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.

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.

Dry yeast, the other form of commercial yeast, is made in much the same way as moist yeast, but, instead of being mixed with a small amount of starch, the yeast culture is combined with a large quantity of starch or meal and then dried. The process of drying kills off some of the plants and renders the remainder inactive; because of this, the yeast requires no special care and will keep for an indefinite period of time, facts that account for its extensive use by housewives who are not within easy reach of the markets. However, because of the inactivity of the yeast plants, much longer time is required to produce fermentation in a bread mixture containing dry yeast than in one in which moist yeast is used. Consequently, the long processes of bread making are brought about by the use of dry yeast. If moist yeast is used for these processes, a smaller quantity is required.

Liquid Yeast.--Some housewives are so situated that they find it difficult to obtain commercial yeast in either of its forms; but this disadvantage need not deprive them of the means of making good home-made bread, for they can prepare a very satisfactory liquid yeast themselves. To make such yeast, flour, water, and a small quantity of sugar are stirred together, and the mixture is then allowed to remain at ordinary room temperature, or 70 degrees Fahrenheit, until it is filled with bubbles. If hops are available, a few of them may be added. When such yeast is added to a sponge mixture, it will lighten the whole amount. Before the sponge is made stiff with flour, however, a little of it should be taken out, put in a covered dish, and set away in a cool, dark place for the next baking. If properly looked after in the manner explained, this yeast may be kept for about 2 weeks.

More certain results and a better flavor are insured in the use of liquid yeast if it is started with commercial yeast, so that whenever this can be obtained it should be used. Then, as just explained, some of the liquid containing the yeast or some of the sponge made with it may be retained for the next baking.

Quality of Yeast.--Of equal importance with the quality of flour is the quality of yeast used in the baking of bread. Yeast is, of course, accountable for the lightness or sponginess of bread, but, in addition, it improves the flavor of the bread if it is of good quality or detracts from the flavor if it is of poor quality. Since the condition of yeast cannot be determined until its effect on the finished product is noted, the housewife should take no chances, but should employ only yeast, whether she uses commercial or liquid, that she knows to be good and reliable. Compressed yeast may be easily judged as to quality. It should be grayish white in color, without streaks or spots, and it should have no sour nor disagreeable odor. If home-made yeast is used and the results obtained are not satisfactory, it may be taken for granted that a fresh supply should be prepared.

YEAST AIDS

As has already been explained, yeast, in order to grow, requires something on which to feed, and the food that produces the most rapid growth is that which contains carbohydrate. Certain of the carbohydrates, however, prove to be better food and produce more rapid growth than others, and these, which are known as yeast aids, are usually added as ingredients in the making of bread. The ones that are most commonly used are sugar and potato water. Sugar is almost always added, but it should be limited in quantity, because a dough mixture that is made heavy with sugar will rise very slowly. Potato water has been found to be a very satisfactory aid, because the starch of the potato is utilized readily by the yeast. If this aid is to be used, the water in which potatoes are boiled may be saved and, when the ingredients required for the making of bread are mixed, it may be added as a part or all of the liquid required. If it is desired to increase the amount of starch in the potato water, a boiled potato or two may be mashed and added to it.