120 Testing Milk and Its Products. 



in normal milk, and its acidity; the larger the number 

 of bacteria per unit of milk, the higher is, in general, 

 the acidity of the milk. The increase in the acidity 

 of milk on standing is caused by the breaking 

 down of milk sugar into lactic acid through the activi- 

 ties of acid-forming bacteria. Since the bacteria get 

 into the milk through a lack of cleanliness during the 

 milking, or careless . handling of the milk after the 

 milking, or both, it follows that an acidity test of new 

 milk win give a good clue to the care bestowed in hand- 

 ling the milk. Such tests will show which patrons take 

 good care of their milk and which do not wash their 

 cans clean, or their hands and the udders of the cows 

 before milking, and have, in general, dirty ways in milk- 

 ing and caring for the milk. The acidity test is always 

 higher in summer than in winter, and is generally high 

 in the case of milk kept for more than a day (Monday 

 milk), or delivered after a warm, sultry day or night. 

 The bacteria have had a good chance to multiply greatly 

 in such milk, even if it be kept cooled down to 

 40°-50° F., and as a result considerable quantities of 

 lactic acid have been formed. The determination of the 

 acidity of fresh milk is explained in detail below (147). 

 130. Method of testing acidity. Methods of meas- 

 uring the acidity or alkalinity of liquids by means of 

 certain chemicals giving characteristic color reactions in 

 the presence of acid or alkaline solutions (so-called 

 volumetric methods of analysis) have been in use for 

 many years in chemical laboratories. They were applied 

 to milk as early as 1872 by Soxhlet,^ and the method 



1 Jour. f. prakt. Chemie, 1872, p. 6, 19. 



