Milk, under normal conditions, is always contaminated with bacteria coming from the most varied sources. If it is produced under clean conditions, the number of bacteria will be small, but in any case, the number of kinds of bacteria that find their way into milk will be large. Many of them find in milk at ordinary temperatures suitable conditions for growth; they use a portion of some of the constituents of the milk as food, producing certain other compounds that are known as "by-products." These by-products impart to milk a taste and odor that is not found in fresh milk. The effect of the action of bacteria may also be made evident by the change in the appearance of the milk. When these various changes become evident to the senses, either by taste, smell or sight, the milk usually is so modified as to be unfit for many ordinary purposes. The preservation of milk, a subject to be treated later, is a study of the ways of preventing or retarding the growth of bacteria in milk, and thus delaying the time when evidences of their action first become apparent. Each class of bacteria produces more or less specific changes in the milk as a result of their growth. Certain bacteria are of the greatest benefit to the butter and cheese maker, while others are distinctly harmful to the manufacturer of dairy products. The changes produced by the different bacteria are called "fermentations" of milk, each being most commonly named from the most important by-product formed. =Acid fermentation of milk.= Fresh milk has a sweet taste and little or no odor, but if it is allowed to stand at ordinary temperatures, it sours; the taste is no longer sweet because the sweetness of the sugar of the milk is masked by the acid produced from the decomposition of a portion of the sugar by the bacteria. The change in odor and taste of milk is apparent long before the appearance is altered and increases in intensity as the acid-fermentation progresses. The first alteration in appearance is most usually one of consistency; the liquid milk is transformed into a semi-solid mass. The terms "curdling" and "sour" are usually synonymous. Milk is, however, often said to be sour as soon as the acid fermentation has progressed to a point where it is evident to taste or smell. This process of souring, or the acid fermentation is so common a change that raw milk which does not show this type of fermentation is looked upon with suspicion, and, usually, justly so. The process in the past was thought to be something inherent in the milk, a natural and inevitable change. It is now known that this is not so, but that it is due to certain kinds of bacteria, and that if these are prevented from getting into milk, it will not sour, but will undergo some other less desirable type of decomposition.