THE MILK CONTRACTOR 299 



which, being lighter than the serum of the milk and having a relatively 

 small surface in proportion to their cubic content, readily rise to the 

 surface to form cream, whereas in the heated milk the clusters are dis- 

 rupted and the single globules fail to rise to the surface because they have 

 relatively large surfaces which meet enough resistance to offset the force 

 of gravity that tends to make them rise. Recently, Peter of the Swiss 

 Dairy School in Berne has shown that the rapidity of the formation as 

 well as the depth of the cream line increases up to a pasteurizing tempera- 

 ture of 141. 8F. beyond which it decreases, but even at 145.4F. was 

 greater than in raw milk. So the usual explanation cannot be accepted; 

 instead it is held that heating decreases the viscosity of milk and so facili- 

 tates the rising of the fat globules but that at temperatures above 145.4F. 

 an abundant though invisible coagulation of the albumen retards or pre- 

 vents the rising of the fat globules. Kilborune investigated the question 

 and found that when milk is cleaned by the centrifugal clarifier the vol- 

 ume of the cream in the milk is reduced 2 to 3 per cent. He found, too, 

 that the volume of cream in bottled milk is affected by several imper- 

 fectly understood factors, viz. : 



1. The temperature to which the milk is heated. 



2. The length of time at which the milk is held at high temperature. 



3. The temperature of the heating medium with which the milk comes into contact 

 during the heating process. 



4. The clarification of the milk. 



5. The amount of agitation to which the milk is subjected especially while hot. 

 This is an important factor. It was noted that in those plants where there was most 

 agitation of the milk and particularly when pumping of the hot milk was practised, the 

 most difficulty with the cream line was experienced. 



Other factors that he believed might possibly affect the cream line are : 



1. The age of the milk at pasteurization. 



2. The grade of the cows that produced the milk. 



3. Whether or not the milk had been frozen before pasteurization. 



As a result of these studies the New York City Board of Health now 

 requires that for milk to be considered pasteurized it must have been 

 exposed to temperatures between 142F. and 145F. for not less than 30 

 min. 



There is some evidence that the method of cooling the milk effects 

 the volume and character of the cream. There are experiments which 

 indicate that in vat pasteurization, if the milk is cooled in the vats, a 

 slow process, the cream line is not very distinct, whereas if the milk is 

 cooled quickly by running it out of the tanks over coolers the cream line 

 is sharply defined and the cream on analysis shows more casein than 

 does the slowly cooled cream. 



Excessive loss from evaporation is to be guarded against in pasteur- 

 ization. For that reason the heated milk should not be exposed to the 



