Fig. 65.—A cream ripener. In this apparatus cream is “ripened,” i.e., undergoes lactic acid fermentation, preparatory to making it into butter.
In the formation of ensilage ([Fig. 66]) the lactic acid bacteria play a very important part, as they do also in “sour mash” distilling, and in many kinds of natural “pickling.” In fact, whenever green vegetable material “sours” spontaneously, lactic acid bacteria are always present and account for a large part of the acid. This property of lactic acid formation is also taken advantage of in the preparation of lactic acid on a commercial scale in at least one plant in this country.
Fig. 66.—Filling a silo on the University farm.
Acetic acid is another common product of acid fermentation. However, in vinegar making the acetic acid is not formed directly from the sugar in the fruit juice by bacteria. The sugar is first converted into alcohol by yeasts, then the alcohol is oxidized to acid by the bacteria ([Fig. 67]). The reaction may be represented as follows:
Dextrose.
C6H12O6 = Ethyl alcohol.
2C2H5OH + 2CO2
C2H5OH + O2 + (bacteria) = Acetic acid.
CH3COOH + H2O.
Butyric acid is generally produced where fermentation of carbohydrates occurs under anaërobic conditions. Some of the “strong” odor of certain kinds of cheese is due to this acid which is formed partly from the milk-sugar remaining in the cheese. Most of it under these conditions comes from the proteins of the cheese and especially from the fat (see [page 101]).
As has been indicated alcohol is a common accompaniment of most acid fermentations, as are the esters of acids other than the chief product. Bacteria are not used in a commercial way to produce alcohol, however, as the yield is too small. There are some few bacteria in which the amount of alcohol is prominent enough to call the process an “alcoholic fermentation” rather than an acid one. In brewing and distilling industries, yeasts are used to make the alcohol, though molds replace them in some countries (“sake” and “arrak” from rice).
