In the making of butter it is necessary to concentrate the milk fat into a small volume. This process, known as creaming, may be accomplished by gravity, if the milk is allowed to stand undisturbed, the fat globules rising slowly to the surface. Much more rapid separation may be secured, by placing the milk in a rapidly revolving container in which it is subjected to centrifugal force, which causes the heavier parts of the milk to pass to the outside of the bowl, while the lighter part, the fat, collects at the center of the revolving bowl. There is an enormous number of fat globules in milk, over 5,000,000,000 in each cubic centimeter, and as these move through the milk serum, they carry with them many of the bacteria. The cream is thus much richer in bacteria than is the skim milk, or even the milk before separation. Besides the mechanical separation in the manner described, the method of creaming is of importance, in determining not only the number but also the kind of bacteria in the cream. =Methods of creaming.= In the shallow-pan method of creaming, the milk is kept at ordinary room temperatures. These temperatures favor especially the growth of the acid-forming bacteria. The milk is usually sour by the time the cream is removed from it; consequently, the bacterial content of the cream is high. Moreover, the cream is exposed to air contamination, and is thus seeded with molds, and those forms of bacteria that are always found in the air. The cream obtained in this manner is likely to contain not only numerous bacteria, but a great variety of forms, some of which undoubtedly are the cause of the poor keeping qualities of butter made from such cream. In the more modern method of gravity creaming, in which the milk is placed in deep narrow cans kept in cold water, the conditions are not favorable for the growth of acid-forming bacteria. If the milk is produced under clean conditions, and is placed in cold water at once, the bacterial content of the cream will be low, and it will be less likely to contain undesirable forms than the cream which is obtained from the shallow pans. In separator cream the bacteria will be represented by the kinds present in the milk at time of separation. If this milk is quite old, the cream will contain large numbers of bacteria; if, however, early separation is made and the milk is clean, the bacterial content of the cream will be low.