12 NATURE AND COMPOSITION OF MILK. 



Normal milk is an opaque, white or yellowish- 

 white fluid, with an odor recalling that of the animal, 

 and a faint sweet taste. The opacity is due partly to 

 the fat globules, but when these are entirely removed 

 the liquid does not become transparent. The reaction 

 of freshly drawn milk is amphoteric, that is, it turns red 

 litmus paper blue and blue litmus paper red. The 

 specific gravity varies between 1028 and 1035. It 

 undergoes a gradual augmentation for a considerable 

 time after the sample has been drawn. The increase may 

 amount to two units. The specific gravity becomes 

 stationary in about five hours, if the milk be maintained 

 at a temperature below 60 F., but at a higher temper- 

 ature it may require twenty-four hours to acquire con- 

 stancy. The change is not dependent on the escape 

 of gases, and is believed to be due to some molecular 

 modification of the casein. 



Unless collected with special care and under condi- 

 tions of extreme cleanliness, milk always contains bac- 

 teria and animal matter of an offensive character, 

 such as epithelium, blood and pus cells, particles of 

 feces and soil. Many minute organisms, especially 

 bacteria, propagate with great rapidity in milk and 

 produce changes in its composition. Some specific 

 organisms, such as the Spirillum cholera, multiply to 

 only a limited extent in ordinary milk, being hamper- 

 ed by the bacteria normally present, but when intro- 

 duced into sterilized milk increase with great rapidity. 



At ordinary temperature milk soon undergoes de- 

 composition under the influence of the microorgan- 



