Fermentation

The process of alcoholic fermentation requires careful control for the production of high quality wines. Requirements include suppression of the growth of undesirable microorganisms, presence of adequate numbers of desirable yeasts, proper nutrition for yeast growth, temperature control for prevention of excessive heat, prevention of oxidation, and proper management of the cap of skins floating in red musts.

Temperature control during alcoholic fermentation is necessary to (1) facilitate yeast growth, (2) extract flavors and colors from the skins, (3) permit accumulation of desirable by-products, and (4) prevent undue rise in temperature, killing the yeast cells.

Optimum temperature for growth of common wine yeast is about 77 F, and in many viticultural areas of the cooler Temperate Zone, grapes are crushed at about this temperature. Fermentation is seldom started at so high a temperature, however, because it is then difficult to prevent the temperature from exceeding 86 F during fermentation.

Extraction of flavors and colors is not a problem in white musts; the crushed grape mass is usually separated from the skins before fermentation. Fermentation of white musts at relatively cool temperature, about 50 to 60 F, apparently results in greater formation and retention of desirable by-products. An undesirable feature of such relatively low-temperature fermentations is the longer period required for completion (six to 10 weeks compared to one to four weeks at higher temperatures) and the tendency for the fermentation to stop while residual sugar remains. In practice white table wines are usually fermented at about 68 F.

In red wine musts, the optimum color extraction consistent with yeast growth occurs at about 72 to 82 F. Alcoholic fermentation produces heat, however, and careful temperature control is required to prevent the temperature from reaching a point (about 86 F) where yeast growth is seriously restricted. At still higher temperatures, growth will stop completely. Modern temperature control is accomplished by use of heat exchangers. Older methods include placing the fermenters in a cold room; using cold pipes in the fermenter; pumping the must through double-walled pipes, with cold water in the surrounding pipe; pumping the must through a sump containing cooling coils; and pumping the coolant through jackets surrounding the tank.

Contact with air must be restricted to prevent oxidation during fermentation. In very large containers, the volume of carbon dioxide given off is sufficient to prevent entry of air. In small fermenters, fermentation traps are inserted, preventing entry of air but permitting exit of carbon dioxide. These traps are particularly desirable during the final stage of fermentation, when carbon dioxide evolution is slow. Following fermentation, small amounts of sulfur dioxide are added to help prevent oxidation. Ascorbic acid is sometimes employed to decrease the oxidation and thus the amount of sulfur dioxide required as an antioxidant, but is not generally recommended.

The use of small fermentation vessels permits a greater percentage of heat loss to the surrounding atmosphere, simplifying temperature control.

Post-fermentation Treatment

With appropriate must composition, yeast strain, temperature, and other factors, alcoholic fermentation ceases when the amount of fermentable sugar available becomes very low (about 0.1 percent). Fermentation will not reach this stage when (1) musts of very high sugar content are fermented, (2) alcohol-intolerant strains of yeast are used, (3) fermentations are carried on at too low or high temperatures, and (4) fermentation under pressure is practiced. Fermentation of normal musts is usually completed in 10 to 30 days. In most cases, the major portion of the yeast cells will soon be found in the sediment, or lees. Separation of the supernatant wine from the lees is called racking. The containers are kept full from this time on by "topping," a process performed frequently, as the temperature of the wine, and hence its volume, decreases. During the early stages, topping is necessary every week or two. Later, monthly or bimonthly fillings are adequate.

Normally the first racking should be performed within one to two weeks after completion of fermentation, particularly in warm climatic regions or in warm cellars, as the yeast in the thick deposit of lees may digest themselves forming off-odors.