When soda is used with sour milk for leavening purposes, the lactic acid in the milk is so acted upon by the soda as to produce gas. However, these two ingredients–soda and sour milk–do not make an absolutely accurate leavening agent, because the quantity of acid in the sour milk varies according to the fermentation that has taken place. For example, sour milk 48 hours old contains more acid than sour milk that is kept under the same conditions but is only 24 hours old.
The proportion of these ingredients that is usually effective in batters and doughs for hot Breads is 1 level teaspoonful of soda to 1 pint of sour milk. So as to derive the best results in using these chemical leavening agents, it will be well to observe that if they are mixed together in a cup the milk will bubble and may, provided the quantity is sufficient, run over. These bubbles are caused by the gas that is formed when the acid and soda meet, and when they break gas escapes, with the result that some of it is lost. Formerly, it was the custom to mix these leavening substances in this way, and then to add them to the other ingredients. Now, however, in order that all gas produced may be kept in the dough mixture, the soda is sifted in with the dry ingredients and the sour milk is added with the liquid ingredients.
10. A point well worth remembering is that sour milk and soda may be substituted for sweet milk and baking powder in a recipe that calls for these ingredients by using 1 teaspoonful of soda to each pint of sour milk. This information should prove valuable to the housewife, especially if she has accumulated a supply of sour milk that should not be wasted. Occasionally it will be found that baking powder and soda are required in the same recipe, but this occurs only when an insufficient amount of soda to produce the desired result is specified.
Woman’s Institute Library of Cookery, Vol. 1
by Woman’s Institute of Domestic Arts and Sciences (Year 1928)