Milk and Us Products. 273 



in oxygen and nitrogen is really an absorption of air and em- 

 phasizes the necessity of maintaining a pure, sweet atmosphere, 

 to which fresh milk is to be exposed. 



Physical properties. Milk is a white, or yellowish white, 

 opaque fluid, with a sweet taste. The specific gravity varies 

 usually from 1.027 to 1.034. The solids other than fat tend to 

 raise the specific gravity, while the fat tends to lower it. As 

 cream may be removed and water added without altering the 

 specific gravity, no safe conclusion as to the quality of the milk 

 can be based on this test alone. When fresh milk is quickly 

 cooled and its specific gravity taken at once, and then again after 

 some hours and at the same temperature, a small but decided 

 rise in density is observable, usually amounting to about 0.0005. 

 This is known as Kechnagel's phenomenon, and has been ex- 

 plained in several ways. It has been ascribed to the escape of 

 gases from the milk ; to a change in the mechanical condition of 

 the casein; and lastly to the solidification of the fat globules. 

 It is suggested that quick cooling does not immediately solidify 

 the fat globules, which are liquid at the temperature of the cow, 

 but that they remain in a super-cooled liquid state. As they 

 slowly solidify, they contract, thereby increasing the density and 

 raising the specific gravity. 



Chemical composition. This varies considerably according 

 to breed, individuality, age, period of lactation and food. The 

 mean composition, according to many American analyses, is 

 as follows: 



Per cent 



Water 87.1 



Fat 3.9 



Sugar 5.1 



Per cent 



Casein 2.5 



Albumin 0.7 



Ash.. 0.7 



It must be remembered that these figures, being averages, im- 

 ply the existence of many values either above or below those 

 given. As a rule the fat is most liable to variation. The fac- 

 tors influencing the composition of milk will be briefly discussed 

 under the following heads: 



