132 MODERN METHODS OF TESTING MILK 



of bacteria upon milk-sugar. In general, when milk 

 contains over .10 per cent, of acid, it may safely be 

 assumed that it contains some lactic acid. The amount 

 of lactic acid present in milk may be approximately 

 found by subtracting .10 from the total amount of 

 acid apparently present. However, in speaking of the 

 acidity of milk, we usually mean the total acidity, and 

 not that due to lactic acid alone. One can not com- 

 monly detect a sour taste in milk that has a total acidity 

 under .3 per cent. 



GENERAL PRINCIPLES OF TESTING ACIDITY 



The method of determining the amount of acid in 

 milk and its products is based upon the well-known 

 chemical action taking place between acids and alka- 

 lis. Whenever we bring together in solution an acid 

 and an alkali, they combine with each other and form 

 a third compound, the acid and alkali disappearing as 

 such. The acid and alkali are said to neutralize each 

 other and the process is called neutralization. For 

 example, if we add together some hydrochloric (muri- 

 atic) acid and sodium hydroxide (caustic soda) in 

 right proportions, we shall have neither hydrochloric 

 acid nor caustic soda, but a new compound, sodium 

 chloride (common salt), which has been formed by 

 the action of the acid and alkali upon each other. The 

 hydrochloric acid used in the experiment tastes sour 

 and biting, while the caustic soda solution has a pecu- 

 liar odor, feels soapy on the skin, and, if strong enough, 

 destroys the skin. After these two compounds are 

 brought together in proper proportions, there is no 



