276 REPORT OF THE CHEMIST OF THE 
completely to a definite coagulum. It would therefore appear ~ 
that in ordinary methods of cream-ripening, when the lactic acid 
is allowed to reach 0.6 per cent., neither calcium casein nor free 
casein is present, but only casein lactate, the substance most 
commonly observed as curdled sour milk. 
(2) Decrease of milk-sugar in cream ripening.—In the three 
experiments the milk-sugar decreased to an extent varying from 
0.72 to 0.88 per ct. in 24 hours. In the first experiment, which 
was continued 72 hours, the amount of milk-sugar decreased very 
little after 24 hours. 
(3) Formation of lactic acid in cream-ripening.—When the 
cream was fresh, the apparent amount of lactic acid was only 
0.14 or 0.15 per ct., which increased in 24 hours to 0.6 to 0.7 per 
ct. In the first experiment, which was continued 72 hours, the 
acid increased very little after 24 hours. 
In connection with these experiments it may be mentioned 
that, in several lots of cream ripened so as to show an acidity of 
38 to 43 per ct., we found much free casein and little casein 
lactate. 
THE ACIDITY OF MILK AND CREAM. 
It may be well in this connection to call attention to the inac- 
curacy involved in attributing the acidity of milk or cream to 
lactic acid alone. The usual method of determining acidity 
in milk or cream is to titrate a given amount of milk with a 
standardized solution of fixed alkali, using phenolphthalein as 
indicator, the alkali being added to the milk until a fairly per- 
manent pink color appears. Lactic acid is not the only com- 
pound in milk that neutralizes alkali. It is well known that 
strictly fresh milk which contains no lactic acid, neutralizes an 
appreciable amount of alkali. The compounds in fresh milk or 
cream that have the power of neutralizing alkali are the follow- 
ing: (1) Calcium casein (milk-casein), (2) acid phosphates 
and citrates, and (8) carbon dioxide. Of these, the calcium 
casein and acid phosphates appear to be most prominent in neu- 
tralizing alkali. While results vary with individual cows, the 
average amount of acidity of fresh milk is about 0.08 per ct., 
calculated as equivalent to lactic acid. If one desires to estimate 
more closely the amount of real lactic acid in milk, it is neces- 
sary only to subtract from the results commonly found .1 per ct. 
