METHODS OF TESTING THE ACIDITY OF MILK 14! 



much confusion exists in this respect, the meaning of 

 a "degree" varying according to the method used. It 

 seems highly desirable on all accounts always to state 

 the results of acidity determination in terms of per- 

 centage of lactic acid ; this method now practically pre- 

 vails in America. In most of the tests in common use, 

 no calculations or only very simple ones, have to be 

 made, because the amount of material taken and the 

 strength of alkali used are such as to enable one to 

 read the results in direct percentage from the burette, 

 as we have already noticed (p. 137). 



METHODS USED IN TESTING ACIDITY 



After having considered the general principles and 

 their application, we do not need to describe the dif- 

 ferent methods in detail further than to point out the 

 distinctive features of each, since the really essential 

 features are the same in all. 



Van Norman's method (Bulletin No. 104, Purdue 

 Univ. Agr. Exp. Sta., 1905) is based upon the use of 

 a fiftieth-normal solution of alkali, prepared by dilut- 

 ing 37 cc. of normal solution of sodium hydroxide 

 (caustic soda) to 1850 cc. (or 20 cc. of normal solu- 

 tion to 1000 cc.), observing precautions already 

 pointed out (Fig. 38). The normal solution must be 

 of guaranteed accuracy and should be obtained only 

 from a reliable chemical or dairy-supply house. The 

 amount of milk, cream, etc. used in testing is that mea- 

 sured by a 17.6 cc. pipette. The alkali may be run into 

 the cup containing the material to be tested either from 

 a burette or from a 100 cc. graduated cylinder until 

 the characteristic pink tint is obtained (p. 139). 



