ion. The yeast is then
pressed, cut into cakes, and wrapped in tinfoil. When fresh, it is of
uniform creamy color, moist, and of a firm, even texture[18]. It should
be kept cold, as it readily decomposes.

176. Dry Yeast is made by mixing starch or meal with fresh yeast until
a stiff dough is formed. This is then dried, either in the sun or at a
moderate temperature, and cut into cakes. By drying, many of the yeast
cells are rendered temporarily inactive, and so it is a slower acting
leaven than the compressed yeast. A dry yeast will keep indefinitely.

177. Production of Carbon Dioxid Gas and Alcohol.--Carbon dioxid and
alcohol are produced in the largest amounts of any of the compounds
formed during bread making. When the alcoholic ferments secreted by the
yeast plant act upon the invert sugars and produce alcoholic
fermentation, carbon dioxid is one of the products formed. Ordinarily
about 1 per cent of carbon dioxid gas is generated and lost during bread
making. About equal weights of carbon dioxid and alcohol are produced
during the fermentation. In baking, the alcohol is vaporized and aids
the carbon dioxid in expanding the dough and making the bread porous. If
all of the moisture given off during bread making be collected it will
be found that from a pound loaf of bread there are about 40 cubic
centimeters of liquid; when this is submitted to chemical analysis,
small amounts of alcohol are obtained. Alcoholic fermentation sometimes
fails to take place readily, because there are not sufficient soluble
carbohydrates to undergo inversion, or other food for the yeast plant.
Starch cannot be converted directly into alcohol and carbon dioxid gas;
it must first be changed into dextrose sugars, and these undergo
alcoholic fermentation. Bread gives no appreciable reaction for alcohol
even when fresh.[64]

[Illustration: FIG. 47.--WHEAT STARCH GRANULES AFTER
FERMENTATION WITH YEAST, AS IN BREAD MAKING.]

If the gluten is of poor quality, or deficient in either gliadin or
glutenin, the dough mass fails to properly expand because the gas is not
all retained. The amount of gas formed is dependent upon temperature,
rapidity of the ferment action, and quality of the yeast and flour. If
the yeast is inactive, other forms of fermentation than the alcoholic
may take place and, as a result, the dough does not expand. Poor yeast
is a frequent cause of poor bread.

The temperature reached in bread making is not sufficient to des