The grains, rye, corn, rice, wheat, from which meal is made, contain only a small quantity of sugar, but, on the other hand, they contain a large quantity of starch which is easily convertible into sugar.  Upon this the tiny yeast plants in the dough feed, and, as in the case of the wines, ferment the sugar, producing carbon dioxide and alcohol.  The dough is thick and sticky and the gas bubbles expand it into a spongy mass.  The tiny yeast plants multiply and continue to make alcohol and gas, and in consequence, the dough becomes lighter and lighter.  When it has risen sufficiently, it is kneaded and placed in an oven; the heat of the oven soon kills the yeast plants and drives the alcohol out of the bread; at the same time it expands the imprisoned gas bubbles and causes them to lighten and swell the bread still more.  Meanwhile, the dough has become stiff enough to support itself.  The result of the fermentation is a light, spongy loaf.

216.  Where does Yeast come From?  The microscopic plants which we call yeast are widely distributed in the air, and float around there until chance brings them in contact with a substance favorable to their growth, such as fruit juices and moist warm batter.  Under the favorable conditions of abundant moisture, heat, and food, they grow and multiply rapidly, and cause the phenomenon of fermentation.  Wild yeast settles on the skin of grapes and apples, but since it does not have access to the fruit juices within, it remains inactive very much as a seed does before it is planted.  But when the fruit is crushed, the yeast plants get into the juice, and feeding on it, grow and multiply.  The stray yeast plants which get into the sirup are relatively few, and hence fermentation is slow; it requires several weeks for currant wine to ferment, and several months for the juice of grapes to be converted into wine.

Stray yeast finds a favorable soil for growth in the warmth and moisture of a batter; but although the number of these stray plants is very large, it is insufficient to cause rapid fermentation, and if we depended upon wild yeast for bread raising, the result would not be to our liking.

When our remote ancestors saved a pinch of dough as leaven for the next baking, they were actually cultivating yeast, although they did not know it.  The reserved portion served as a favorable breeding place to the yeast plants within it; they grew and reproduced amazingly, and became so numerous, that the small mass of old dough in which they were gathered served to leaven the entire batch at the next baking.