330 
cording to these observers it then contained formic acid. They also 
reached the conclusion that the discoloration and coagulation of 
milk by heat is due to the oxidation of lactose in the presence of the 
alkaline salts of the milk, one product of the oxidation consisting of 
formic acid, which, like other acids, precipitates the caseinogen. 
The latter undergoes no further change except that it is discolored 
b}^ the products of the decomposition of lactose. 
Bruno Bardach (12) has also studied the coagulation of milk by 
heat. He found that about twelve hours’ heating at 100° C. was 
required in order to coagulate perfectly fresh milk, whereas at 150° 
C. it coagulates in three minutes, and at 130° C. in one hour. He 
found only the merest traces of formic acid to be formed at 130° C. 
He concludes from his study of the subject that the coagulation of 
milk b}^ heat is a complex process; that it is brought about by the 
action at the high temperature of the small quantities of acid which 
are formed from the lactose, and which ordinarily are powerless to 
coagulate the original unchanged casein (caseinogen), and that it is 
only after the casein (caseinogen) has been changed by the action of 
heat that such small amounts of acid can cause its coagulation. 
The j)art played by calcium salts in the acid coagulation of milk 
has been studied by Loevenhart (13). According to this author the 
very small quantities of acid required to effect the coagulation of 
milk at temperatures at or beloAV boiling accomplish this change by 
rendering the calcium salts normally present in milk available for 
the coagulation of the caseinogen. Therefore the temperature at 
which a given specimen of slightly sour milk will coagulate on heat- 
ing depends partly upon the degree of acidity and also upon the 
nature and amount of the calcium salts present in the milk. 
Von Soxhlet (14) has also recently investigated the coagulation 
which occurs on boiling faintly acid milk. He observed that at the 
commencement of the souring of milk boiling causes a coagulum 
to form. This occurs when only one-eighth of the amount of acid 
necessary to produce coagulation at ordinary temperatures is present. 
It depends, according to this author, on the formation of an insoluble 
compo'und of caseinogen with soluble calcium salts, the acid first 
produced forming inonocalcium phosphate from the dicalcium phos- 
phate present in the fresh milk. 
The fact that milk occasionally curdles in the pasteurizing appa- 
ratus during pasteurization makes the accumulation of data bearing 
on this particular phase of the subject a matter of considerable im- 
portance. During our recent investigations of the Washington milk 
supply we incidentally made a number of observations on the coagu- 
lation of slightly sour milk at or below boiling. The results of these 
observations, aranged in the order of diminishing acidity, are given 
in the following table : 
