Wild yeasts, as those of the air came to be called, have been studied, as have also the other organisms found with them, such as bacteria and molds. All do not act alike; even all yeasts do not. The yeast now used in bread-making was found to serve that purpose well. It has since been separated and so used. It is not secured entirely free from other organisms, but when conditions favorable for its growth are provided, the result sought in bread-rising is obtained.

The conditions for growth of the yeast-plant are suitable temperature and food. The yeast-plant multiplies by budding.

Yeast-plant developing during the process of fermentation a, b, c, d, successive stages of cell multiplication. (After Green).

The temperature most favorable for this is between 70° and 900 F. At 131 ° F. and at freezing temperatures yeast-action is destroyed. At other temperatures not between 70° and 900 F. the action may go on slowly, but too slowly for a favorable result in food. Retarded yeast-activity permits other changes to occur through the development of other organisms. These may destroy the value of a food. In bread-rising the temperature needed for yeast-activity may be secured and maintained by keeping the pan of dough in a pan of water comfortable for the hand. (A thermometer should be used whenever possible).

The food of the yeast-plant is present in bread as now made. Sugar enables yeast to act as a leaven. Some starch of flour is converted into sugar in the form yeast uses. As it uses the sugar, the sugar is broken up. One of the products of this action is carbon dioxid gas. The formation and expansion of this as it is heated produce lightness. The process of breaking up the food-substances of the yeast-plant into carbon dioxid gas and alcohol is called alcoholic fermentation.