1636 



The Cornell Reading-Courses 



The taste and odor of milk are sweetish, although pure milk has little 

 of either. If there are strong flavors and odors they are due to the food 

 eaten by the animal or to contamination of one kind or another, such as 

 may result from dirty stables or unclean utensils. 



Freshly drawn milk has a double, or amphoteric, reaction. It acts as 

 an acid on an alkali and as an alkali on an acid, turning blue litmus paper 

 red and red litmus paper blue. After standing for a time the reaction 

 usually becomes decidedly acid, due to the action of lactic-acid bacteria. 

 These bacteria, or germs, work on the milk sugar and convert it into 

 lactic acid. 



When milk is boiled in an open vessel a heavy scum of dried milk forms 

 on the surface. If this scum is removed another will appear as soon as 

 the milk is boiled, and such a membrane will form as long as any milk 

 is left. If milk is boiled in a closed vessel the scum does not form. 



For all ordinary purposes milk may be said to have six constituents, 

 which, together with the percentage of each, are as follows: 



Percentage 



Water 87.0 



Sugar 5 • ° 



Fat 40 



Casein 2.6 



Albumen -7 



Ash 7 



For many reasons, some of which are not well understood, the compo- 

 sition of milk is variable. The physical condition of the cow, the kind of 

 food given, the housing, and the age of the cow are four things that may 

 affect the composition. Wing, quoting from Koenig, gives the follow- 

 ing variation of normal milk: 



Percentage 



Water . . 

 Sugar. . . 



Fat 



Casein . . 

 Albumen 

 Ash 



Of course, single variations may be found that go either above or below 

 the limits given in the table; but the figures presented are the results of 

 a large number of analyses of samples of normal milk. 



