Doughs are made light or porous in the following ways:

(a) By the production (and expansion by heat) of carbon dioxid gas from the baking soda in baking powder or baking soda, combined with some acid substance.

(b) From carbon dioxid gas produced by the growth of yeast - a plant. .(c) From the expansion of entangled air, incorporated in the dough by means of beaten eggs, especially the white, and by the beating batters, and by folding thick doughs, (d) From the expansion of water to steam.

Two level teaspoons of baking powder are equivalent to one-half teaspoon of baking soda combined with one and one-fourth (i. e., slightly rounded) teaspoon of cream of tartar; or one cup of thick sour milk, or one cup of molasses, in place of the cream of tartar.

Two cups of flour made into a soft dough requires two to four level teaspoons of baking powder.

Batters and muffin mixtures require somewhat more baking powder to the flour than soft doughs.

One teaspoon less of baking powder may be used for each egg added.

The yeast plant grows best at 75° to 90° F. It changes sugar into alcohol and carbon dioxid 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 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.