Yeast is a collection of living, one-celled organisms that partake of the nature of plant rather than animal life. These organisms may be produced by cultivation. In a proper environment - with necessary warmth, moisture, and complex food to feed upon - these microscopic fungi bulge a little upon one side. This bulge takes on an oval shape, and soon separates from the parent cell as a distinct organism. Other cells quickly follow from the parent cell and from the new cells or buds; and thus the yeast-plant grows. The little yeast plants or cells are vigorous and tenacious of life, living under most adverse circumstances; but these are killed on exposure to a temperature of about 212° Fahr. They endure cold much better, as life is simply suspended in a temperature of about 30° Fahr. The most favorable temperature for their growth is between 65° and 75° Fahr. A cake of compressed yeast, one of the best forms in which a housekeeper can secure a supply of yeast, is a collection of yeast plants massed together without the presence of suitable conditions for growth. By the exclusion of air and heat, the plants may be kept for some days alive and in vigorous condition for future growth.

In making bread, we soften the cake in liquid to separate the plants, and then stir them into the flour. Salt may be added as a matter of taste. It retards, perhaps, the growth of the plant. The little plants, finding in the starch and gluten of the flour a complex food which they enjoy, begin to feed and grow or bud; and chemical changes take place. Starch is changed to sugar, and sugar to alcohol and carbon (carbonioacid gas). The gas, in its efforts to escape, expands the tenacious elastic cell walls of gluten in which it is entangled, and lifts up the dough. If this dough be subjected to heat (212° Fahr. at the centre), the alcohol and carbon dioxide will be driven off, the cell walls fixed, and sweet bread produced. But, if the dough be left to itself, this change, which is called alcoholic fermentation, will be followed by another change. The alcohol breaks up into acetic acid and water; and, if baked, the resultant bread will be sour.

A good yeast cake is of a light even color. There is an absence of dark streaks through it.