lOO DAIRY TECHNOLOGY 



makes creaming very difficult, because the small globules 

 are unable to overcome the viscosity of the milk serum. 

 In unheated milk the fat globules are well clustered, and 

 present a relatively small surface as compared with their 

 volume. The creaming of milk is but slightly affected by 

 temperatures up to 150° F., but subjecting it to a tempera- 

 ture of 160° F. for 20 minutes retards it very markedly. 



Pasteurizers. — The pasteurization of milk may be 

 accomplished by heating at a low temperature for a long 

 time or at a high temperature for a short time. One 

 class of machines, known as the intermittent or batch 

 pasteurizer, is so operated that the milk may be heated 

 at any temperature for any desired length of time. These 

 machines are commonly vats or other receptacles in which 

 the milk is heated and agitated mechanically. Cooling 

 is accomplished by passing cold water through the agita- 

 tor or jacket as the case may be. These machines are 

 very efficient, but it can readily be seen that their capacity 

 is limited, and hence they are hardly practicable except 

 in a small business. 



The other class of pasteurizers is known as the con- 

 tinuous or flask machine, because there is a continuous 

 flow of milk through the machine and it is heated instan- 

 taneously. The milk is heated to a high temperature and 

 immediately passed on to the cooler. However, it has 

 been shown by experiment, that all the milk is not heated 

 to the same temperature, and hence some of it will not 

 be thoroughly pasteurized and some of it will be over- 

 pasteurized. 



A test^ of one of the commonly used machines revealed 

 the fact that some of the milk passed through in 15 seconds, 

 most of it was held 30 seconds and some of it 60 seconds. 

 ' Russel and Hastings — Oiillines of Dairy Bacteriology. 



