32 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



gravity, see page 228.) Determination of the specific 

 gravity of the milk solids and of the per cent, of fat 

 in the milk solids will assist in detecting milk which has 

 been skimmed or skimmed and watered. ( See page 246 

 for methods.) The milk solids of normal market milk 

 have a specific gravity of 1.31 to 1.36, and the per cent, 

 of fat in the milk solids is 20 to 34. When milk is 

 skimmed or skimmed and watered, the specific gravity 

 of the milk solids is increased, while the per cent, of fat in 

 the solids is decreased. 



The influence of disease on the specific gravity of 

 individual milk is not constant, but the specific gravity 

 is usually lowered. However, the specific gravity of 

 individual cow's milk cannot be made use of to discover 

 diseased conditions, because milk from different cows 

 shows such great variations under normal conditions. 

 The specific gravity of milk is lower when it is drawn 

 from the udder than it is several hours later. 



Refraction. Rays of light passing through one me- 

 dium into another of different optical density, as through 

 air into milk, are broken or refracted at the point of con- 

 tact of the two media. The degree of refraction, or the 

 refractive power compared with that of air, is called 

 the refractive index. Since the calcium caseinate and 

 fat contained in milk prevent the light rays from passing 

 through it, these substances must be removed before the 

 refractive index can be determined. The refractive index 

 of milk, so-called, is really the refractive index of the 

 milk serum or whey, i.e., the milk minus the calcium 

 caseinate and fat. 



The refractive power of the milk serum depends upon 

 the quantity of lactose or of lactose and salts present. 

 Adding water to milk reduces the proportion of these 



