I liked this and thought I'd post it here.

Yeast - from http://KingArthur.com.

Yeast is a living plant, and as such requires suitable conditions to
thrive. These conditions include moisture, oxygen, food, and appropriate
temperatures.

Under suitable conditions, the yeast will reproduce, and it will generate
fermentation. Fermentation is the conversion of sugars into alcohol and
carbon dioxide, by yeast and bacteria. When food, water, oxygen, and a
proper temperature occur, the life cycle of yeast will become activated.

In Nature, there are dozens of Genus of yeast, hundreds of species, and
thousands of subspecies or strains. Saccharomyces cerivisiae is the
strain that has been chosen for commercial yeast, because it has
characteristics that favor rapid gas production. Commercial yeast is
available in a number of forms, from cream yeast (a liquid form of
compressed yeast, it is usually delivered in tank trucks to storage bins,
and is used in very large operations), to compressed yeast (also called
cake yeast or fresh yeast), and finally dry yeast.

As we have noted, yeast requires moisture, oxygen, a suitable
temperature, and food in order to multiply and generate fermentation.
Generally speaking, bread dough is an ideal environment for the yeast,
providing all the necessary conditions for its needs.

• Water is needed by the yeast in order for it to absorb nutrients. It is
well known by bakers that salt retards the activity of yeast
fermentation. This is due to osmotic pressure exerted on the yeast cells
by the salt. Salt, being hygroscopic (attracting moisture), draws water
out of the yeast cell, reducing the amount available to the yeast, and
this is why there is a decrease in fermentation from the presence of
salt. Sugar acts the same way. It too is hygroscopic, and once a dough
contains more than 9% sugar, a decrease is noted in the rate of
fermentation.

• Oxygen, obtained mostly by the mixing of the dough, enables the yeast
to metabolize nutrients and to multiply. Although yeast requires oxygen
for its reproduction, in reality there is almost no reproduction
occurring in bread dough, and the rise we see is almost entirely due to
gas production during fermentation. Available oxygen is used up within a
matter of minutes after dough mixing, and fermentation occurs in an
anaerobic environment.

• Dough temperature is crucial for yeast activity. For commercial yeast,
the optimum temperature for fermentation is 90°F or even higher. It is
important to note, however, that a dough temperature in this range is
inappropriate; although fermentation would be favored, it would occur at
the expense of flavor development, which requires lower temperatures.
Wild yeasts prefer a narrower temperature zone than commercial yeast, and
in general perform better at slightly lower temperatures than commercial
yeast.

• During fermentation, food is provided to the yeast by the conversion of
starches (by amylase enzymes) into sugar. The yeast ferments the sugar,
and as a result of this fermentation, carbon dioxide gas and alcohol are
produced. The CO2 is trapped by the gluten network in the dough, and
provides volume to the baked loaf. The alcohol is largely evaporated
during the baking of the bread. Another by-product of fermentation is
heat. The forms of yeast most commonly used by the baker are fresh yeast
(also known as cake yeast or compressed yeast), active dry yeast, and
instant dry yeast. When converting from fresh to dry, it is important to
adjust the weight of the yeast. Although it is best to follow the
conversion ratio provided by the manufacturer, there are general
conversion guidelines that may prove helpful.

• To convert from fresh yeast to active dry yeast, multiply the fresh
quantity by .4. Active dry yeast must be hydrated in warm water before
being incorporated into a dough.

• To convert from fresh yeast to instant dry yeast, multiply the fresh
quantity by .33. Instant yeast can be incorporated into the dough without
first rehydrating it; however, it is sensitive to ice or ice-cold
temperatures, and if the water temperature of the dough is cold, it is
best to mix the dough for a minute or two before adding the yeast. In
order to maintain dough yield, most manufacturers suggest making up the
weight difference between dry yeast and fresh with additional water.


AN INTERESTING RELATIONSHIP

There is an interesting relationship in what we call San Francisco
Sourdough between the wild yeast, Candida milleri, and the dominant
lactobacillus strain, Lactobacillus sanfranciscensis. C. milleri cannot
utilize maltose during fermentation, while L. sanfranciscensis is happy
to use it. And once it does, it excretes glucose. This is fortunate for
C. milleri, because it is fond of glucose, and ferments this simple sugar
readily. At the same time, competing bacterial species are inhibited by
the presence of so much glucose, and this is to the benefit of L.
sanfranciscensis, whose development is therefore favored.

A last factor in this relationship pertains to acidity. L.
sanfranciscensis produces a lot of acetic acid, which contributes
significantly to the flavor we associate with sourdough bread. C. milleri
is more tolerant of an acidic environment than many yeast varieties. The
high level of acidity prevents competing yeasts from dominating the
culture, much to the benefit of C. Milleri.