Under the conditions in which milk is drawn, it is practically impossible to secure the same without bacterial contamination. The result of the introduction of these organisms often changes its character materially as most bacteria cause the production of more or less pronounced fermentative processes. Under normal conditions, milk sours, i. e., develops lactic acid, but at times this more common fermentation may be replaced by other changes which are marked by the production of some other more or less undesirable flavor, odor or change in appearance. In referring to these changes, it is usually customary to designate them after the most prominent by-product formed, but it must be kept in mind that generally some other decomposition products are usually produced. Whether the organisms producing this or that series of changes prevail or not depends upon the initial seeding, and the conditions under which the milk is kept. Ordinarily, the lactic acid organisms grow so luxuriantly in the milk that they overpower all competitors and so determine the nature of the fermentation; but occasionally the milk becomes infected with other types of bacteria in relatively large numbers and the conditions may be especially suitable to the development of these forms, thereby modifying the course of the normal changes that occur. The kinds of bacteria that find it possible to develop in milk may be included under two heads: 1. Those which cause no appreciable change in the milk, either in taste, odor or appearance. While these are frequently designated as the inert bacteria, it must not be supposed that they have absolutely no effect on milk. It is probably true in most cases that slight changes of a chemical nature are produced, but the nature of the changes do not permit of ready recognition. 2. This class embraces all those organisms which, as a result of their growth, are capable of producing evident changes. These transformations may be such as to affect the taste, as in the sour milk or in the bitter fermentations, or the odor, as in some of the fetid changes, or the appearance of the milk, as in the slimy and color changes later described. ~Souring of milk.~ Ordinarily if milk is allowed to stand for several days at ordinary temperatures it turns sour. This is due to the formation of lactic acid, which is produced by the decomposition of the milk-sugar. While this change is well nigh universal, it does not occur without a pre-existing cause, and that is the presence of certain living bacterial forms. These organisms develop in milk with great rapidity, and the decomposition changes that are noted in souring are due to the by-products of their development. The milk-sugar undergoes fermentation, the chief product being lactic acid, although various other by-products, as other organic acids (acetic, formic and succinic), different alcohols and gaseous products, as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.