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Re: yeast
- To: Aquatic-Plants at actwin_com
- Subject: Re: yeast
- From: James Folsom <jamesfolsom at home_com>
- Date: Thu, 08 Nov 2001 17:33:34 -0500
- References: <200111082048.fA8Km2219387 at actwin_com>
- User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:0.9.2) Gecko/20010726 Netscape6/6.1
In a fermentation setting they will not reproduce unless they have
sufficient stored energy as fermentation is only a subsistence
lifestyle. That is to say that production of alcohol from carbohydrates
does not provide enough energy for budding. When Oxygen is present they
can furthur oxidize the alcohol and derive far more energy. Most of
which the cells don't need so they store it for use during times when
oxygen is not present.
Brewers yeast is produced in aerobic conditions that promote energy
storage. So that the little yeast that is introduced into the wort will
reproduce rapidly to produce the number of cells needed to make alchol
in a timely fashion. The yeast from the first batch of alchol can be
reused several more times if they use all of it. But in subsequent
fermentations the yeast has no more stored energy and will not
reproduce. Eventually the yeast is completly depleted and won't be
useful for fermentation. This is where we get bread yeast from.
Also in terms of the yeast in the bottom being less active: This may
not be strictly true. If your talking top flocculating yeast then that
probably is the case. But bottom flocculating yeast like to be on the
bottom thus there may be less active yeast toward the top as well.
Also, If your gonna reuse the yeast its best to keep all of it because
of the reduced ability to reproduce. Keeping Only a small portions
should theoretically not work. But having said that: 1% glucose causes
metabolite repression. I.e. even if oxygen is present the yeast won't
engage in aerobic metabolism. So in a glucose solution spent yeast
cannot reproduce. I suspect that in the sucrose solutions that we tend
to use that metabolite repression does not occur, and the oxygen that is
introduced with fresh solution is enough to at least partially allow
some reproduction.