92 



Dairy Bacteriology. 



1. A temperature above the maximum growing- point 

 (42-45 C.) and below the the thermal death-point (52- 

 68 C.) will prevent further growth, and consequently 

 fermentative action. 



2. A temperature above the thermal death-point de- 

 stroys bacteria, and thereby stops all changes. This tem- 

 perature varies, however, with the condition of the bac- 

 teria being much higher for spore-bearing species. 



Attempts have been made to employ the first principle, 

 viz., prolonged heating above growing temperature, but 

 when milk is so heated, its physical appearance is changed. 

 The methods of heating most satisfactorily used are 



known as sterilization 

 and pasteurization, in 

 which a degree of tem- 

 perature is used approxi- 

 mating the boiling- and 

 scalding-points respect- 

 ively. 



92. Characteristics 

 of heated milk. When 

 milk is subjected to the 

 action of heat, a num- 

 ber of changes take place 

 in its reactions. 



1 . Thinner body . Milk , 



FIG. 15. Microscopic appearance of nor- 



mal milk. The fat globules are grouped but more especially 



together into tiny clots. The consistency cream heated to a tem- 

 or body 01 cream is in part due to this char- m 



acteristic. perature exceeding 150 



F., becomes much thin- 

 ner, a condition due to a change in the grouping of the 

 fat globules. 1 In normal milk, the butter-fat is massed 

 together in microscopic clots as in fig. 15. In heated 

 1 Babcock & Russell, 13th Kept. Wis. Stat., p. 73, 1896. 



