324 
localities, and in this connection he has observed that the coagula- 
bility by lactosemm may be restored to boiled milk by the addition 
of soluble calcium salts. 
Ham mars ten observed that milk curdles when it is heated to 130° 
to 150° C. (see p. 344). Cazeneuve and Haddon (11) observed that 
milk which had been coagulated at 130° C. became very acid. Ac- 
cording to these observers it then contained formic acid. They also 
reached the conclusion that the discoloration and coagulation of 
c* 
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 
by 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 by 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 coagu- 
late 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 part 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 below 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 
compound of caseinogen with soluble calcium salts, the acid first 
produced forming monocalcium 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 
