( Originally Published 1921 )
Yeasts belong to the fungi, one of the lowest order of non-flowering plants, without leaves or stems. Like all other plants, they require warmth, moisture, and food in order to grow ; and when properly supplied with these, they multiply rapidly.
Pasteur found, by experimentation, that when yeast from fresh grape juice was watched under the microscope,' "two cellules had furnished eight, including the two mother cells, in the course of two hours." Fermentation proceeds slowly at a temperature 50° F.; but from seventy to ninety degrees, it grows rapidly. Fermentation may be arrested by the exhaustion of either the fermenting agent (yeast) or the food supply (starch or sugar), or by exposure to heat at the temperature of boiling water. When not well nourished, the yeast cells begin to break up and die, and finally decompose with an offensive odor.
Yeast converts sugar into alcohol, carbon dioxide, and other products of fermentation; and it is believed that leavened bread owes its flavor largely to these organic substances. Dried yeast cakes are made by mixing about 20% starch with the yeast for the purpose of keeping it. Pure yeast (washed and pressed into cakes, as compressed yeast) will keep for weeks in a cold place, such as an ice box; but it will spoil in a few days at best if not kept cold. In the making of all the homemade liquid yeasts, essentially the same principles are involved,— the introduction of a dried yeast cake, or a small quantity of lively yeast, into a mixture of some kind of starch, such as potato, or flour, or both. Under proper conditions of warmth, the small amount of yeast begins to supply itself with food by converting the starch into dextrin, and multiplies itself with great rapidity, and will continue to do so as long as there is material to supply it with the means of growth.
While the growth of yeast under normal conditions is rapid, its decay is equally so; and unless preserved by some means, the yeast plants will die, and the mixture become sour. If not to be used immediately, yeast should be placed in some receptacle as nearly air-tight as possible, and set in a cool cellar or refrigerator, where it can be kept at a temperature not conducive to fermentation. Thus kept, the little yeast plants will remain dormant until again surrounded by favorable conditions for growth.
The starch of potato seems to furnish better material for the growth of yeast than that of flour. The potatoes should be perfectly mature when used for this purpose; new ones will positively not answer the purpose. Sugar helps to nourish the yeast plant, and a small amount is usually employed in making yeast.
The most convenient yeast is that sold as compressed yeast. It should be used only when' fresh, its freshness being determined by its light color and the absence of dark streaks. When compressed yeast is unobtainable, very satisfactory results follow the use of liquid yeast.
I cake dried yeast 2 cups potato water cup (4 level tablespoons) sugar
Drain the water from boiled potatoes at noon, arid when it is cooled to about too°, add the. sugar and the yeast cake broken up. Put in a glass jar and set in a warm place until the evening. The liquid should measure 2 cups, and should be covered with a thick foam before it is used for bread. Salt and shortening retard the action of yeast, hence are omitted in setting a sponge, and are added in mixing the dough. Use 4 measures of water to 1 measure of the above liquid yeast when set at night, and 2 measures of water to I of yeast if set during the day.