THE PHYSICAL PROPERTIES OF MILK 53 



4, the depth of the cream layer; 5, the viscosity of the milk; 6, the 

 size of the fat globules; 7, centrifugation. 



1. The Origin of the Milk. The milk, from different animals 

 has different creaming properties. Separation is rapid and nearly 

 complete in human milk, for example, an opalescent % bluish-white 

 fluid remaining under a layer of yellow fat. In cow's milk sep- 

 aration is slower and less complete. Below the characteristic 

 yellow layer of cream there remains a white or bluish-white 

 fluid which is nearly as opaque as the original milk. In goat's 

 milk there is usually no spontaneous creaming unless the milk 

 has been previously heated, while heating of cow's milk retards 

 creaming. 



The cause of these differences in creaming must be sought in 

 tfhe size of the fat globules and in the physical condition of the milk 

 plasma. Milks with relatively large fat globules form a cream 

 layer more rapidly than milks with small fat globules, other con- 

 ditions being equal. Immediately after milking the casein is in 

 a swollen condition and hinders the rising of the fat to a lesser 

 degree than later, when it has contracted. Probably there is a 

 similar difference between the milks from different animals. 

 Furthermore, the quantity of casein in the different milks is not 

 the same. Human milk, for example, contains much less casein 

 than either cow's or goat's milk, and we may assume that the ris- 

 ing globules meet with less resistance. However, a full explana- 

 tion is still lacking. 



2. Age of the Milk. It has been stated that the fat globules 

 rise more rapidly in fresh milk than in old milk, because the casein 

 gradually contracts and offers greater resistance to the upward 

 movement of the fat. Moreover, when milk ages it becomes 

 sour and the casein is precipitated. In this condition fat rises 

 but slowly. Agitation of any kind, such as results from handling 

 and transporting milk, retards creaming. The best results in 

 creaming are obtained right after milking (Fig. 16). 



3. Temperature. Creaming is slower in cold milk than in 

 warm milk, for two reasons. Milk increases in viscosity when the 

 temperature is lowered and the fat congeals; the spheric shape is 

 lost and the surface becomes wrinkled. Both these conditions 

 retard the rising. 



If t'he temperature of a can of milk is not uniform throughout, 

 currents are created and these carry fat globules, especially the 

 small ones, out of their upward path. Therefore cooling should 

 commence from the bottom so as to prevent the formation of 

 currents. 



When milk is heated the fat clusters break up, small globules 

 are isolated, and some coalesce (Fig. 17). Clusters of fat rise 



