CHAPTER XX 



MILK PRODUCTS 



Cream. It has been shown that the specific gravity of milk 

 serum containing 8-5 per cent, of solids free from fat is i'o34, 

 and that of the fat itself is about 0*93. It is in consequence 

 of this difference of specific gravity that the fat globules tend 

 to rise to the surface, and accumulate in the upper layer which 

 is then called cream. 



The rate of motion of the fat globules depends upon a 

 number of circumstances, but whatever it may be, it is obvious 

 that the length of time in which a fat globule will reach the 

 surface will vary according to the depth of the liquid. If the 

 velocity were the same throughout, a globule would take twice 

 as long to come to the surface from a depth of eight inches as 

 it takes when the depth is only four inches. 



It has long been customary, therefore, to set milk for cream 

 in wide shallow vessels. This method has the disadvantage 

 of exposing a large surface to the air ; numerous bacteria may 

 enter into the milk, and in warm weather rapidly turn it 

 sour. 



The larger globules rise much faster than the smaller, 

 because their movement is not retarded to the same extent by 

 the frictional resistance of the serum. This resistance is much 

 greater than that of pure water, because the " casein " is not 

 properly dissolved, but merely in a state of colloidal suspension. 

 The resistance becomes greater the longer the milk stands, 

 owing to the formation of lactic acid, which gradually alters 

 the casein, and finally coagulates it when, of course, the 

 motion of the globules is entirely arrested. The milk should, 

 therefore, be set for cream immediately after it is drawn. The 



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