20 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



in which they are present vary very much. The fat shows 

 a greater variation than the other solids. In milk rich 

 in total solids, the fat content is frequently considerably 

 above the average, while the per cent, of solids not fat 

 only slightly exceeds the average. On the other hand, 

 in thin, poor milk the fat per cent, may be far below 

 the average, while the per cent, of solids not fat is not 

 very far from the average. Milk from individual cows 

 shows a greater variation in composition than different 

 samples of market milk, which is a mixture of the milk 

 from a number of cows. 



Individual milk may show the following variations 

 in the proportion of the different constituents: fat, 2.5 

 to 7.5 per cent. ; lactose, 4 to 5.8 per cent. ; casein, 2 to 5 

 per cent.; albumin, 0.39 to 0.95 per cent.; globulin, a 

 trace; salts, 0.35 to 1.21 per cent.; water, 83 to 89 per 

 cent. 



The fat content shows the greatest range of varia- 

 tion and the lactose the least. These variations must be 

 taken into consideration in collecting samples of milk 

 for certain tests and in judging the results of tests for 

 adulteration and skimming. They are due to a number 

 of causes. 



Some of these causes, such as the breed, individuality, 

 and stage of lactation, are more or less regular and con- 

 stant in their operation. As a rule, cows of the Jersey 

 and Guernsey breeds give milk richer in fat than Hoi- 

 steins and Ayrshires, but some individuals of the Hoi- 

 stein and Ayrshire breeds give milk with a higher fat 

 content than some Jerseys and Guernseys. Early in the 

 stage of lactation, when the milk flow is most abundant, 

 the proportion of solids, especially the fat, is less than it 

 is later, when the milk flow has decreased. Instances 



