METHODS OF EXAMINING MILK 255 



10 grammes of powdered phenolphthalein in 300 c.c. of 

 90 per cent, alcohol. The one-tenth normal (Yio) solu- 

 tion of sodium hydroxide (NaOH) should be prepared 

 and standardized as directed by the U. S. Pharmacopoeia. 

 Each cubic centimetre of this solution contains sufficient 

 sodium hydroxide (0.004 gramme) to neutralize 0.009 

 gramme of lactic acid. 



After thoroughly mixing the milk or cream to be 

 tested, 50 c.c. are measured into the beaker or cup with 

 the pipette. The pipette is then filled to the 50 c.c. mark 

 with water, preferably distilled, which is added to the 

 milk or cream. This is done for the purpose of rinsing 

 all milk or cream from the pipette. Five to ten drops of 

 the phenolphthalein solution are then placed in the beaker 

 or cup and the fluid is mixed by stirring with a glass rod. 

 The tenth normal sodium hydroxide solution is run into 

 the beaker or cup drop by drop, stirring frequently, until 

 a pink color appears and remains permanently. The 

 amount of sodium hydroxide solution used is then read 

 off from the burette. 



The result of the test may be expressed in degrees 

 or in per cent of acidity. Degrees of acidity correspond 

 to the number of c.c. of tenth-normal sodium hydroxide 

 solution used to the hundred c.c. of milk or cream. The 

 per cent, of acidity may be obtained by the following 

 calculation: 



_ 

 No. c.c. milk or cream used 



For example, if 9 c.c. of the one-tenth normal solu- 

 tion are used in testing 50 c.c. of milk or cream, then 



X 100 9 X 0.018 0.16 per cent, acidity. 



