A UAIKY LABORATORY GUIDE 43 



EXPLANATION. ACIDITY OF MILK 



The acidity of milk is of two kinds — apparent 

 and real acidity. The apparent acidity is due to 

 the acid reaction of the acid phosphates and casein. 

 The real acidity is due to the presence of lactic 

 acid (C.jHoO.j ), which is produced by the action of 

 bacteria upon the sugar of the milk. The follow- 

 ing reaction is supposed to be the one which takes 

 place: 



Ci2H220ii + H.O=4C3H603 



The apparent acidity, according to Van Slyke, 

 does not go above .08 per cent to .1 per cent, and is 

 i)f minor importance so far as dairy work is con- 

 cerned. In determining the acidity of milk it is as- 

 sumed that all of the acidity is due to the presence 

 of lactic acid. 



The real acidity will ordinarily go as high as i 

 per cent, and in some casesi higher. Usually, how- 

 ever, when from .8 per cent to i per cent acidity 

 is reached the lactic acid organisms will cease 

 working. If a part of the acid is neutralized the 

 organisms 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 

 \er\' 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 finding the amount of acid in the milk. 

 The process by which this is done is called titration. 



