THE PHYSICAL PROPERTIES OF MILK 



When milk is freshly obtained from the udder it is opaque, 

 white in color, with more or less of a yellowish tint, and has a 

 characteristic sweetish taste and odor. If a small amount of 

 fresh milk is placed under the microscope the field is almost filled 

 with homogeneous disks of strongly refractive power and greatly 

 varying diameter. They appear structureless and without a mem- 

 brane. Close observation of these disks shows that they fre- 

 quently form clusters which consist of disks of varying size. In 

 reality, the disks are spheres and consist of milk-fat. Besides 

 these "fat globules," bacteria, cells, and particles of foreign matter 

 may be seen. All these substances are suspended in a fluid — 

 the milk plasma (Fig. 15). 



Fig. 15. — Fat globules and bacteria. Note the relative size of the tat globules 

 of milk and the lactic acid bacteria (Russell and Hastings). 



The fat globules in milk are in the form of an emulsion, which 

 means that they are distributed throughout the fluid in small 

 microscopic globules and held in suspension. Artificial emulsions 

 can easily be made by mixing an oil with some substance like gum 

 arable, gum tragacanth, or extract of malt and some water, and 

 by agitating the mixture violently in a mortar. An emulsion of 

 this kind is white and opaque and under the microscope simulates 

 the appearance of milk. 



If milk is allowed to stand quietly the physical appearance 

 changes rapidly. A yellowish layer gradually accumulates on the 

 surface and increases in depth for some time, after which it dimin- 



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