MILK 7 



Freezing Point of Milk. Milk freezes at 31F. As a can of milk 

 freezes, the solids, except fat, are excluded from the ice so that the portion 

 that remains unfrozen contains more casein, milk sugar and ash than 

 the frozen, while the latter carries more fat because it rises to the top 

 and is entangled by the ice in freezing. So samples taken for analysis 

 from partly frozen cans do not represent the true composition of the milk. 



Electric Conductivity of Milk. Because of the dissolved salts that 

 it contains, milk passes the electric current, the electric conductivity 

 being dependent on the degree of dissociation of the salts which it carries. 

 The resisting power of milk varies between 180 and 210 ohms. 



Refractive Index of Milk. The refractive index of milk varies with 

 the composition; in normal milk the refractive index ranges from 1.3470 

 to 1.3515. A minimum of 1.3435 is rarely observed. 



Fat Globules in Milk. Under the microscope the most conspicuous 

 elements of milk are the fat globules. They have a pearly luster and 

 vary in size from 0.001 to 0.01 mm. in diameter averaging about 0.005 

 mm. and become smaller and smaller from the first milk to the strippings. 



As the period of lactation advances, the number of large globules 

 decreases and of small ones increases. Apparently the age of the cow 

 has no relation to the size of the globules. Breed affects the size of the 

 globules, the Holsteins and Ayrshires giving small, the Brown Swiss 

 medium and the Shorthorn, Guernsey and Jersey large ones, but in this 

 matter individual cows differ markedly. The fat globules are arranged 

 in groups or clumps in the milk instead of being distributed uniformly 

 throughout it, a matter of practical importance, for when these clumps 

 are broken up, either by heating or centrifugalizing the milk, it does not 

 cream well. 



Cellular Content of Milk. Stained milk smears under high powers 

 of the microscope reveal numerous cells that have caused much discus- 

 sion. They are now considered to be: (1) epithelial cells from the milk 

 ducts and milk cistern; (2) mononuclear and polynuclear cells; and (3) red 

 blood cells. At one time large numbers of these cells were believed to 

 indicate inflammatory conditions in the udder and it was proposed to 

 exclude from the market, milk that showed more than a certain number 

 of cells per microscopic field, but it is now recognized that perfectly 

 healthy animals at times shed epithelial cells in large numbers and the 

 proposal is regarded as untenable. The presence of large numbers of 

 polymorphonuclear cells together with long-chained streptococci is 

 strongly indicative of mammitis and warrants an investigation of the 

 herd or cow supplying the milk. 



The Enzymes of Milk. Besides the constituents already mentioned 

 milk contains certain enzymes or soluble ferments derived from living 

 cells. The most important of them are galactase, lipase, lactokinase, 

 catalase, peroxidase and reductase. The origin of the ferments is not 



