The products formed in the decomposition of meat and eggs are not only offensive but may also be injurious to the health of the consumer. Milk that has been fermented by the desirable kinds of acid-forming bacteria is not harmful. It is consumed in a variety of forms (buttermilk, cottage cheese) as a common article of food and its use is rapidly increasing. The preparation of the pure culture buttermilks or artificially soured milks that are now so frequently recommended for digestive troubles rests upon an acid fermentation of this type.
Undesirable acid-forming bacteria. Other types of bacteria capable of forming substances that impart to milk an offensive odor and a disagreeable taste not infrequently appear instead of the desirable group. Instead of producing from the sugar of milk large quantities of lactic acid, these types generate other acids, such as acetic and formic, which impart a sharp taste to the milk. Besides the acids the bacteria of this group form gases from the sugar of the milk. Some produce small amounts of gas; others so much that the curd will be spongy and will float on the surface of the whey. The fermentation caused by them is often called a "gassy fermentation" and is dreaded by butter and cheese makers since the gas is indicative of bad flavors that will appear in the product. Gas may also be produced in other types of fermentations to be discussed later.
This class of bacteria enters the milk with the dust, dirt, and manure, in which materials they are especially abundant. No spores are formed; hence they are easily killed by heating the milk. They grow both in the presence and in the absence of free oxygen. High temperatures favor their growth, most rapid development taking place at 100° to 103° F.
Spontaneous fermentation of milk. The normal souring of milk is due to a mixture of these two groups of bacteria. The relative proportions existing between the two in any sample of milk is dependent on a number of factors, most important of which is the degree of cleanliness exercised in the production of the milk. Where careless conditions obtain under which dust and manure particles find their way into milk, it becomes more abundantly seeded with gas-generating bacteria, and consequently, the type of fermentation is undesirable. If, however, the milk is drawn into clean utensils and care is taken to exclude dirt, the pure lactic acid types are able to control the character of the changes produced, and a clean, pleasant tasting liquid results. It will be seen that things are well arranged by nature; one of the most important food products undergoes a type of decomposition that is not offensive and when produced under clean conditions, the sour milk is as healthful a food as is the fresh product. Thus there is every reason for cleanliness in the production of milk, for cleanliness' sake and because clean milk means better products, and greater returns to everyone, producer and dealer.
There are other kinds of acid-forming bacteria in milk but they are of small importance compared with those just discussed. Some of the bacteria derived from the inside of the udder of the cow form acid, but these forms grow very slowly in milk at ordinary temperatures, and have no influence on the keeping quality.
Fig. 21.—Different Types of Curds.
The flask on the left shows the soft curd produced by the bacteria that curdle the milk without the production of acid. The flask on the right shows the gassy curd formed by butyric acid bacteria in heated milk.
Sweet curdling fermentation of milk. Samples of milk are sometimes found that are curdled, but which do not taste sour, or have the normal odor of sour milk. The curd is usually soft and the taste bitter. It is evident that the curdling cannot be due to the same factors as in the normal souring of milk. Such a change is similar to the action of rennet which is used to curdle the milk in cheese making. This ferment will curdle perfectly sweet milk, producing a curd that looks like that formed in the acid fermentation of milk. The cause of these sweet curdling milks, which appear from time to time, is due to the introduction of certain bacteria which have the power of secreting an enzyme resembling that found in rennet. In such cases the milks curdle prematurely especially when warmed. The curd may gradually disappear, for the bacteria also produce another enzyme that digests the curd, and thus renders it soluble. When this advanced phase becomes evident, it is often called the digestive fermentation of milk. This change is produced largely by putrefactive bacteria of various kinds that find their way into milk with dust and dirt. Many of them are spore formers; hence, are not killed when milk is heated, as in pasteurization, while the acid-formers are destroyed. Pasteurized milk is thus likely to undergo the sweet-curdling fermentation, if it is kept for any length of time. Raw milk rarely undergoes this type of decomposition, since the rennet-forming bacteria under ordinary conditions are unable to develop in competition with the acid-forming bacteria.
Butyric acid fermentation of milk. A fermentation that is much less frequently noted than the two previously discussed is known as the butyric fermentation, since butyric acid is the principal by-product. The causal bacteria cannot compete with the ordinary acid-forming bacteria in raw milk; hence it is most frequently noted in pasteurized milk, since the organisms produce spores and are not killed by the heating. Pasteurized milk under the action of the butyric acid bacteria undergoes a gassy fermentation, developing a pronounced acidity and the disagreeable odor of butyric acid, which resembles that of rancid butter. The butyric acid bacteria are anaerobic, and thus can grow in butter and cheese away from the air.