3. Sources of yeast:
Yeast was first found as wild yeast in the air, but now it may be obtained at grocery stores, in three forms:
(1) Liquid yeast.—The plants are put into a starchy liquid. This will keep only a few days, as the starch sours.
(2) Dry yeast.—The plants are put into a starchy paste and the mixture is dried. This form will keep for months, because it is perfectly dry but, for the same reason, it takes the plants a long time to become active when used.
(3) Compressed yeast.—The plants are put into cakes of a starchy mixture and left moist. They will keep only a few days. Good compressed yeast is a pale fawn colour, smells sweet, breaks clean, and crumbles easily.
4. Experiments with yeast:
Make a yeast garden by using the plants obtained at the grocery store as follows:
Take half a cup of lukewarm water to give the plants moisture, a teaspoonful of sugar for immediate food, and the same of wheat starch (flour) for a reserve food. Beat the mixture to infold oxygen, and then put in one-quarter cake of yeast plants.
Divide the mixture among a number of test-tubes, so that each group of four pupils has three.