Yeast, as it is used in bread-making, varies in form. It may be liquid, compressed, or dry. The form is not important, save as this affects the purity or keeping quality of the yeast. Though the gas produced by the development of yeast is not the only significant effect of its growth, it is this that makes yeast a rising-agent and valuable for leavening mixtures. Yeast must therefore be so prepared and kept as to prevent the formation and escape of this gas before the yeast is introduced into the mixture to be raised by it. Bread made light by forcing carbon dioxid gas directly into it lacks the flavor of yeast-bread.

Yeast cells greatly magnified (After Conn and Buddington).

Hop.

Yeast is a natural leaven. It leaves practically no residue.

When yeast is home-made, it is prepared by cooking potatoes in water in which a few hops have been boiled. Some sugar and flour are added, and the mixture fermented by a little yeast called the starter. Home-made yeast may contain many bacteria and wild yeasts that do not produce essentially advantageous changes in food.

The yeast of commerce is a by-product of distilleries or breweries. The usual form is that of compressed yeast. This is wrapped in tin foil and should be kept in a cool place. It decomposes easily and produces therefore unfavorable changes when not fresh. Dry yeast is the same yeast-product mixed with starch or meal and dried. Yeast when dried thus is made inactive for a while. It therefore acts less promptly in a mixture than does compressed yeast but keeps indefinitely.