, the typical smell of the oxidized wine was tentatively ascribed to 2-nonanon and 2-undecanone (Ferreira and Bertrand 1996).
In another study on white wines undergoing barrel-aging, 2,5-furandicarbaldehyde, furyl hydroxymethyl ketone, and hydroxymaltol have been identified as further chemical markers of oxidative aging, especially of the honey descriptor resulting therefrom. However, it is not known to which extent these compounds are derived from wood. Their concentration decreases by post-fermentation yeast lees stirring (Lavigne-Cruège et al. 2000). It is not clear whether the latter effect is due to adsorption of these compounds by yeast lees or to consumption of dissolved oxygen by post-fermentation yeast cells (Section 2.5.6).
Under conditions of accelerated aging, 22 new odor-active compounds were identified for the first time after oxidation of six different white wines. Four of them were present in all wines and 14 in more than half of them. Several of these compounds displayed a repulsive, oxidized smell. Using methods of sensory profiling analysis and multivariate statistics, 15 of the discriminated odor attributes proved to be affected by oxidation, whereby the overall aroma pattern changed by 60%. There was a sensory oxidation pattern common to all oxidized wines (Escudero et al. 2000 a).
The aroma profile of oxidized or 'evolved' white wines is ascribed to higher aldehydes5suchas methional. It is produced by oxidation of coupled oxidation of methionol with ethanol, or via Strecker degradation of the amino acid methionine mediated by ortho-quinones6formed during wine oxidation (Escudero et al. 2000 b, Ferreira 2003 a).
Both alcohols and amino acids act as precursors of malodorous carbonyls
In simple terms, the Strecker degradation of amino acids involves their interaction with dicarbonyl7compounds. In the presence of α-dicarbonyl compounds, the amino acid is decarboxylated8 and deaminated9, thus forming an aldehyde with one carbon atom less than the original amino acid and known as "Strecker aldehyde". According to this pathway, 3-methylbutanal (malty aroma), phenylacetaldehyde (honey aroma) and methional (cooked vegetable aroma) are formed from the corresponding amino acids leucine, phenylalanine and methionine, respectively (Hofmann and Schieberle 2000).
In a wider sense, many other dicarbonyl compounds including ortho-quinones can be used. Indeed, in wine conditions, Strecker degradation was shown to occur to a greater extent via the reaction of ortho-quinones with the amino acids when metal ions are present as in any wine (Monforte et al. 2018). As already stated, these quinones are generated by oxidation of phenols, which act as primary oxygen acceptors in wine (Section 2.2.1.). Under comparable oxidation conditions, catechin, a flavonoid phenol, yielded more phenylacetaldehyde from phenylalanine than nonflavonoid phenols (Oliveira et al. 2017).
After the sensory role of the Strecker aldehydes was revealed, subsequent studies showed that the intensity of the cooked vegetables odor also correlates positively with the concentrations of 2-nonenal, benzaldehyde, furfural, and eugenol, while acetaldehyde levels are not significantly influenced by oxidation (Escudero et al. 2002). Further research confirmed the importance of methional as a key compound in the aroma pattern of oxidized white wines, altogether with phenylacetaldehyde, 3-(methylthio)-propionaldehyde, and sotolon (4,5-dimethyl-3-dihydroxy-2(5H)-furanone) (Ferreira et al. 2002, 2003 a, 2003 b, Ferreira 2007), benzaldehyde, furfural, and other higher aldehydes resulting from oxidation of unsaturated fatty acids10(Ferreira et al. 1997, Culleré et al 2007) or amino acids (Bueno et al. 2016).
As a summary of compositional data, it can be stated that the typical aroma of oxidative aging is caused by a large variety of carbonyls, among which higher aldehydes are the most important compounds, and sotolon as a volatile lactone11. For simplifying the analytical quantification of that kind of aroma, 2-phenylacetaldehyde, methional, sotolon, and 3-methylbutanal can be used as chemical markers (Pons et al. 2015, Mayr et al. 2015).
It is not yet clearly established whether the main pathway of the formation of all these aldehydes is the Strecker degradation of their corresponding amino acids or direct oxidation of their corresponding alcohols. However, from a practical point of view, the conditions of their generation and their sensory impact are much more important: Their concentration strongly correlates with oxygen uptake, temperature, and the intensity of odor descriptors referred to as boiled potatoes, farm-feed, hay, straw, wood, honey, etc.
Microbiological approach to reduce aging-relevant amino acids
Based on the assumption that the higher aldehydes involved in the smell of oxidative aging are primarily formed by the Strecker reaction of their corresponding amino acids, first attempts have been made to reduce the concentration of these amino acids by microbiological means. Specifically, Saccharomyces cerevisiae yeast strains were selected on the basis of their high consumption of amino acids such as phenylalanine, methionine and leucine.
Following fermentations of real must with these yeasts, chemical and sensory analyses were performed after one and three months of an aging process under periodic aeration at room temperature. Some of the yeast strains left behind lower residual concentrations of pro-oxidative amino acids after fermentation. Thus, they allowed production of less 'evolved' wines with lower mounts of higher aldehydes and lower sensory scores for oxidation-related aroma descriptors (Balboa-Lagunero et al. 2013).
It would make sense to continue such trials on a pilot scale and bring them to technical maturity in view of the current trend to reduce SO2 additions. They would open the way to new perspectives in research on yeast, which generally focuses on other topics such as fermentation kinetics or the production of fruity aromas.
Variable acceptance threshold of oxidative aging
As already stated, under comparable oxidation and storage conditions, the relative concentrations of oxidation-related higher aldehydes depend on the respective kinds and amounts of precursors and further intrinsic factors of each wine. They are determined by viticultural conditions, fruit ripeness, vinification and even yeast strain. This explains why oxidative aging displays somewhat variable aroma profiles. It might explain furthermore why some tasters perceive oxidative aging as less repulsive in wines made from very ripe fruit than in wines made from less ripe grapes. Fruit quality, consumers' expectations, traditions, and cultural environment determine to what extent the flavor of typical aging is accepted.
Figure 1 gives an extreme example of typical aging in an unoaked white wine intended to be fruity. The aroma profile changed completely during the first year of bottle storage. As a side effect, astringency and bitter ratings also increased. The reason for this is explained in section 2.2.1.
Limited effect of sulfur dioxide on higher aldehydes
The formation rates of the oxidation-derived higher aldehydes such as methional and phenylacetaldehyde are strongly dependent on the individual wine for a given amount of oxygen consumed. Most of them display a formation rate that correlates positively with the concentration of their corresponding amino acids, but not with their corresponding alcohols. Indeed, they are to a large extent complexed as bisulfite adducts at the free SO2 levels usually found in wines. Cleavage of the bound forms occurs during the first steps of oxidation as a consequence of equilibrium shifts caused by SO2 depletion. As the free SO2 level decreases, the concentration of the free odor-active fraction increases. At low levels of free SO2, de novo formation can also be observed with amino acids as the most important precursors (Grant-Preece et al. 2013, Ferreira et al. 2015, Bueno et al. 2016).
The entirety of research results show that higher aldehydes responsible for the smell of oxidative aging are ultimately generated by several pathways altogether:
All three reactions are fostered by oxygen uptake as it can occur before or after bottling.
Unfortunately, the data obtained on the inverse relationship between higher aldehydes and SO2 can hardly be used to remedy white wines affected by oxidative aging. Practical experience shows that increasing the free SO2 level from for example 20 to 50 mg/L only marginally improves their aroma. This observation suggests that higher aldehydes are less reactive with SO2 than acetaldehyde, which disappears entirely upon SO2 addition. This behavior has a simple explanation:
Indeed, it has been shown that the dissociation constant of the SO2-adduct with higher aldehydes is much higher than that with acetaldehyde. In other words, higher aldehydes have a lower potential to be bound by SO2. As a result, it is difficult to decrease their concentrations below the detection threshold by adjusting free SO2 to levels typically used in wine (Grant-Preece et al. 2013). Higher aldehydes and free SO2 can co-exist. Details and exact data are given further below in connection with what is called ‘free acetaldehyde’.
Oxidation accelerates the breakdown of fruity aroma compounds
Alongside the formation of new odor-active compounds, oxidative aging can lead to a breakdown of existing molecules contributing to fruity aroma attributes of young wines. The previously discussed hydrolysis of fermentation-derived esters is the major driver of changes in the aroma profile during the first weeks and months after alcoholic fermentation. Eventually, it proceeds until the equilibrium between esters and their corresponding alcohols is achieved (Garofolo and Piracci 1994). Theoretically, this equilibrium is not directly influenced by oxygen. However, research upon the impact of oxygen on the concentration of these esters led to conflicting results.
Variable oxygen permeation through different bottle closures (Section 2.6.7.) did not affect the concentration of the esters at least over the range of mild oxidation conditions obtained by the use of standard closures (Ugliano et al. 2015). In contrast, bottling with air-containing bottle headspace resulted in lower ester concentrations than bottling under inert conditions (Patrianakou and Roussis 2013). This effect is explained by a degradation of esters via the Fenton reaction (Waterhouse et al. 2016), which ultimately leads to a nonspecific oxidation by oxygen radicals (Section 2.2.1.).
Sensory consequences also occur when oxygen uptake during wine storage causes modification or decrease of terpenols and norisoprenoids, both grape-derived aroma compound groups conveying a floral-fruity smell (Rapp and Mandery 1986, Ferreira et al. 2002). However, the greatest losses of fruity attributes observed upon exposure to oxygen should be primarily due to oxidative degradation of sulfur-containing compounds, more precisely the polyfunctional aromatic thiols which can significantly add to the varietal character of some wines and can be dominant in Sauvignon wines.
Breakdown of fruity compounds enhances the effect of off-flavor
The whole bulk of research shows that oxidative aging comprises, besides the synthesis of new molecules known for their off-flavor, also the degradation of fruity-floral aroma compounds that are generallyexpected and looked for in young wines. Furthermore, it reveals that some reactions causing the sensory perception of typical aging are able to occur under anaerobic conditions. However, it also emphasizes the importance of oxygen uptake and subsequent oxidation reactions, which cause the formation of the multifarious carbonyls already discussed as new odor-active compounds.
At a low concentration level, the compounds responsible for typical aging can be considered to contribute to aroma complexity. When their concentration increases, they adversely affect wine quality more and more until they ultimately become responsible for the aroma feature of typically aged or 'evolved' wines.
How far a white wine should be aged primarily depends on personal preference. If a consumer accepts so-called mature whitewines despite their having lost much of their original fresh-fruity varietal character, typical aging is of less concern. Contrariwise, if one prefers the wine possessing a fruity elegance and freshness or showing a distinctive varietal character, then flavor stability is a key issue. This explains why most fruity white wines are recommended for early consumption.
Difference between oxidative aging and the smell of free acetaldehyde
In the worst case, the odor of the multifarious carbonyls produced upon oxidative aging can be accompanied and partially masked by free acetaldehyde with its typical smell reminiscent of bruised apples and sherry. Acetaldehyde is the most important aldehyde present in wine and primarily producedby yeast as a secondary product of alcoholic fermentation in a concentration range of 5 to 100 mg/L. In addition, a few mg/L can be produced as a result of oxidation of ethanol when oxygen is picked up during storage (Sections 2.2.1 and 2.5.1).
Only free acetaldehyde is odor-active
In this context, it is crucial to correctly interpret acetaldehyde concentration data. Headspace measurements by gas chromatography only record free acetaldehyde, which does not occur in the presence of free SO2, whilst colorimetric and enzymatic measurements record both the free and the bound form.
There is considerable confusion in the literature concerning the sensory threshold of acetaldehyde, which is reported to be somewhere between 10 and 100 mg/L, depending on the source and the kind of measurement, and without specifying SO2 levels. In actual fact, most winemakers and many consumers are able to sensorially detect concentrations as low as 1 mg/L free acetaldehyde in model solution or after removal of free SO2 from wine. The presence of free acetaldehyde is a common wine fault related to the absence of any free SO2 as it might occur after its depletion by oxygen ingress or in wines without added sulfites. When wine without added SO2 is to be produced (Section 2.5.2), it is paramount to minimize acetaldehyde levels for sensory reasons since no SO2can be supplied to bind it.
Free acetaldehyde can cause a sensory bias: Oxidative aging proceeds in the presence of free SO2 used as the traditional antioxidant in the wine industry, but it is strongly accelerated in its absence as long as oxygen is available. It is imperative to distinguish the sherry-like smell of free acetaldehyde from that of oxidative, typical aging, even though both of them might occur simultaneously. Unfortunately, there are not yet any analytical means to quantify free acetaldehyde for routine control. Most acetaldehyde measurements performed by winery or other labs provide total acetaldehyde regardless of the extent to which it is bound or free. Instead, a screening procedure for free acetaldehyde can be used. It is based on its SO2 binding.
Screening test for free acetaldehyde
A stock solution containing 10,000 mg/L SO2 is prepared by dissolving 18 g/L freshpotassium metabisulfite (K2S2O5) in one liter of water. Adding 0.5 mL of that solution to 100 mL wine sample provides 50 mg/L SO2. After five minutes, wine odor is evaluated and compared to an untreated control. The reduction or disappearance of a bruised apple/sherry like odor is indicative of free acetaldehyde.
This approach can be upgraded to test how much SO2 must be added to achieve a desired level of free SO2. For that purpose, increasing amounts of SO2 stock solution are added to wine samples and SO2 levels measured a few hours later.
Oxidative aging by smell can hardly be remedied
In contrast to the smell of free acetaldehyde which is easy to deal with in wine stabilization by addition of SO2, the higher aldehydes responsible for the smell of oxidative aging are much less reactive. As discussed earlier, they do bind to sulfur dioxide to produce an odorless addition product, but the extent of this reaction is not satisfactory in remedying white wines affected by oxidative aging (Grant-Preece et al. 2013). The reactions leading to their formation are sparsely reversible under practical winemaking conditions. This is the reason why the smell of oxidative aging cannot be effectively reversed by addition of the commonly used reducing agents such as sulfur dioxide or ascorbic acid.
Because of the diversity and low reactivity of the compounds involved, they are difficult to remove from wine. Since they do not carry any phenolic groups in the molecule, they cannot be removed by phenol-specific fining agents such as PVPP or caseinates. Such fining agents are only able to lower odorless phenols acting as catalysts in the formation of oxidative aging and the astringency they cause (Sections 2.2.1 and 2.4.7), but they do not improve aroma (Schneider 2006 a).
Some minor improvement of wines affected by typical aging can be achieved by fining with activated charcoal (0.05 to 0.3 g/L) or yeast lees obtained from young wines. However, charcoal has well-known side-effects feared for unspecifically stripping out any aroma compounds. More specific fining agents are not available, nor do other fining materials commonly used in the wine industry show any effect on oxidative aging as perceived by smell. This makes it all the more important to prevent or at least mitigate it, which will be described in detail in the following chapters.
2.2. Influence of the phenolic composition
Introduction: Phenolic substances are deeply involved in oxygen-related aging reactions as they are the primary oxygen acceptors even in the presence of free sulfur dioxide. Although these phenols are odorless and non-volatile, their reaction with oxygen leads to odor changes in the wine. The underlying chemical pathways, the impact of the most critical phenolic fraction of white wines on their shelf life and aging, and its analytical assessment in quality control are outlined in this section.
2.2.1. Oxidation of phenols in wine
There are countless organic compounds in wine that are potential targets for oxidation processes, but only a few of them are accessible to direct oxidation by molecular dissolved oxygen. These are phenolic substances and heavy metal ions such as iron and copper. When wine picks up oxygen, phenols are the primary reactants that are oxidized and able to bind large amounts of oxygen. Their oxidation initiates the oxidation of other compounds, including SO2, in a downstream reaction called coupled oxidation.
Phenolic compounds possess a common structure comprising at least one aromatic benzene ring12 with one or more hydroxyl (−OH) substituents. The most important wine phenols are presented in section 2.2.2. Those containing a 1,2-dihydroxyl substitution pattern, the so-called vicinal phenols, are the most easily oxidized. This process is more rapid at higher pH due to a higher percentage of dissociated phenolate13anions that can react with oxygen.
Phenols (PhOH) are weak acids, and their dissociation yielding phenolate anions (PhO-) can be written according to the general formula
PhOH → PhO_+ H+
The oxidation of phenols leads to their corresponding quinones. As a result, phenolic hydroxyl (−OH) substituents are replaced by quinoid (=O) substituents. This oxidation is promoted by heavymetal ions such as iron and copper, and requires only trace concentrations thereof. This requirement is easily met since wines contain much higher concentrations of these metals than needed for oxidation. It initiates a cascade of chemical transformations. Figure 2 depicts the reaction schema in a very simplified way, with R representing an undefined carbon chain.
Figure 2: Oxidation of phenols, formation of hydrogen peroxide, and oxidation of alcohols by the Fenton reaction.
R
Fe3+
O2
Fe2+
Fe3+
alcohol
OH°
hydroxyl radical
Fe2+
H2O2
aldehyde
H2SO3
H2SO3
H2SO4
H2SO4
R
H2O
When phenols oxidize, hydrogen peroxide (H2O2) is also generated in one of the first steps. In the competitive scenario of wine with the simultaneous presence of free SO2 (shown as H2SO3) and divalent iron (Fe2+), the major part of hydrogen peroxide is scavenged by SO2 which, in turn, is oxidized to sulfate (shown as H2SO4). Free SO2 is also able to partially reduce quinones back to the phenols they originated from.
Reactions of hydrogen peroxide produced upon phenol oxidation
A minor fraction of the peroxide produced via phenol oxidation is converted into hydroxyl radicals through a reaction cascade that is catalyzed by heavy metals. The hydroxyl radicals are able to oxidize a wide array of wine constituents that do not react directly with oxygen. This reaction is known as the Fenton reaction. It leads to an unspecific oxidation of wine components at rates that are proportional to their concentrations, thus oxidizing ethanol to acetaldehyde, glycerin to glyceraldehyde, tartaric acid to glyoxylic acid, etc.
In the broadest sense, this reaction is also responsible for the oxidation of higher alcohols to the corresponding higher aldehydes as soon as hydrogen peroxide is generated by the oxidation of phenols (Wildenradt and Singleton 1974, Singleton 1987, Waterhouse and Laurie 2006, du Toit et al. 2006, Danilewicz 2007, 2012, Elias and Waterhouse 2010, Oliveira et al. 2011). Thus, it explains to a large extent the involvement of higher aldehydes in the aroma profile of wines affected by oxidative aging.
The direct reaction of sulfur dioxide (SO2) with molecular oxygen (O2) is very slow. Free SO2 protects wine against oxidation only in an indirect way by trapping intermediate peroxide and, thus, impeding the Fenton reaction. In other words, wanting to protect wine against oxidation means first and foremost controlling the Fenton reaction (Elias and Waterhouse 2010). Under practical conditions, this undertaking is barely achievable because it would require the total absence of heavy metal ions. However, they are present in all wines in sufficient amounts. For that reason, control and limitation of oxygen uptake after primary fermentation and particularly after filtration (Section 2.6) is of major importance to protect white wine against oxidative aging.
Reactions of quinones produced upon phenol oxidation
The quinones generated upon phenol oxidation deserve further consideration. They are unstable and react in different ways:
As mentioned before, they are partially reduced back by SO
2
to their corresponding phenols they stem from. Thereby, SO
2
is oxidized to sulfate. However, this reaction is not complete. Its extent depends on the initial level of free SO
2
. In the absence of SO
2
, it does not occur at all. The incompleteness of this reaction is one of the reasons why a part of the oxygen remains irreversibly bound to the wine matrix instead of being scavenged by SO
2
, and why a given amount of dissolved oxygen in wine oxidizes less SO
2
than expected from stoichiometric calculations (Danilewicz et al. 2008, Danilewicz 2016, Waterhouse et al. 2016). The extent to which SO
2
protects wine against oxidation is discussed in detail in section 2.5.1. Similarly to SO
2
, ascorbic acid
is also able to convert quinones back to the original phenols when it is added to the wine (Section 2.5.3).
A quinone combines spontaneously with a remaining phenol, and in the process the resulting dimers
15
can rearrange their structure through an enol
16
-like conversion reaction to form a new diphenol. For example, a quinone-phenol dimer can be converted into a new diphenol dimer. Thus, the original phenolic hydroxyl (−OH) groups are regenerated, and the quinoid state is abolished. Several reaction pathways of this kind have been postulated (Singleton 1987); one of them is represented in figure 3. It is a particular case in chemistry in which oxidation is reverted without the action of a reducing agent. When the quinone is already a dimer, the same reaction generates the trimer of a phenol, a trimer of a quinone generates the tetramer of a phenol, etc. The regenerated phenolic hydroxyl groups are available for further oxidation. Therefore, this reaction pattern is called regenerative polymerization
. It explains why more oxygen can be taken up than would be expected from stoichiometric calculations based on the original form and concentration of the phenol molecules.
Figure 3: Regeneration of a phenolic OH-group through polymerization of a quinone with a phenol.
+
R
R
R
R
Nucleophilic addition
reaction
17
with sulfur-containing amino acids
and peptides, in particular with glutathione
, which is an important thiol in wine and musts. The resulting addition product is colorless (Section 2.4.3).
Nucleophilic addition reaction with amino acids according to the previously discussed pathway of Strecker degradation, leading to the formation of Strecker aldehydes participating in the off-flavor of oxidative aging (Section 2.1).
Nucleophilic addition reaction with the sulfite form of SO
2
, leading to a sulfonate addition product. This reaction is not to be confused with the reduction of quinones back to the original phenols, in which sulfite acts as a reducing agent.
Nucleophilic addition reaction with hydrogen sulfide
(H
2
S) involved in the reduction flavor (Section 3.1) of wines affected by that defect. This reaction possibly explains the occasional effectiveness of oxygen additions sometimes used to remove this off-aroma.
Nucleophilic addition reaction with desirable aroma thiols
that are responsible for the distinctive varietal aroma of some cultivars such as Sauvignon blanc,
explaining the sensitivity to oxygen uptake of wines obtained from these cultivars.
In the competitive reaction scenario of wine, the aforementioned quinone reactions are in an equilibrium, which depends on concentrations and relative reaction rates of the initial compounds (Nikolantonaki and Waterhouse 2012). The addition of reducing agents used in winemaking, in general SO2 and ascorbic acid in some cases, has the purpose of shifting the equilibrium towards reactions 1 and 5, thus diverting quinones from undergoing reactions 2, 4, and 7 that are responsible for the sensory perception of oxidative aging. Sections 2.5.1, 2.5.3 and 2.5.5 cover in detail to what extent this is possible.
2.2.2. Total phenols and the importance of flavonoid phenols
Phenolic chemistry of grapes and wine can be quite daunting for normal winemakers not only because of its complicated reaction mechanisms but also because of the intricate chemical nomenclature. However, phenols are important in defining wine style andmodulating aging.
The broad class of phenols originating from the grapes are composed essentially of two groups – flavonoids and nonflavonoids. Together they constitute the total phenol content, which is around 200 mg/L in standard white wines.
Nonflavonoid phenols present in the grape
In grapes, most of the nonflavonoid phenols are dissolved in the berry pulp, with some of them also present in the skins. Because of their easy extractability from grape pulp, their levels are relatively constant in white and red wines regardless of skin contact prior to pressing. They comprise mainly derivatives of hydroxybenzoic acids based on the C6-C1 structure of benzoic acid, and derivatives of hydroxycinnamic acids based on the C6-C3 structure of cinnamic acid (Figure 4). The various acids are differentiated by the substitution pattern of their benzene ring. Furthermore, most of them are esterified with sugars or organic acids such as tartaric acid.
Figure 4: General structures of benzoic acids and cinnamic acids.
cinnamic acid
benzoic acid
The derivatives of hydroxycinnamic acids, particularly caftaric acid, and their oxidation products are the most abundant class of phenolics in white juices and wines with concentrations of 100 to 200 mg/L. They are also called hydroxycinnamates.
Basically all grape-derived nonflavonoid phenols display a bitter taste and elicit a sensation of astringency. However, they are hardly able to do so in wine to an appreciable extent, because their concentration is close to or below their taste threshold. Therefore, they must not be confused with tannins. They do not play a direct role in 'phenolic taste', color, or aging reactions of wine. At best, they collectively contribute to weight and volume on the palate (Singleton and Noble 1976, Arnold et al. 1980, Vérette et al. 1988, Smith and Waters 2012). This is demonstrated when specific phenol-absorbing fining agents are inadvertently used on white wines whose phenolic make-up consists nearly solely of nonflavonoids; the wines become thin and meager since there are no other phenols able to react with the fining material.
Nonflavonoid phenols from external sources
Another group of nonflavonoid phenols, commonly called hydrolyzable tannins, are not present in the grape, but can be detected in wines aged in wooden barrels, treated with oak alternatives, or certain commercial tannins. These hydrolyzable tannins include ellagitannins and gallotannins, which release ellagic acid and gallic acid after hydrolysis, respectively. They display an astringency frequently perceived as extraordinarily strong immediately after their addition, though this tendency tends to slowly decrease during aging due to their breakdown caused by oxidation and hydrolysis. However, their chemical structure and properties differ from grape-derived tannins, which are made up of flavonoid phenols.
Despite not being naturally present in grapes, hydrolyzable tannins constitute the major share of commercially available tannins legalized as wine additives. One of the reasons for this is their putative action as a reducing agent, which is covered in section 2.5.4.
Flavonoid phenols
Flavonoid phenols possess a common C6-C3-C6 skeleton composed of three rings (A, B, and C) according to the general structure
They branch into different sub-categories that differ by their degree of unsaturation18 and the substituents of their lateral B-ring. Almost solely one category of them, the flavan-3-ols, can be found in white wines. Although flavan-3,4-ols and flavonols are also present in the skins of grapes grown under hot-climate conditions, they are only detected in trace amounts in finished white wines. Hence, only flavan-3-ols, also known as catechins, will be covered in this book. These are (+)-catechin, (-)-epicatechin, (-)-epigallocatechin, (-)-epicatechin-3-gallate, and (-)-epigallocatechin-3-gallate (Figure 5). The gallate esters refer to a binding with gallic acid.
Figure 5: Molecular structure of catechin, epicatechin, and their gallates.
(+)-catechin and (-)-epicatechin are the predominant flavan-3-ols in white wines. In contrast to these colorless monomeric forms of flavan3-ols, their dimeric and polymeric forms, the so-called procyanidins, have been reported to be too low in concentration to be detected (Carando et al. 1999 a) or to account for only a minor proportion of all catechins in white wines (Lea et al. 1979, Carando et al. 1999 b). Nevertheless, some polymerization of flavan-3-ols does occur as will be shown below.
In the grape, flavonoid phenols occur to a limited extent dissolved in the pulp juice (Section 2.3.2), but in much higher concentrations in the solid tissues of skins, seeds, and stems, from which they can be extracted from during skin contact and by mechanical impact on the must during transport and pressing. Depending on grape processing and juice treatment, they are present in normal white wines at highly variable concentrations ranging from approximately 3 to 40 mg/L. They can strongly contribute to astringency and bitterness in white wines when their concentration exceeds a certain level (Section 2.4.1).
Polymerization of flavonoid phenols
During wine aging, flavonoid phenols undergo polymerization, which can be either oxidative or non-oxidative. Polymerization is the combination of many small monomeric molecules to form fewer dimeric or even larger molecules. It is accelerated by previous oxidation of the phenolic compounds (Section 2.2.1). It proceeds the faster the higher the initial concentration of the flavonoids. The reason is easy to explain: In order to react one with another, two molecules must first collide in the three-dimensional space in accordance with the laws of probability. The probability that this happens increases as their number grows.
Differences between flavonoid polymerization in juice and in wine
It is important to note that with regard to enological consequences, there are fundamental differences between the polymerization of flavonoids in juice and in wine:
Thus, the question of oxidation of must or its prevention is of great enological importance and is therefore dealt with in detail in the following sections.
Sensory properties of flavonoids
The notion of flavonoid phenols as opposed to total phenols might appear unusual to most winemakers, because it is widely neglected in white wine enology. However, whilst nonflavonoids do not play a major role in white wine mouthfeel and aging behavior, flavonoid levels describe very precisely the concentration of astringent phenols that pass from the grapes into the must and that the wine industry generally aims at minimizing in white wines. As shown below, it is an extremely important parameter for assessing the long-time behavior of these wines and their astringency.
The role of flavonoids in white wine color
White wines without significant amounts of flavonoids are stable in color and not able to produce significant browning under conditions of oxygen uptake during aging even in the absence of free SO2 (Rossi and Singleton 1966, Simpson 1982, Lee and Jaworski 1988, Fernandéz-Zurbano et al. 1995, 1998). Conversely, flavonoid concentrations strongly correlate with the browning
