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It has been determined that yeast is a microscopic plant existing everywhere in the air and in dust; consequently, it is found on all things that are exposed to air or dust. In order that it may grow, this plant requires the three things necessary for the growth of any plant, namely, food, moisture, and warmth.

Carbohydrate in the form of sugar proves to be an ideal food for yeast, and 70 to 90 degrees Fahrenheit is the temperature at which the most rapid growth occurs. When these conditions exist and a sufficient amount of moisture is provided, yeast grows very rapidly and produces fermentation.

The changes that take place when yeast causes fermentation can be detected very readily by observing the fermenting of fruit juice. As every housewife knows, the first indication of a ferment in fruit juice is the appearance of tiny bubbles, which collect on the sides and the bottom of the vessel containing the fruit and then gradually rise to the top. These bubbles are a form of gas called carbon-dioxide, or carbonic-acid, gas.

If, after they appear, the juice is tasted, it will be found to be slightly alcoholic and to have a somewhat sour or acid taste. The gas, the acid, and the alcohol thus produced are the three results of the action of the ferment.

When yeast is used in the making of bread out of wheat flour, the changes just mentioned take place. To understand the action of this plant, it will be necessary to remember that wheat contains a large proportion of starch. This substance, however, cannot be acted on by the yeast plant; it must first be changed into sugar. The yeast that is added to the flour changes some of the starch into sugar and transforms the sugar into alcohol and carbonic-acid gas.

This gas, which is lighter than the dough, rises, and in its efforts to escape expands the elastic, glutinous dough into a mass of bubbles with thin walls until the dough is two or three times its original bulk. The yeast plants, though, must be well distributed throughout the dough; otherwise, there are likely to be no bubbles in some places and large bubbles with thick walls in others.

The gas thus formed is prevented from escaping by the toughness or the elasticity of the gluten, and the spaces that it leaves are what produce a light, porous loaf. When the expansion has gone on long enough, the formation of gas is checked and the ferment is killed by baking the dough in a hot oven. During the baking, the alcohol is driven off by heat, some of the starch is browned and forms the crust, and so little acid is produced in the short time in which the yeast is active that it is not noticeable.

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.