29
Optimizing White Wine Aromas

29.1 Introduction

Wine producers often speak about making the “best wine,” but such statements obscure the fact that consumers (and critics) do not have uniform opinions in their wine preferences. For example, Sauvignon Blanc wines can possess a diverse range of aromas (e.g., tropical/fruity versus green/herbaceous), and controlled studies have identified specific consumer segments that prefer each of these styles [1]. “Optimization” of aromas or other sensory attributes of a wine – whether by changing vineyard practices, the winemaking process, or even packaging decisions – first requires defining a desired outcome, which is often one that the winemaker believes will lead to better sales.

Because of legal restrictions regarding additives and production practices (Chapter 27) the chemistry of a finished wine is closely related to the initial grape composition, which will in turn be dependent on grape variety, growing region, and viticultural practices. Despite the marketing parlance that the best winemakers “let the fruit speak for itself,” winemakers have a high level of influence over wine quality and style, and research has been conducted into all manner of aspects, from harvest decisions to pre‐fermentation operations and adjustment of juice/must composition, and then to fermentation and storage conditions (Chapters 21 to 25).

Several techniques for optimizing wine aromas – particularly white wine aromas – have been described throughout the book, such as the practice of fermenting at low temperatures to increase “fruity” aromas due to esters (Chapter 22.1) and minimizing oxygen exposure post‐fermentation to avoid oxidation of varietal thiols that impart tropical aromas (Chapter 24). Selection of yeast (and bacteria) will also critically affect many white wine sensory attributes (e.g., References [2] to [8]). This chapter focuses on one emerging research area within this broad topic – optimization of white wine aromas through yeast selection, with particular emphasis on varietal thiols.

29.2 Enhancement of varietal thiols

The highly potent odorants known as polyfunctional (varietal) thiols contribute to the tropical aromas of many wines, and particularly Sauvignon Blanc (Chapter 10). As discussed previously, these thiols appear to be formed by microbial metabolism of grape‐derived, non‐volatile precursors (S‐conjugates) in the juice (Chapter 23.2). Certain consumer segments prefer Sauvignon Blanc with strong tropical aromas and less intense green/herbaceous aromas [1, 9]. As noted by Swiegers et al., vineyard management is effective at controlling the primary odorants responsible for herbaceous aromas (particularly methoxypyrazines, Chapter 5), but the secondary aromas of “tropical”‐smelling varietal thiols and other compounds are controlled through fermentation parameters [10]. Because only a small fraction of the S‐conjugate pool is liberated during a typical fermentation (Chapter 23.2), there has been considerable interest in characterizing the ability of various yeast strains to release different varietal thiols during fermentation [10–12].

As shown in Figure 29.1, yeast strains can vary both in the total amount of key varietal thiols they produce (3‐mercaptohexan‐1‐ol [3‐MH], 3‐mercaptohexyl acetate [3‐MHA], and 4‐mercapto‐4‐methylpentan‐2‐one [4‐MMP]), but also in their relative concentrations. This information can allow winemakers to optimize production of wines like Sauvignon Blanc with certain characteristics, that is, more or less 3‐MHA (“passionfruit”) as compared to 4‐MMP (“cat pee”), depending on the targeted style. Due to the relative stability of many of the thiols under reductive conditions, the sensory effects of yeast strain can still be evident after some years of bottle aging [13].

Figure 29.1 Influence of yeast strain on varietal thiol concentrations for (a and b) a single Sauvignon Blanc juice fermented on a 20 L scale (left panel), and (c and d) four different Sauvignon Blanc juices fermented on a 225 L scale (4‐MMP was undetectable in each wine using Davis 522 strain, right panel).

Data from References [10] and [12]

Key enzymes involved in the liberation of varietal thiols during fermentation have been identified through studies of genetically modified (GM) yeasts, and non‐commercial GM yeasts overexpressing certain enzymes can produce greater amounts of varietal thiols (Chapter 23.2). This knowledge has helped drive research in molecular breeding¹ to produce yeasts with superior varietal thiol production in commercial wine strains [14, 15].

29.3 Cofermentation and spontaneous fermentation

The genetic diversity of commercial S. cerevisiae yeast strains provides tools to optimize wine aroma attributes. However, an even greater diversity may be accessible through the myriad indigenous yeasts (including non‐Saccharomyces) present in and around wineries and vineyards. Winemakers may choose not to inoculate with a commercial yeast, but instead execute a spontaneous (or wild) fermentation (more properly, a spontaneous cofermentation, since multiple species/strains will be present).² Alternatively, commercial non‐Saccharomyces yeasts – or hybrids of Saccharomyces and non‐Saccharomyces yeasts – may be employed alone or in cofermentation with conventional yeasts [16–20].

The use of spontaneous fermentation or commercial non‐Saccharomyces yeasts often produces sensorially distinct wines.³ These wines are not necessarily appealing to a broad range of consumers – the use of “native” Saccharomyces isolated from oak trees yielded wines with strong sulfurous aromas and low fruitiness [21]. However, other work with spontaneous or non‐Saccharomyces fermentations has shown characteristics that would be appealing to particular segments, such as greater wine complexity or increased intensity of certain fruity aromas [18, 20, 22]. Unsurprisingly, sensory differences often appear to be related to differences in common fermentation‐derived metabolites (e.g., fatty acid ethyl esters, acetate esters, higher alcohols, fatty acids), but grape‐derived aroma compounds can also be influenced. A comparison of three indigenous yeast strains (inoculated separately) and a spontaneous fermentation using Albariño juice showed that one strain in particular produced substantially higher amounts of C₁₃‐norisoprenoids (β‐damascenone and β‐ionone) and monoterpenoids (geraniol and linalool), yielding a wine that was rated highest in quality by a sensory panel [23].

Cofermentations have also been demonstrated to result in unexpected outcomes due to interactions between different yeasts. In one study, Sauvignon Blanc wines produced by coinoculated commercial S. cerevisiae and non‐Saccharomyces natural isolates were compared to the same wines produced from yeast monocultures (Figure 29.2). Concentrations of grape‐derived compounds (C₁₃‐norisoprenoids, monoterpenoids, and varietal thiols) produced from cofermentation were not necessarily intermediary to other values, for example, β‐damascenone production was double in an Mp‐Sc cofermentation as compared to monocultures of Mp or Sc. However, in some cases monocultures stood out as higher producers, especially Mp and Sc for 3‐MH and 3‐MHA, and Cz for linalool, geraniol, and β‐damascenone (Figure 29.2) [24]. Significantly, differences in varietal thiol production (and sensory characters) have also been observed for Sauvignon Blanc fermented with combinations of two or three commercial S. cerevisiae strains [25]. Considering that many yeast suppliers offer over 30 strains, winemakers potentially have thousands of possible combinations to explore even if they limit themselves to no more than three strains per cofermentation. Future work is expected to lead to a better molecular understanding of why yeast strain interactions occur, and how they can be employed by winemakers to optimize for particular outcomes.

Figure 29.2 Influence of yeast species and coinoculation on concentrations of (a) monoterpenoids and β‐damascenone, and (b) varietal thiols, in fermentations of Sauvignon Blanc juice. Sc, Saccharomyces cerevisiae; Mp, Metchnikowia pulcherrima; Td, Torulaspora delbrueckii; Cz, Candida zemplinina.

Selected data from Reference [24]

References

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Notes

1 Despite GM yeasts having been used for research over many years, a number of countries ban their use in commercial winemaking. Among other things, consumer acceptance is a major hurdle. Nonetheless, GM yeast research has allowed for certain genetic markers to be identified that relate to varietal thiol production. Desirable traits can then be selected for using non-GM approaches, such as classical breeding between different yeast strains and backcrossing of hybrids with the commercial parent strain.

2 As a caveat, avoiding inoculation does not guarantee that commercial yeasts will not be present. Several genotyping studies on spontaneous fermentations indicate that the major yeasts at the end of fermentation are often identical to (or closely related to) commercial wine strains, since these yeasts are able to tolerate the higher ethanol environments.

3 By their very nature, spontaneous fermentations with indigenous yeasts are relatively unpredictable, so obtaining a reproducible outcome could be difficult (especially when compared to inoculated fermentation).