Preservation of Milk. 119 



resistant. If, however, the growing organisms are de- 

 stroyed, the milk will keep much longer than if it had 

 not been so treated. 



The process of pasteurization was first used by the 

 French bacteriologist, Pasteur, for the treatment of the 

 wines of his native district which were likely to undergo 

 undesirable types of fermentations due to bacteria. 

 From the wine industry it was applied in the brewing 

 industry, and was later found to be of the greatest serv- 

 ice in the dairy industry. The process of pasteurization 

 may be briefly defined, as the heating of milk to tempera- 

 tures, varying from 140 F. and upward for a longer or 

 shorter time, and subsequently cooling to a low temper- 

 ature, so as to prevent the germination of the spores that 

 are not destroyed by the heating. 



Effect of heat on milk. When milk is heated it un- 

 dergoes more or less profound changes, depending on 

 the temperature and time of heating. Some of these 

 changes are of practical importance, since they are more 

 or less evident, and objectionable to the consumer. 



In raw milk the fat globules are largely found in 

 larger or smaller aggregates, rather than uniformly dis- 

 tributed throughout the serum. The surface of a mass 

 of fat globules is smaller in proportion to the volume of 

 the mass than is the case with single globules, hence 

 globule clusters encounter less resistance in their passage 

 through the serum, either as they rise to the surface in 

 gravity creaming, or in the separator bowl. If these 

 clusters are broken up, so that the globules ere uniformly 

 distributed, the milk will cream much less rapidly and 

 completely. In the process known as ' ' homogenization " 

 of milk, the individual fat globules are broken into such 

 small globules, that they cannot overcome the viscosity 



