Ke'.v York Agkicultukal Experiment Station. 139 



Acidity. — As the result of experience in the pasteurization of 

 milk by the discontinuous method it has been found impractical 

 to attempt to pasteurize milk intended for immediate consump- 

 tion when the acidity is over 0.2 per ct. calculated as lactic acid. 

 During a considerable portion of the time our mixed milk has 

 had an acidity above this figure. 



The custom of expressing acidity in milk by percentages of 

 lactic acid has little to recommend it other than that it is com- 

 mon practice. It is a well-known fact that a part of the reaction 

 called acidity is due to the union of the alkali with the casein 

 and that lactic acid is only one of a number of acids found in 

 milk. 



The fact always determined is the neutralization as indicated 

 by phenolphthalien of a certain amount of normal solution of 

 alkali. In our results this observed fact is expressed by the num- 

 ber of cubic centimeters of normal alkali neutralized by a litre 

 of milk (n/l). For convenience of comparison the per ct. of 

 lactic acid erroneously assumed from this data is given in paren- 

 thesis. 



The acidity of our mixed milk ranged from 18.9 n/l (0.17 per 

 ct.) to 40 n/l (0.36 per ct.) and during a considerable portion of 

 the time it was above 22.2 n/l (0.2 per ct.). Thus the conditions 

 of acidity under which the pasteurizing was done were not what 

 would be considered ideal, but rather those which would be found 

 in an average creamery. As the acidity of the mixed milk ap- 

 proached 40 n/l (0.36 per ct.) a considerable layer burned fast 

 to the sides of the milk chamber in the pasteurizer and the accu- 

 mulation in the separator bowl was increased. 



On Feb. 7 the milk with an acidity of 35.5 n/l (0.32 per ct.) 

 was pasteurized at 85° C. (185° F.) very successfully; but the 

 next attempt with whole milk having an acidity of 40 n/l (0.36 

 per ct.) quickly accumulated a layer on the walls of the pasteur- 

 izer and clogged the separator bowl after passing only about 80 

 pounds of milk. 



