Doughs are made light or porous in the following ways:

(a) By the production (and expansion by heat) of carbon dioxide gas from the combination of baking soda with some acid substance.

(b) From carbon dioxide gas produced by the growth of yeast - a microscopic plant.

(c) From the expansion of entangled air, incorporated in the dough by means of (1) beating batters; (2) folding thick doughs; (3) beating air into eggs, especially the whites - then folding them into the mixture. (d) From the expansion of water to steam.

Proportions

Use two level teaspoons baking powder to one cup of flour.

Use one teaspoon less of baking powder for each egg added after two have been used.

Use one-half teaspoon soda with one and a quarter teaspoon cream of tartar.

Use one-half teaspoon soda with one cup sour milk.

Use one-half teaspoon soda with one-half cup molasses.

Two teaspoons baking powder are equivalent to one-half teaspoon soda in the above combinations.

The yeast plant grows best at 75 to 90 degrees F. It changes sugar into alcohol and carbon dioxide gas. Flour contains a small proportion of sugar and during bread making some of the starch is changed into sugar, but the yeast begins to act more quickly if a little sugar or glucose is added at first. Salt and fats in quantity hinder the growth of the yeast. Low temperatures stop the growth almost completely; high temperatures kill the plant.

When eggs are used as leavening agents, the whites are beaten separately, as they will hold much more air than the yolks, and folded into the mixture the last thing, breaking as few air cells as possible.

When air is depended on for leavening agent, all materials are kept as cold as possible. Cold air expands more on heating than warm air. In pastry making, heat also melts the fat, so that the dough cannot be handled.