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 1.8429 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 
