502 QUANTITATIVE ESTIMATION OF BACTERIA IN MILK 



higher degree of acidity should not be permitted to be sold as fresh, 

 sweet milk, and health authorities should include the determination 

 of acidity among the measures governing the sale of milk. 



Determination of the Acidity of Milk. The acidity of milk can be 

 determined as follows: Place 50 c.c. of milk into a beaker and add 

 2 c.c. of a 2 per cent, alcoholic solution of phenolphthalein as an 

 indicator. Then add from a burette (gradually under constant shaking 

 or stirring with a glass rod) decinormal solution of sodium hydroxide 

 (^ sol. NaHO) until all acid has been neutralized and the fluid 

 retains a very faint pink color. This, of course, means that the 

 phenolphthalein in solution now indicates that all acid has been 

 neutralized and that a trace of alkaline decinormal solution of sodium 

 hydroxide has been added in excess. Each cubic centimeter of -^ sol. 

 NaHO is equivalent to 0.009 gram of lactic acid. 



In order to obtain the amount of acidity in 50 c.c. of milk each 

 cubic centimeter of decinormal solution used out of the burette to 

 effect complete neutralization must be multiplied by 0.009. The 

 product should again be multiplied by 2 in order to obtain the amount 

 of acidity per 100 c.c. of milk. 1 The rule, therefore, when 50 c.c. 

 of milk have been used is simply: "Multiply the number of cubic 

 centimeters of -^ sol. NaHO used out of the burette by 0.018; the 

 result will be the percentage of acidity in milk." In this calculation 

 the total acidity is expressed as lactic acid; part of the acidity may be 

 due to other fatty acids, such as butyric, succinic, formic, acetic, but 

 this is immaterial, because for practical purposes it is quite sufficient 

 to know the total acidity. 



For example, in the test of 50 c.c. of milk, 9.3 c.c. of -^ sol. NaHO 

 out of the burette have been used in order to accomplish complete 

 neutralization; the result then is 9.3 X 0.018 = 0.167 per cent, of 

 acid in the milk tested. It is generally held that market milk should 

 not show more than 0.2 per cent, of total acidity, because any excess, 

 as a rule, indicates that lactic-acid fermentation has pretty well begun. 



Recording of Results of Bacterial Counts in Round Numbers. As a 

 rule, bacterial counts of milk are expressed from the number of 

 colonies counted on the plates, in round numbers, so that: 



Counts below 100,000 are rounded off in terms of 10,000; for 

 instance, 20,000. 



Counts between 100,000 and 500,000 are rounded off in terms of 

 50,000; for instance, 350,000. 



Counts between 500,000 and 1,000,000 in terms of 100,000; for 

 instance, 700,000. 



Counts between 1,000,000 and 2,000,000 in terms of 200,000; for 

 instance, 1,400,000. 



Counts between 2,000,000 and 5,000,000 in terms of 500,000; for 

 instance, 2,500,000. 



1 It is, of course, customary to express the acidity in per cent., i. e., for 100 c.c. of milk. 



