MILK 17 



to stand undisturbed they rise to the top and in a very 

 short time form a layer on the top of the fluid, which 

 is known as the cream layer or the cream line. By many 

 consumers the quality of milk is judged solely by the 

 thickness of the cream layer. When the cream is re- 

 moved the remaining fluid is called skim milk; or it is 

 called separator milk when the cream is removed by a 

 centrifugal apparatus known as a separator. 



The fat globules vary in size with the breed, the stage 

 of lactation, the feed, at different periods of the same 

 milking, and with the individual. In the milk of Jersey 

 and Guernsey cows the fat globules are larger than they 

 are in the milk of Holsteins and Ayrshires. The cream 

 rises more rapidly when the globules are large than when 

 they are small. 



Moderately high temperatures also favor the sepa- 

 ration of the fat globules from the remainder of the 

 milk; therefore when milk is to be run through a sepa- 

 rator it is usually warmed to 32 C. (90 F.). On the 

 other hand, higher temperatures delay or entirely pre- 

 vent the formation of a cream layer. Temperatures 

 above 70 C. (158 F.) destroy the cream line entirely. 

 A temperature of 65 C. (149 F.) for ten minutes has 

 no effect, but as the time of exposure at this tempera- 

 ture is increased the formation of the cream layer is 

 delayed more and more, until finally, after forty min- 

 utes' exposure, it does not form at all. Milk may be 

 heated at 63 C. (145.4 F.) for thirty minutes and at 

 60 C. (140 F.) for as long as fifty minutes without 

 affecting the cream line. The cream does not rise in 

 homogenized milk because the fat globules have been 

 broken up into fine particles. Such milk is said to be 

 more palatable and more digestible than ordinary milk, 



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