52 



MILK 



ishes somewhat. If microscopic examinations are made of this 

 yellowish layer, of the whiter portion below and of the very 

 bottom, the three resulting slides represent a widely different 

 appearance. They differ from each other and from the slide made 

 of whole milk. The shde from the surface is fairly crowded wkh 

 fat globules and may also contain many bacteria and body cells. 

 The sUde from the middle portion shows fewer fat globules and of 

 smaller average size than the globules of the first slide. Bacteria 

 and cells are relatively scarce. The third sUde— from the bottom 

 —contains few fat globules, but large numbers of bacteria, cellsf 

 and other substances, such as Nissen corpuscles, and frequently 

 particles of dirt. These mucroscopic studies show that milk is 

 not a homogeneous fluid and that by gravity alone it can be sepa- 

 rated into three more or less distinct layers. 



Further microscopic examination of the surface shows that 

 the fat globules are larger nearer the surface than below. In the 

 course of time they press against each other from below so that 

 the layer becomes more compact. This is the cause of the dimin- 

 ished size of the surface layer after the maximum has been reached. 

 The milk below this layer is white and retains its color and opacity 

 indefinitely. 



Lloyd poured Shorthorn milk into a vessel 15 inches high and 

 after twelve hours measured the size of fat globules in different 

 parts of the cream. The following table gives the results, namely, 

 that the average size increases with approach to the surface: 



THE SIZE OF FAT GLOBULES AT DIFFERENT PARTS OF THE CREAM IN A BOTTLE 



The surface layer is the cream layer; the process of its forma- 

 tion is called "creaming"; the milk below the cream is skimmed 

 milk; while t'he fresh unchanged milk is the whole milk. When 

 milk is obtained from more than one animal it is called mixed milk. 



The milk-fat in cow's milk usually has a specific gravity of 

 about 0.93 at 15° C, but sometimes it is higher. Whole milk 

 has a specific gravity of 1.027 to 1.034. The smaller weight of 

 the fat is the cause of its rising to form the cream layer. This is 

 clearly a physical phenomenon and is influenced by a variety of 

 factors. The most important of these factors are: 1, The origin 

 of the milk; 2, the age of the milk; 3, the temperature of the milk; 



