ly take place. Experiments show about 2 per cent of loss
of dry matter under ordinary conditions. These losses are not confined
to the carbohydrates alone, but also extend to the proteids and other
compounds. When 100 pounds of flour containing 10 per cent of water and
90 per cent of dry matter are made into bread, the bread contains about
88 pounds of dry matter. In exceptional cases, where there has been
prolonged fermentation, the losses exceed 2 per cent[64].

[Illustration: FIG. 46.--BREWERS' YEAST.]

174. Action of Yeast.--Yeast is a monocellular plant requiring sugar
and other food materials for its nourishment. Under favorable conditions
it rapidly increases by budding, and as a result produces the well-known
alcoholic fermentation. It requires mineral food, as do plants of a
higher order, and oftentimes the fermentation process is checked for
want of sufficient soluble mineral food. The yeast plant causes a
number of chemical changes to take place, as conversion of starch to a
soluble form and alcoholic fermentation.

C_{6}H_{10}O_{5} + H_{2}O = C_{6}H_{12}O_{6}.

C_{6}H_{12}O_{6} = 2 C_{2}H_{5}OH + 2 CO_{2}.

Alcoholic fermentation cannot occur until the starch has been converted
into dextrose sugar. The yeast plant is destroyed at a temperature of
131 deg. F. It is most active from 70 deg. to 90 deg. F. At a low
temperature it is less active, and when it freezes the cells are ruptured.
A number of different kinds of fermentation are associated with the growth
of the yeast plant, and there are many varieties of yeast, some of which
are more active than others. For bread making an active yeast is desirable
to prevent the formation of acid bodies. If the work proceeds quickly,
the rising process is completed before the acid fermentation is far
advanced. If fermentation is too prolonged, some of the products of the
yeast plant impart an undesirable taste and odor to the bread, and
hinder the development of the gluten and expansion of the loaf.

175. Compressed Yeast.--The yeast most commonly used in bread making
is compressed yeast, a product of distilleries. The yeast floating on
the surface of the wort is skimmed off and that remaining is allowed to
settle to the bottom, and is obtained by running the wort into shallow
tanks or settling trays. It is then washed with cold water, and the
impurities are removed either by sieving through silk or wire sieves,
or, during the washing, by fractional precipitat