In the absence of SO2 addition, the indigenous microflora is relied upon to initiate and then carry out a spontaneous or natural fermenta-tion.This is one of the most well studied of all fermentations, and much is now known about the ecology of wine and the yeasts that participate in the wine fermentation. In reality, however, the yeast fermentation is but one of two distinct fermentations that occur in wine making. Yeasts, of course, ferment sugars to ethanol, CO2, and small amounts of other end products. A second fermentation, called the malolactic fermentation, is carried out by specific lactic acid bacteria that are either naturally present or added for this purpose. The malolactic fermentation, to be discussed later, is now regarded as nearly as important to wine quality as the ethanolic fermentation.
As noted above, the surface of grapes usually contain less than 104 yeast cells per grape (or per ml of juice).This number may increase during ripening on the vine, especially if the temperature is warm. Although ten or more yeast genera may be represented, the primary organism most frequently isolated from grape surfaces and the fresh must is Kloeckera apicu-lata. In contrast, S. cerevisiae, the yeast most responsible for the wine fermentation, is rarely observed on grapes. Rather, S. cerevisiae and other related strains are introduced into the must during grape handling and crushing steps directly from the equipment.The must is inoculated, in other words, by the yeasts originating from the grape surface as well as by those residing on the winery equipment.
Despite the large amount of available carbohydrates and other nutrients, the must is actually quite a selective environment.The pH is typically below that which many organisms can tolerate, and the organic acids present in the must have considerable antimicrobial activity. The high sugar concentration and resulting high osmotic pressure can also inhibit many of the indigenous organisms. Eventually, the CO2 formed during the early stages of fermentation makes the environment anaerobic, restricting growth of aerobic organisms. Likewise, enough ethanol is produced to provide selection against ethanol-sensitive organisms. Finally, sulfites, if added, have antimicrobial activity against a wide range of yeasts and bacteria. Thus, as the environment changes, yeast species that are numerically dominant may be displaced by other species or strains better suited to the environment at any particular time. Microbial ecologists refer to such arrangements as successions. One reason why the outcome of a natural wine fermentation is so difficult to predict is that successions rarely occur exactly the same way in different musts, since the wild flora is never entirely the same.
Members of the genus Saccharomyces represent less than 10% of the initial yeast popula-tion.The early yeast population is dominated by K. apiculata. This organism produces up to 6% ethanol from glucose and fructose; however, it has a low ethanol tolerance, being inhibited by 3% to 4% ethanol.Thus, even though K. apicu-lata is among the first to grow in the must, its numbers are not usually maintained beyond the first several days. However, it is thought that this organism does produce small amounts of acids, esters, glycerol, and other potential flavor components, some of which may or may not be de-sirable.As the ethanol concentration increases and the Eh (or redox potential) is reduced, due to CO2 formation, the environment begins to select for S. cerevisiae and other various ethanol-tolerant Saccharomyces spp.
According to current yeast taxonomy (Chapter 2), most wine yeasts are now classified simply as Saccharomyces cerevisiae. Thus, although different Saccharomyces strains may be present in musts during the course of a fermentation or in musts of varying composition (e.g., high sugar, low pH, etc.), these strains are all S. cerevisiae. It is interesting to note that
S. cerevisiae is especially well-adapted to the wine environment and appears to have been the predominant wine strain for thousands of years (Box 10-5).
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