MILK 33 



substances, and therefore reduces the refractive index. 

 Hence the refractive index may be used to detect watered 

 milk. (See method on page 249.) The refractive index 

 of normal market milk ranges from 1.3429 to 1.3445. 

 On the scale of the Zeiss dipping refractometer, the 

 reading for normal milk ranges from 37.3 to 41.5. 



Viscosity is manifested by the adherence of milk to 

 the sides of a glass vessel. It increases as the tempera- 

 ture of the milk is lowered, and vice versa. It is de- 

 creased by skimming and by the addition of water. Dur- 

 ing the colostral period and near the end of lactation it 

 is greater than at other times during the lactation period; 

 it is also increased in disease or injuries of the genital 

 organs, especially the udder. 



Boiling momentarily, or heating for a longer time 

 at lower temperatures, decreases the viscosity of milk, 

 causing it to appear thinner than normal raw milk. 

 Cream heated at 60 C. (140 F.) for twenty minutes 

 appears thinner and less viscous than raw cream with 

 the same fat per cent, and will not "whip" readily. Vis- 

 cogen, a mixture of cane sugar and lime, has been added 

 to heated cream to overcome this change. This prepara- 

 tion has also been used to increase the viscosity of raw 

 cream of low fat per cent., and also to increase the con- 

 sistency of skimmed or watered milk. The addition of 

 viscogen to cream or milk is illegal unless the product is 

 sold as visco-cream or visco-milk. The specific gravity 

 of watered or skimmed milk is increased by the addition 

 of viscogen, and the per cent, of solids not fat, especially 

 the salts, is also increased. The acidity is reduced. 



Starch is also added to cream to increase the body 

 or consistency. 



Freezing Point. The freezing point of milk is 

 3 



