TESTING ACiniTY OF MILK AND MILK PRODUCTS IO3 



of acid to be formed in cream-ripening is, therefore, to 

 be governed more by the amount of calcium casein in 

 the cream than by any other constituent. The less fat 

 there is in normal cream, the more casein there will be, 

 and the greater the per cent, of acidity needed. The 

 more fat there is in cream, the less calcium casein there 

 will be, and the less the amount of acid needed. These 

 statements conform to practical experience. Thus, 

 it is found that in cream containing 25 per cent, of 

 fat, it is necessary to produce nearly .7 per cent, of 

 acid in order to get the results sought by ripening, 

 while, in cream containing 35 per cent, of fat, less 

 than .6 per cent, of acid is sufficient. 



To ascertain how much acid should be formed in 

 cream before churning, the following rule is suggested 

 by Van Norman (Bulletin 104, Purdue Univ. Agr. 

 Exp. Sta.) : From 100 substract the per cent, of fat 

 in the cream tested and multiply the result by .9, or, 

 expressed as a formula, (100 — per cent, of fat in 

 cream) X.9. For example : 



Cream with 20 per cent, fat requires .72 per cent, of 

 acidity. 



Cream with 25 per cent, fat requires .67 per cent, of 

 acidity. 



Cream with 30 per cent, fat requires .63 per cent, of 

 acidity. 



Cream with 35 per cent, fat requires .58 per cent, of 

 acidity. 



The use of .9 as a factor for multiplying may not 

 suit all conditions and some other factor, .8 for exam- 

 ple, may be used. Each operator may experiment and 



