This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
The most satisfactory temperature for the growth of yeast is from 75° to 95° F. It ceases to grow when the temperature is below 3 0 ° F. and is killed at about 212° F. Yeast should not be softened in very cold water if immediate activity is desired.
Compressed Yeast - A cake of fresh compressed yeast breaks with a clean edge and has no odor of putrefaction. It is creamy yellow and uniform in color. When old, compressed yeast becomes slightly slippery, is streaky, and has an unpleasant odor. Only fresh compressed yeast should be used in bread-making. In compressed yeast the yeast plants are alive and ready for action, hence bread-making with compressed yeast requires less time than with dry yeast.
Dry Yeast - Dry yeast is a mass of yeast plants mixed with corn-meal and dried. As yeast will live for some time and yet can not grow without moisture, these dry cakes will keep for many weeks. The dried plants are inactive and even when warmth and moisture, food and air are supplied, they take some time to become active again.
Liquid, Railroad or Starter Yeast - This consists of potato water, sugar and salt, in which yeast plants are in an active condition. The starter must be stored in a cool temperature to retard the action of the yeast. The disadvantage of liquid yeast lies in the fact that other yeasts than those best suited for bread-making may be thriving there also, and soon bread made from this perpetual yeast may have a characteristic flavor. Starter should be thrown out occasionally and remade with a fresh yeast cake.