MILK AS A FOOD 35 



The milk of Holstein cows is more dilute; that is, contains 

 more water than the milk of Jerseys and Guernseys. 



Fat. The fat is a very important but by no means the 

 chief constituent of milk. Very many people think that 

 fat is the only valuable part of milk, for the reason that the 

 cream layer is so plainly in evidence, and so much emphasis 

 has been laid upon the butter-fat standard. As a matter of 

 fact, even the richness of milk cannot always be gauged by 

 the cream line. The casein and milk sugar are not visible, 

 but are just as important as the fat. 



Fat occurs in milk in the form of an emulsion; that is, 

 it is not dissolved in the milk as the sugar and salts are 

 dissolved, but is suspended in the milk serum in the form of 

 small droplets or globules. The fat is lighter than the milk 

 and therefore, when milk stands, it rises and floats upon 

 the surface just as pieces of wood float upon the surface 

 of water. The fat rising thus to the top of the milk as a dis- 

 tinct layer is known as "cream" or "top milk." 



Cream is obtained in two ways : When milk is allowed to 

 stand, the top layer is known as "gravity cream," to dis- 

 tinguish it from "separator cream" obtained by centrifu- 

 gation. The heavier portions of milk, that is, the water, 

 casein, and sugar, are thrown as far from the centre as pos- 

 sible by the centrifugal force of the rapidly rotating ma- 

 chine and the lighter cream is thus forced to the centre 

 where it can be drawn off. It takes at least twelve hours 

 to obtain gravity cream, while separator cream may be ob- 

 tained at once from perfectly fresh milk. Further, the fat 

 can be separated by centrifugal force much more com- 

 pletely than by gravity. 



Cream or top milk does not consist of fat alone, but of all 

 the constituents of the milk. It is simply milk rich in fat. 



Upon shaking milk or cream the fat globules gradually 

 coalesce into larger drops, and these drops unite with other 

 drops until lumps are formed butter. 



The fat in milk consists of a mixture of different neutral 



