Fig. 2.—Effect of different fermentations of milk: U, Curd pitted with gas holes; G and O, gassy curds which float; K, smooth, solid desirable curd.
22. Acid fermentation of milk.—By far the most common and important fermentation taking place in milk is due to the action of the lactic acid-forming bacteria on the milk-sugar or lactose. The bacteria that bring about this fermentation may be divided into several groups on the basis of their morphology, proteolytic activity, gas production, temperature adaptation and production of substances other than lactic acid. The larger number of organisms producing lactic acid in milk also produce other organic acids in greater or less abundance. Inasmuch as lactic acid is the principal substance produced, they are called lactic acid organisms. This group contains different kinds of organisms which may be subdivided into small groups as follows:
| (a) | Bacterium lactis-acidi group |
| (b) | Bacterium colon-aërogenes group. |
| (c) | Acid peptonizing group. |
| (d) | Bacillus bulgaricus group. |
| (e) | Acid cocci or weak acid-producing group. |
23. Bacterium lactis-acidi group.—There are many strains or varieties in this group which are closely related in their activities. They are universally present in milk and are commonly the greatest causal agent in its souring. They are widely distributed in nature. At a temperature of 65° F. to 95° F., these bacteria grow and multiply very rapidly; at 70° F. (approximately 20° C.) these forms usually outgrow all others. The total amount of acid produced in milk by these organisms varies from 0.6 of one per cent to 1 per cent acid calculated as pure lactic acid. These forms coagulate milk to a smooth curd of uniform consistency. In addition to the lactic acid, there are produced traces of acetic, succinic, formic and proprionic acids, traces of certain alcohols, aldehydes and esters. Substances other than lactic acid are not produced by organisms of this group to such an extent as to impart undesirable flavors to the milk. The action of this group on the milk proteins is very slight. They produce no visible sign of peptonization. The B. lactis-acidi group of organisms are essential to the production of the initial acidity necessary in most types of cheese. The practical culture and utilization of them for this purpose under factory conditions are discussed in Chapter IV, entitled "Lactic Starters."
24. Colon-aërogenes group.—This group takes its name from a typical species, Bacterium coli communis, which is a normal inhabitant of the intestines of man and animals, and from Bacterium coli aerogenes, which is similar in many respects to B. coli communis. The initial presence of these bacteria in milk is indicative of fecal contamination or unclean conditions of production. These organisms, however, grow and develop in milk very rapidly at high temperatures of handling. The total acidity produced by these forms is less than that by the Bacterium lactis-acidi group. Of the acid produced, less than 30 per cent is lactic acid; the other acids are formic, acetic, proprionic and succinic. The large percentage of these acids, with comparatively large amounts of certain alcohols, aldehydes and esters, invariably impart undesirable flavors and odors to the milk. Members of this group uniformly ferment the lactose with the production of the gases, carbon dioxide and hydrogen. The milk is coagulated into a lumpy curd, containing gas pockets.
25. Acid peptonizing group.—These are often associated with colon organisms. The group includes those bacteria which coagulate milk with an acid curd and subsequently partly digest it. They grow and multiply rapidly at a temperature between 65° and 98° F. They impart undesirable flavors and odors to the milk, which appear to be due to the formation of acids other than lactic acid, and to action on the milk proteins.
26. Bacillus bulgaricus group.—These organisms grow best at a temperature of 105° to 115° F. They will develop at lower temperatures, but not so rapidly. They survive heating to 135° F. without loss of vigor, as occurs in Swiss cheese-making. They produce from 1 to 4 per cent of acid in milk, which is practically all lactic acid. They do not produce gas. They impart no undesirable flavors to the milk.
27. Acid cocci or weak acid-producers.—This group of organisms is not very well defined. It consists mostly of coccus forms, commonly found in the air and in the udder. Their presence in the milk may indicate direct udder contamination. These are regarded as of little importance, unless in very large number, and they have been only partially studied. They produce little or no lactic acid, and small amounts of acetic, proprionic, butyric and caproic acids. These forms rarely create enough acid to coagulate milk.