One of the very few practical uses of the gaseous fermentation of carbohydrates is in making “salt rising” bread. The “rising” of the material is due not to yeasts but to the formation of gas by certain bacteria which are present on the corn meal or flour used in the process ([Fig. 63]).
Fig. 63.—A loaf of “salt rising” bread. The porous structure is due to the gas formed by bacilli and not by yeasts.
Another is in the formation of the “holes” or “eyes” so characteristic of Swiss and other types of cheese ([Fig. 64]).
Fig. 64.—Ohio Swiss cheese. The “eyes” are due to gas formed by bacteria during the ripening of the cheese.
A great many organic acids are formed during the “acid fermentation” of carbohydrates by bacteria. Each kind of bacterium, as a rule, forms several different acids as well as other substances, though usually one is produced in much larger amounts, and the kind of fermentation is named from this acid. One of the commonest of these acids is lactic. The “lactic acid bacteria” form a very large and important group and are indispensable in many commercial processes. In the making of butter the cream is first “ripened,” as is the milk from which many kinds of cheese are made ([Fig. 65]). The chief feature of this “ripening” is the formation of lactic acid from the milk-sugar by the action of bacteria. A similar change occurs in the popular “Bulgarian fermented milk.” The reaction is usually represented by the equation:
Milk-sugar.
C12H22O11 + H2O + (bacteria) = Lactic acid.
4C3H6O3
It is not probable that the change occurs quantitatively as indicated, because a number of other substances are also formed. Some of these are acetic and succinic acids and alcohol. Another industrial use of this acid fermentation is in the preparation of “sauer kraut.” These bacteria are chiefly anaërobic and grow best in a relatively high salt concentration. They occur naturally on the cabbage leaves.
