Linda F. Bisson

Yeast and Biochemistry of Ethanol Fermentation

The transformation of grape juice into wine is essentially a microbial process. As such, it is important for the enologist to have an understanding of yeast and fermentation biochemistry as the fundamental basis of the winemaking profession. The alcoholic fermentation, the conversion of the principal grape sugars glucose and fructose to ethanol and carbon dioxide, is conducted by yeasts of the genus Saccharomyces, generally by S. cerevisiae and S. bayanus. The current use of the old term bayanus for the yeast closely related to S. cerevisiae is controversial (see Section A3c); but we expect bayanus to become once more an accepted appellation (Vaughn-Martini and Kurtzman 1985).

The Role of Sulfur Dioxide in Wine

While the use of sulfur dioxide in winemaking dates back to Egyptian and Roman times (Bioletti 1912), the full extent of its role in wines is often not understood because of the multiple activities and reactions in which it is involved. It had been estimated in the early part of this century that the free forms possessed approximately 50 times the antiseptic activity of the bound forms (Bioletti 1912). While most of the sulfur dioxide found in wine is deliberately added to the must, juice, or wine, significant amounts are normally produced by yeast during fermentation (Weeks 1969; Bidan and Collon 1985).

The Physical and Chemical Stability of Wine

The major physical instability in bottled wines continues to be the precipitation of the tartaric salts, potassium bitartrate, and calcium tartrate. Prevention of this precipitation in bottled wines is desirable because consumers find it objectionable and an indication of poor quality control. Precipitation of these salts can be due to one or more reasons, such as the incomplete stabilization in the cellar, the use of a nonrepresentative sample for the stability test, the use of an inappropriate stability test, the removal of colloidal materials at the point of final filtration that have previously inhibited the precipitation and natural chemical changes, especially the polymerization of phenolic pigments. The initial instability is caused by supersaturated levels in juices that are augmented by the decrease in solubility due to ethanol and the low temperatures used for wine storage.

Red and White Table Wines

There are several aspects of wine fermentations that are of general interest and these will be considered in detail before consideration is given to the major wine types. The addresses of equipment companies mentioned in this chapter can be found in Appendix I.

Preparation of Musts and Juice

The natural variation in almost every aspect of grape composition is a major feature of wines and is the cause of the seasonal, varietal, and regional differences that they display. The application of scientific understanding to the production of the best possible wines requires that we take steps to protect the most desirable components of the juice, sometimes by intervening with natural reactions and sometimes to overcome natural deficiencies or imbalances that exist by nutrient additions and physical treatments.

Sugar and Glycerol Transport in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the process of transport of sugar substrates into the cell comprises a complex network of transporters and interacting regulatory mechanisms. Members of the large family of hexose (HXT) transporters display uptake efficiencies consistent with their environmental expression and play physiological roles in addition to feeding the glycolytic pathway. Multiple glucose-inducing and glucose-independent mechanisms serve to regulate expression of the sugar transporters in yeast assuring that expression levels and transporter activity are coordinated with cellular metabolism and energy needs. The expression of sugar transport activity is modulated by other nutritional and environmental factors that may override glucose-generated signals. Transporter expression and activity is regulated transcriptionally, post-transcriptionally and post-translationally. Recent studies have expanded upon this suite of regulatory mechanisms to include transcriptional expression fine tuning mediated by antisense RNA and prion-based regulation of transcription. Much remains to be learned about cell biology from the continued analysis of this dynamic process of substrate acquisition.

Viticulture for Winemakers

Viticulture is a science and technology separate from enology and winemaking per se. It has its own extensive literature. References that give more detail than possible here as well as additional perspectives include fairly recent examples in English from several different countries (Coombe and Dry 1988; Jackson and Schuster 1987; Pongracz 1978; Wagner 1976; Weaver 1976; Winkler et al. 1974). Examples in other languages from major wine-producing countries can also be useful (Champagnol 1984; Galet 1988; Huglin 1986; Reynier 1989; Altman 1987; Currle et al. 1983; Kadisch 1986; Ferraro-Olmos 1983; Larrea-Redondo 1981; Fregoni 1985; Saracco 1984). Although wine-makers should know as much as possible about viticulture, the professions are so demanding that it is difficult to be fully expert in both.

The Fining and Clarification of Wines

The purposes of clarification and fining during wine processing include removal of excessive levels of certain wine components, achieving clarity, and making that clarity stable especially from a physicochemical viewpoint. The noun fining is used in winemaking to describe the deliberate addition of an adsorptive compound that is followed by the settling or precipitation of partially soluble components from the wine. The materials used for these reasons are collectively referred to as fining agents, even though the solutes that they address and the mechanism of their removal vary considerably. The need to employ such treatments is often determined not only by compositional aspects of the musts but also by the winemaking practices that have been employed.

Microbiological Spoilage of Wine and its Control

This chapter includes the descriptions and origins of various kinds of microbiological spoilage organisms—and the prevention of their presence and the control of their growth if present. It is important for the winemaker to know which spoilage has occurred in any given instance and to understand potential spoilage problems, but obviously it is better to forestall spoilage than to diagnose it. The taxonomic identifications of the yeasts are given in Chapter 4, and the lactic acid bacteria in Chapter 6. For the aerobic bacteria, the taxonomies are given at the end of this chapter.

Juice and Wine Acidity

The acidity of a juice or wine, in particular the pH, plays an important role in many aspects of winemaking and wine stability (see also Chapters 3, 8, and 12). The ability of most bacteria to grow, the solubility of the tartrate salts, the effectiveness of sulfur dioxide, ascorbic acid, and enzyme additions, the solubility of proteins and effectiveness of bentonite, the polymerization of the color pigments, as well as oxidative and browning reactions are all influenced by the juice or wine pH. The titratable acidity is an important parameter in the sensory evaluation of finished wines. It and the pH are also important in aging reactions.