Fermentation is the process by which, under influence of air, warmth, moisture, and some ferment, sugar (or dextrose, starch converted into sugar) is changed into alcohol (CsH5HO) and carbon-dioxide (CO.). The product of all fermentation is the same. Three kinds are considered, — alcoholic, acetic, and lactic. Where bread dough is allowed to ferment by addition of yeast, the fermentation is alcoholic; where alcoholic fermentation continues too long, acetic fermentation sets in, which is a continuation of alcoholic. Luetic fermentation is fermentation which takes place when milk sours.
Liquid, dry or compressed yeast, may be used for raising bread. The compressed yeast cakes done up in tin foil have long proved satisfactory, and are now almost universally used, having replaced the homemade liquid yeast. Never use a yeast cake unless perfectly fresh, which may be determined by its light color and absence of dark streaks.
The yeast plant is killed at 212° F.; life is suspended, but not entirely destroyed, at 32° F. The temperature best suited for its growth is from 65° to 68° F. The most favorable conditions for the growth of yeast are a warm, moist, sweet, nitrogenous soil. These must be especially considered in bread making.
The most well-known and commercially significant yeast are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker’s yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins such as niacin, folic acid, riboflavin, and biotin.
The use of steamed or boiled potatoes, water from potato boiling, or sugar in a bread dough provides food for the growth of yeasts, however, too much sugar will dehydrate them. Yeast is inhibited by both salt and sugar, but more so with salt than sugar. Fats such as butter or eggs slow down yeast growth, however others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.