24. Commercial Yeast.--When yeast plants are deprived of water and food, they cease to multiply. However, under these conditions, they may be kept alive so that when water and food are again provided they will increase in number and carry on their work. Advantage has been taken of these characteristics of yeast, for although at one time the making of yeast was entirely a household process, it has now, like butter, cheese, canned fruit, etc., become a commercial product. The first yeast put on the market was collected from the surface of the contents of brewers' vats, where it floated in large quantities; but as this was an impure, unreliable product composed of various kinds of bacteria, it is no longer used for the purpose of making bread. At present, yeast is carefully grown as a pure yeast culture, or product. It is marketed in such a way that when proper food, such as soft dough, or sponge, and a favorable temperature are provided, the plants will multiply and act on the carbohydrate that they find in the food. In fact, the purpose of the well-known process of "setting" a sponge is to obtain a large number of yeast plants from a few.
Commercial yeast is placed on the market in two forms--_moist_ and _dry_. Each of these yeasts has its advantages, so that the one to select depends on the method preferred for the making of bread as well as the time that may be devoted to the preparation of this food.
25. Moist yeast, which is usually called _compressed yeast_, consists of the pure yeast culture, or growth, mixed with starch to make a sort of dough and then compressed into small cakes, the form in which it is sold. The moist condition of this kind of commercial yeast keeps the plants in an active state and permits of very rapid growth in a dough mixture. Consequently, it proves very useful for the rapid methods of making bread. It is soft, yet brittle, is of a grayish-white color, and has no odor except that of yeast.
Since the plants of compressed yeast require very little moisture to make them grow, an unfavorable, or low, temperature is needed to keep the yeast from spoiling; in fact, it is not guaranteed to remain good longer than a few days, and then only if it is kept at a temperature low enough to prevent the plants from growing. This fact makes it inadvisable to purchase compressed yeast at great distances from the source of supply, although it may be obtained by parcel post from manufacturers or dealers.