1. Description of yeast:
Yeast is a one-celled plant which can be seen only with a microscope. Under good conditions it becomes very active and multiplies rapidly by a process called budding. It is used by the housekeeper for the carbonic acid gas it gives off.
Yeast plants magnified
2. Conditions necessary for the activity of yeast:
(1) Oxygen
(2) Water
(3) Food.—This must be sugar, or starch which it will change into sugar. Potato starch is more easily used by yeast than flour starch. It uses also some nitrogenous food and mineral matter.
(4) Heat.—The yeast plant thrives in a heat of about the same temperature as our bodies. A little extra heat will only make it grow faster; but excessive heat will kill it.
Freezing will not kill the plant, though cold makes yeast inactive.