Wine making WORLD OF MICROBIOLOGY AND IMMUNOLOGY600•
mature local vineyards, especially those established in North
America, rely on yeast strains that are injected into the crushed
grape suspension. The growth of the yeast will then occur in
the nutrient-rich mixture of the suspension.
The fermentation process begins when the yeast is
added to the juice that is obtained following the crushing of
the grapes. This process can be stunted or halted by the poor
growth of the yeast. This can occur if conditions such as tem-
perature and light are not favorable. Also, contaminating
microorganisms can outgrow the yeast and out compete the
yeast cells for the nutrients. Selective growth of Sacchromyces
cerevisiaecan be encouraged by maintaining a temperature of
between 158 and 167°F (70 and 75°C). The bacteriathat are
prone to develop in the fermenting suspension do not tolerate
such an elevated temperature. Yeast other than Sacchromyces
cerevisiaeare not as tolerant of the presence of sulfur dioxide.Thus the addition of compounds containing sulfur dioxide to
fermenting wine is a common practice.
The explosion in popularity of home-based wine mak-
ing has streamlined the production process. Home vintners can
purchase so-called starter yeast, which is essentially a powder
consisting of a form of the yeast that is dormant. Upon the
addition of the yeast powder to a solution of grape essence and
sugar, resuscitation of the yeast occurs, growth resumes, and
fermentation starts. In another modification to this process, the
yeast starter can be added to a liquid growth source for a few
days. Then this new cultureof yeast can be used to inoculate
the grape essence and sugar solution. The advantage of the
second approach is that the amount of yeast, which is added,
can be better controlled, and the addition of liquid culture
encourages a more efficient dispersion of the yeast cells
throughout the grape solution.Barrels used to age wine in the wine making process.womi_W 5/7/03 10:18 AM Page 600