112 THE COAGULATION OF CONDENSED MILK, 



The supposition that the co.agulation was entirely an acid 

 coMgulation was, however, shown to be wrong by the following 

 experiment. Infected milk, with and without the addition of 

 chrtlk, was poured into separate Petri dishes which were 

 incubated at 22° for three days, when the}'- were transferred to 

 the incubator at ST''. Four hours later, the chalk-test had 

 coagulated while the other had partly coagulated. The reaction 

 of the coagulated milk with the chalk was neutral, while the 

 partly coagulated milk was acid. The neutral reaction of the 

 coagulated milk with chalk shows that the thickening was not 

 caused by the formation of acid, but resulted from the action of 

 an enzyme secreted by the micrococcus. 



The coagulated milk is more acid than uncoagulated milk of 

 the same maker; for example, a 20% solution upon being tested 

 showed an acidity to phenolphthalein equal to 42 c.c. of —^ per 

 100 grm., while an uncoagulated milk equalled 24 c.c. Much the 

 same fact was shown in an experiment in which dilute acid was 

 added to 50 c.c. of the 20% solution of the milk in a bottle. The 

 volume of — ■ acid required to produce incipient coagulation, 

 visible as minute specks upon the side of the bottle after shaking, 

 was noted. The coagulated milk required 9*6 c.c, and the 

 uncoagulated ll'l c.c. These experiments should be considered 

 in conjunction with the fact that the tins had been opened, and 

 the milks exposed to the air for some time, the coagulated milk 

 for 7 days, the uncoagulated for a month. 



Experiments were made to determine the lethal temperature, 

 but at the time tliis was done, the micrococcus had apparently 

 deteriorated, as an exposure in milk at 63° for 10 minutes sufficed 

 to kill it. A fortnight later the lethal temperature was found 

 to be 61°. These temperatures cannot be taken as conclusive, 

 and as the death-point was not increased by subsequent sub- 

 culture, the lethal temperature remains unknown. 



With regard to the cultural and other characters of the coccus, 

 it measures 1/i, stains well and is Gram-positive. Upon agar, 

 there forms a porcelain-white, raised and fat-glistening growth. 

 Bouillon becomes turbid and a coherent sediment is produced; 



