122 



Testing Milk and Its Products. 



termining the acidity of milk, and is the solution labeled 

 Neutralizer of the Manns' test. 



The indicator used is a solution of phenolphtalein, a 

 light yellowish powder; its compounds with alkalies are 

 red, in weak alkaline solutions pink colored, while its 

 acid compounds are colorless. The phenolphtalein solu- 

 tion used is prepared by dissolving 10 grams in 300 cc. 

 of 90 per cent, alcohol (Mohr). 



132. In testing the acidity of either 

 milk or cream it is necessary to meas- 

 ure out with exactness the quantity of 

 liquid to be tested; Manns recom- 

 mended using a 50 cc. pipette. This 

 amount of milk or cream is measured 

 into a clean tin, porcelain or glass cup, 

 a few drops of the phenolphtalein so- 

 lution are added, and the Neutral- 

 § izer (or alkali solution) is cautiously 

 dropped in from a burette, the point at 

 which the solution stands before any is 

 drawn off being noted. By constant 

 stirring during this operation it will be 

 noticed that the pink color formed by 

 the addition of even a drop of alkali 

 solution will at first entirely disappear, 

 but as more and more of the acid in 

 the sample becomes neutralized, 

 the color will disappear more 

 slowly, until finally a point is 

 reached when the pink color re- 



FiG. 41. Apparatus used 



In Manns' test. mains permanent for a time. No 



