In milk, certain groups of bacteria are commonly present, but many others which happen to get into it live and multiply rapidly. A favorable temperature is very necessary for such organisms to multiply. There is a range of temperature, more or less wide, at which each group of bacteria grows and multiplies with the greatest rapidity. This range varies with the different groups, but most of them find temperatures between 75° F. and 95° F. the most favorable for growth. Excessive heat kills the bacteria. Low temperatures stop growth, but kill few if any bacteria. Temperatures of 50° F. and lower retard the growth of most forms of bacteria found commonly in milk. Many forms will slowly develop, however, below 50° and some growth will occur down to the freezing point. Milk held at 50° F. or lower will remain in good condition long enough to be handled without injury to quality until received in the cheese factory. In the place of seeds, some groups of bacteria form spores. The spores are exceedingly resistant to unfavorable conditions of growth, such as heat, cold, drying, food supply and even chemical agents. This property makes it difficult to destroy such bacteria. +21. Groups of bacteria in milk.+--Milk when first drawn usually shows an amphoteric reaction; that is, it will give the acid and alkaline reactions with litmus paper. Under normal conditions, milk soon begins to undergo changes, due to the bacteria. Changes produced in this way are called "fermentations"; the agents causing them, "ferments." Normally the acid fermentation takes place first, and later other fermentations or changes begin, which, after a time, so decompose the milk that it will not be suitable for cheese-making or human consumption. The following grouping of the organisms in milk is based on their effects on the milk itself[11]: I. Acid-producing types. II. Peptonizing types. III. Inert types. IV. Alkali-producing types. V. Butyric fermenting types. Each type of bacteria produces more or less specific changes in the milk. As a general rule, the predominance of one of these types is an aid in the interpretation of the quality of the product at the time of analysis, such as the age, the temperature at which it has been held, the conditions under which it was produced and, in some cases, the general source of the contamination. The reaction due to certain bacteria is utilized in the manufacture and handling of dairy products; other groups have deleterious effects. (See Fig. 2.) [Illustration: 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.]