This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
A leavening agent is a substance that forms bubbles of gas in a batter or dough, thus leavening it or making it porous. During the baking the expansion of the gas produced stretches the batter; then the heat coagulation of the protein of the flour or of the flour and eggs sets the dough structure.
Yeast produces carbon dioxide gas in doughs, but with yeast a period of time is necessary for enough gas to be produced to leaven the dough sufficiently before the product can be baked.
Steam. One of the simplest ways of leavening is by steam. One volume of water forms more than 1600 volumes of water vapor, so that steam may have considerable leavening ability. Steam is most effective as a leavening agent in thin batters in which the proportion of water is large, such as popovers and cream puffs. Medium batters, of which griddle cakes and muffins are examples, are partially leavened by steam and partially by the gas produced from the added baking powder. Steam does not aid to such a great extent in leavening the batters and doughs containing little water both because of the small proportion of water used and because of the large amount held by the starch and protein.
Soda. Sodium bicarbonate or soda may be used for a leavening agent. When heated it gives off gas as follows:
The residue, sodium carbonate, gives a disagreeable taste and produces a yellow color in the baked product. For this reason some substance is used with soda that combines with it to give an end product that does not affect the flavor or color of the food so decidedly as does sodium carbonate.
Hydrochloric acid. The following reaction is typical of baking powder reactions in that an acid substance is combined with the soda to produce the gas. According to Hart, the first patent for a baking powder, taken by Dr. Whiting in 1837 in England, was for the formula given below. In the baking powders of today an acid salt, in powder form, replaces the acid of this formula.
Baking ammonia. Baking ammonia has been used for years in baked products and is still used in many bakeries for some goods. Baking ammonia is ammonium bicarbonate; when heated it breaks up to form ammonia, water, and carbon dioxide. Since two gases are produced it is a very efficient leavening agent. The chief objection to its use is that, unless all the ammonium bicarbonate is broken up and all the ammonia gas driven off, the taste and smell of ammonia remain in the baked product. For this reason it is used in baked products like cookies and cream puffs, that are small and thin or have thin walls.
The author was interested in looking over several hundred cooky recipes sent in from all over the United States for a magazine cooking contest in 1927. The surprising thing about them was the length of time some of the recipes must have been used by, or handed down to, a family, for great numbers of them called for leavening agents in common use years ago. There were many that called for baking ammonia, frequently with no definite proportions, for the recipe would state to use three or five cents worth of ammonia.
Baking ammonia is in rather coarse crystals as it is obtained at the drug store, and therefore produces better results if dissolved in the liquid before it is added to the dough.
Sour milk. The use of sour milk and soda to produce carbon dioxide gas in baked products antedates the use of baking powder. As the acidity of the milk varies with its age and degree of sourness, the amount of soda needed to neutralize the acidity will vary. Some recipes call for more soda than is needed to neutralize the acid of the milk, thus an excess is left that forms sodium carbonate with its resulting bad after-taste and yellow color. This is probably the reason that many present-day housekeepers do not like to use it. Then some cooks do not learn to vary the proportion of soda to use with the milk.
Slightly sour or "blinky" milk will combine with very little soda, but very sour milk will require a larger quantity. An average amount for ordinary sour milk is 1/2 teaspoon of soda per cup of sour milk. This produces gas equivalent to the amount produced by about 2 teaspoons of baking powder. As this does not always produce as porous a texture as is desired, some baking powder may also be added. It is preferable from the color and the flavor obtained in baked and steamed batters to use too little soda to neutralize the acid in the milk, rather than an excess. In fact, sour milk can be used in muffins, biscuits, and cakes with baking powder and no soda, with far less perceptible taste than when excess soda is used.
Many recipes state in the directions to dissolve the soda in a small amount of water. This seems to hold over from the days when soda like the baking ammonia was coarse and not finely powdered, and the solution in water prevented the formation of brown spots. This is a good method if the soda is added directly to the batter, for it tends to lump in the package. But finely powdered soda such as is used at the present time may be sifted with the flour, though it is true that different brands may differ in fineness of division.
Some recipes state to mix the soda with the sour milk or molasses before combining with the other ingredients. This is not a good practise since gas begins to evolve as soon as the two are mixed and thus a large portion of the gas is useless as a leavening agent.
Molasses. Daniels and Heisig have reported the acidity of molasses, honey, and corn sirup. Cooking molasses and sorghum, like sour milk, vary in acidity, so that the amount of soda required to neutralize the acid varies. Some require 1/2 teaspoon of soda per cup, others 3/4, and some 1 teaspoon. An average amount is about 3/4 of a teaspoon per cup of molasses. However, the strong flavor of the molasses may mask the flavor caused by the excess soda to a certain extent, so that one is often safe in using the amount required for the molasses containing the highest proportion of acid, i.e., 1 teaspoon per cup.