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 68° 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, 
2 
