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. [Illustration: 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. [Illustration: 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. Ethyl alcohol. Acetic acid. C₆H₁₂O₆ = 2C₂H₅OH + 2CO₂ C₂H₅OH + O₂ + (bacteria) = 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). [Illustration: FIG. 67.--A vinegar ripener. The tank shown opened at the side is filled with a special type of beech shavings which thus provide a very large surface. The apple juice which has been previously fermented with yeast, which converts the sugar into alcohol, is allowed to trickle through the openings at the top over the shavings. The acetic acid bacteria on the shavings rapidly oxidize the alcohol to acetic acid. The vinegar is drawn off below.]