CHAPTER VII. 



BACTERIA AND BUTTER MAKING. 



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 separa- 

 tion. 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 tempera- 

 tures. 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 



