WHITE WINE ENOLOGY

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...