288 BACTERIOLOGICAL ANALYSIS OF MILK. 



color is retained in the milk. The number of c.c. of the 

 alkaline solution gives directly the per cent, of acid. Thus if 

 13 c.c. of alkaline solution are required the acid present is 

 .13%. This direct reading makes this method very con- 

 venient, and it is fairly accurate, accurate enough, indeed, for 

 most work upon the acidity of milk. 



DETERMINATIONS OF THE AMOUNT OF DIRT IN MILK. 



The dirt which makes its way into milk is a source of bac- 

 teria and its determination is, therefore, closely associated 

 with bacteriological problems. There is no very simple 

 method of determining the amount of dirt in milk, and none 

 gives the amount very accurately. The methods in use will 

 give somewhat closely the amount of insoluble dirt that may 

 find its way in the milk, but none of the soluble material is 

 detected. The methods of determining this filth are two, one 

 depending upon gravity and the other upon centrifugal force. 



Gravity Method. By this method a measured quantity of 

 milk, usually a liter, is allowed to stand in a tall cylinder for 

 several hours. During this time the solid dirt will mostly 

 settle to the bottom of the jar and can be seen as a sediment. 

 By means of a tube the milk is carefully siphoned off from 

 the sediment to within about one half inch of the bottom, 

 and then filtered water is added to fill the cylinder. The 

 material is allowed to sediment again for several hours, and 

 again the supernatant liquid is siphoned away. This opera- 

 tion is repeated several times, until the liquid is practically 

 clear. After the operation has been repeated until the dirt 

 has been thoroughly washed, the sediment which has settled 

 in the bottom of the cylinder is filtered from the remaining 

 liquid. The filter paper used should have been previously 

 accurately weighed with a chemical balance, after having been 

 thoroughly dried by being kept for a day or two in an ordi- 

 nary chemist's desiccator. The sediment is collected upon 

 the filter paper, and the paper carefully dried, first by heat 



