Testing the Acidity of Milk and Cream. 109 



between the bacteria found in normal milk, and its acid- 

 ity; the larger the number of bacteria per unit of milk, 

 the higher 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 influence 

 of acid-forming bacteria. Since the bacteria get into 

 the milk through lack of cleanliness during the milking, 

 or careless handling of the milk after the milking, this 

 being kept under conditions that favor the multiplica- 

 tion of the bacteria contained therein, it follows that an 

 acidity test of new milk will give a good clue to the care 

 bestowed in handling the milk. Such a test 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 dirty ways 

 generally in milking and caring for the milk. The acidity 

 test is always higher in summer ttian in winter, and is 

 generally high in 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 chance to multiply 

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

 to 40-50F., 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 (143). 

 127. Methods of testing acidity. Methods of measuring 

 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, 1 and the method worked out 



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



