THE TESTING OF MILK AND CREAM 133 



the acid phosphates and casein. The real acidity is due to the 

 presence of lactic acid (CsHeOa), which is produced by the 

 action of bacteria upon the sugar of the milk. The following 

 reaction is supposed to be the one which takes place : 



Ci 2 H 22 O u + H 2 = 4C 3 H 6 3 



The apparent acidity, according to Van Slyke, does not go 

 above .08 per cent to .1 per cent, and is of minor importance 

 so far as dairy work is concerned. In determining the acidity 

 of milk it is assumed that all of the acidity is due to the pres- 

 ence of lactic acid. 



The real acidity will ordinarily go as high as 1 per cent and 

 in some cases higher. Usually, however, when from .8 per cent 

 to 1 per cent acidity is reached, the lactic acid organisms will 

 cease working. If a part of the acid is neutralized, the organ- 

 isms will again commence the production of acid. 



Lactic acid is important in the manufacture of dairy products. 

 For example, butter is churned from cream which is soured or 

 "ripened" by lactic acid. The presence and amount of lactic 

 acid is very important all through the process of cheese-making. 

 In many cases the nature of the product depends on the amount 

 of acid present during the successive steps of manufacture. 



For these reasons it is necessary to have some means of find- 

 ing the amount of acid in the milk. The process by which this 

 is done is called titration. It is a principle of chemistry that 

 an alkali will neutralize an acid. In order, therefore, to find 

 the acid in the milk, we take a known quantity of the milk and 

 measure into it an alkali whose strength we know. The in- 

 strument used to measure the amount of alkali used is called a 

 burette, and the unit of measure is the cubic centimeter. It is 

 commonly graduated as fine as tenths of a cubic centimeter. 

 One can tell when all of the acid is neutralized by means of an 

 indicator. The indicator used most in dairy work is phenol- 

 phthalein, which is colorless in acid and pink in alkali. If two or 



