Yeast Modulation of Wine Flavor
Introduction
It is the defining characteristic of flavor that distinguishes the differences among the thousands of wines produced throughout the world. Many wine‐tasting professionals describe flavor as the simple aroma of the fermenting grape must or young wine or the bouquet of the more complex compounds that evolve as a result of fermentation, élevage, and aging. Wine connoisseurs might refer to grape aromas as primary aromas, to those originating during fermentation and oak aging as secondary aromas, and to bottle‐aging aromas as either tertiary aromas or bouquet. There is little consistency in the use of these terms, and many authorities differ about the point in a wine's life cycle at which an aroma becomes a bouquet. The words flavor, aroma, and bouquet are often used interchangeably—a tradition that has been followed in this review.
A synergistic relationship exists between the grape and the yeast in determining flavor in wine, which relationship the grape grower and winemaker attempt to harmonize, but this is a complex process with an almost infinite number of parameters that can be altered to produce wines with vastly different flavor profiles. A good wine is largely dependent on the grape grower's and winemaker's scientific and intuitive abilities to manipulate parameters in such a way that the end product is balanced and rich in flavor. Therefore, the production of a wine to the specification set by the targeted market requires an integration of science, economics, and artistry—a blend of individual creativity and innovative technology (Bisson et al., 2002).
It is well known that grapes of different varieties/cultivars display characteristic aromas that are distinctive of the wines (Rapp 1986, Schreier 1976). However, it can be shown that although some volatile aroma substances arise from components of the grapes, many of these compounds are changed and a further substantial portion of wine flavor substances are formed during yeast fermentation (Lambrechts and Pretorius, 2000). Therefore, wine has more flavor than the grape juice it is fermented from, and the importance of yeast and other wine‐related microorganisms is central to the development of wine flavor (Fig. 1). Different biosynthetic pathways are interactive during the formation of the aroma of alcoholic beverages, and different factors play their part in the formation of the total aroma. The quantitative development of flavor compounds found in grapes and produced during yeast fermentation fluctuates considerably because of the influence of various factors including the cultivar (Baumes 1986, Van Wyk 1979), the soil quality, the quality and ripeness of the grapes (Houtman 1980a, Houtman 1980b), the yeast strain used in the ferment (Delteil 1992, Howell 2004, Lambrechts 2000, Pretorius 2000, Soles 1982), the pH of the must (Marais, 1978), and the nature and prevailing temperature of yeast fermentation (Daudt and Ough, 1973). Further, numerous technological aspects and vinification methods (Gómez 1994, Voilley 1990), for example, the nature of grape crushing, as well as of the mash and must treatment and skin contact time (Falqué and Fernández, 1996), will also significantly influence the final aroma. Bacteria present on the grapes and in the must before and throughout fermentation exert their largest effect on wine flavor after completion of alcoholic fermentation, specifically during and after malolactic fermentation (Davis et al., 1985).
Despite this complexity, advances in wine research have increasingly been able to assist winemakers in predetermining the outcome of their product by applying basic scientific principles. Today, one cannot imagine winemaking without at least some scientific input. For example, the pH meter, once seen by some as a threat to the art of winemaking, is now indispensable (Fornachon 1943, Hooke 1994). In the same way, research in the area of flavor science will make it possible for winemakers to optimize the flavor profile of a wine by analyzing their grapes, selecting the right yeast based on that information, and setting the correct conditions for fermentation, taking into account the characters of both grape and yeast.
The importance of yeast in wine flavor is well known and is intensively researched (Lambrechts 2000, Pretorius 2000, Pretorius 2003, Pretorius 2004, Pretorius 2002). However, we have only scratched the surface when it comes to characterizing and optimizing these precious little bugs for their ability to produce a world‐class wine abounding in flavor. Improvements can be made by tailoring wine yeasts for their ability to biosynthesize flavor compounds at optimal concentrations as well as to release desired grape‐derived flavor compounds or modify grape‐derived flavor compounds without affecting the general fermentation performance. This tailoring could assist winemakers in their effort to consistently produce wine to definable specifications and styles.
This chapter reviews the scientific knowledge of the role of microorganisms, especially yeast, in the development of wine flavor. Specific attention will be given to the contribution of esters, higher alcohols, volatile thiols, volatile phenols, and monoterpenoids to the flavor profile.
Section snippets
Wine Yeast
The yeast Saccharomyces cerevisiae, the most important microorganism in wine, is a single‐celled fungus able to grow in high sugar (between 220 and 250 g/L) and low pH (pH 3–4) and to survive in the presence of a high ethanol concentration. These attributes give this yeast a massive competitive advantage in grape must, where it ferments high concentrations of glucose and fructose to ethanol and carbon dioxide.
Several unique features of yeast metabolism contribute to its competitive advantage
Esters
Most esters found in alcoholic beverages are secondary metabolites produced by S. cerevisiae during the fermentation (Engan, 1974). Their synthesis is linked to the lipid and acetyl‐CoA (coenzyme A) metabolism (Fig. 2), and they constitute one of the largest and most important groups of compounds affecting flavor in fermented beverages (Fujii 1994, Peddie 1990). The most significant esters are ethyl acetate (fruity, solvent‐like), isoamyl acetate (isopenthyl acetate, pear‐drops), isobutyl
Volatile Thiols
The sulfur‐containing volatiles 4‐mercapto‐4‐methylpentan‐2‐one (4MMP) and 3‐mercaptohexanol (3MH) are present in a cysteine conjugate in grape juice (e.g., sauvignon blanc) and are released during fermentation by the action of yeast (Tominaga 1995, Tominaga 1998). After reports showing that 4MMP was present as a nonvolatile, nonglycosylated bound precursor in must, a mechanism of release was suggested. A cell‐free enzyme extract of the bacteria Eubacterium linosum can release 4MMP from
Concluding Remarks
The consistent production of wines to definable specifications and styles is increasingly important. The infinite number of flavor profiles of bottled wines results from the synergy between grapes and yeast. In the way the conductor of an orchestra produces beautiful music from the musicians and their instruments, the winemaker works with his grapes and yeasts to produce a magnificent harmony in a wine.
The blending of the precise amounts of different flavor compounds to produce the distinct
Acknowledgments
The authors are grateful to Mariska Lilly, Kate Howell, Nydia Potgieter, Annél Smit, and Heather Smyth for their contributions. Research conducted at The Australian Wine Research Institute is supported by Australia's grape growers and winemakers through their investment body, the Grape and Wine Research and Development Corporation, with matching funds from the Australian Government.
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