Thanks again for another interesting question. The simple answer to
your question is to ensure proper fermentation and prevent spoiling
the wine. The explanation can be found below:
"When yeasts enter a new environment, they exist for a while in what
is known as a lag period. During this period, which can last from
minutes to hours, the yeasts basically test the environment to
determine whether or not it is suitable for colonization. If they
determine that it is, they end the lag period and begin reproducing.
Yeast can reproduce at an alarming rate in a favorable environment.
This rate, at least for a period, is typically one logarithmic unit
per two hours, meaning that 1 yeast cell can theoretically become 8 in
six hours, 64 in 12 hours, 512 in 18 hours, and 4096 in 24 hours.
Luckily, when their population density reaches about 150,000,000 per
milliliter of host liquid, they settle down and maintain a relatively
steady population. This continues, all other conditions remaining
constant, until they deplete all available oxygen, use up all
available nutrients, or the alcohol they produce becomes intolerably
concentrated. Most then die off, but if both OXYGEN and nutrients are
still available then BRETTANOMYCES, DEKKERA AND ZYGOSACCHAROMYCES
BAILII, if present, may now decide to end their lag period and begin
multiplying, ruining what would otherwise be an almost finished wine."
(Exerpted from The Winemaking Home Page by Jack B. Keller, Jr. - The
Miracle Of Yeast, Copyright © 2003
"A stuck fermentation is difficult to restart and dangerous because
once the yeast stop growing, the protective layer of carbon dioxide is
removed. The door is open for spoilage yeast and bacteria
(BRETTANOMYCES, LACTOBACILLUS AND ACETOBACTOR) to consume the
remaining sugar in turn producing vinegar, earthy, barnyard or
(Exerpted from Enology International - Going Wild: Wild Yeast In
Winemaking by Jordan P. Ross, Copyright © 2002
The air we breathe carries oxygen and other microbes that could ruin
the fermentation process in winemaking, which could result in
unfinished or bad wine. Even worse, one could end up making vinegar
instead of wine. By minimizing the amount of air present during
fermentation, the risk of spoilage is reduced, if not virtually
How Winemaking Works
You can find more references through the last link below.
Keyword searches at HowStuffWorks.com
yeast winemaking - in Google Web search
I hope this helps you. Should you have any comments/questions, please
feel free to post your clarification before rating this and I'll
attend to you as soon as possible. Thanks for asking.