114: Dairy Bacteriology. 



ization has made considerable headway, not only in sup- 

 plying a milk that is designed to serve as children's food, 

 but even for general purposes. 



Conditions that determine pasteurizing limits. Consid- 

 erable latitude with reference to temperature limits is per- 

 missible in pasteurizing, but there are certain conditions 

 which should be met, and these, in a sense, fix the limits 

 employed. They are as follows: 



1. Physical requirement. Inasmuch as it is undesirable 

 to have any material change in taste and appearance in 

 pasteurized milk from that normally found in the raw pro- 

 duct, the pasteurizing temperature should be limited to the 

 degree of heat that can safely be employed without any 

 danger of imparting a cooked or scalded flavor to milk. If 

 the exposure is made for any considerable period of time, 

 say fifteen to twenty minutes, this change in taste appears 

 to be quite permanent when the milk is heated to 158 F. 

 This condition, therefore, determines the maximum limit 

 that should be used in pasteurizing, if one is to avoid 

 the production of a cooked flavor. Even below this tem- 

 perature a slight change in flavor occurs, although it dis- 

 appears upon chilling the milk- Where access of air is 

 excluded during heating, this cooked taste does not develop 

 so markedly. 



2. Biological requirement. To be of value in increasing 

 the keeping quality of milk and to insure freedom from 

 disease bacteria, it is necessary, in all cases, to exceed the 

 thermal death-point of at least the actively developing bac- 

 teria in the milk. For most bacteria this limit is constant 

 and quite sharply defined, ranging from 130 to 140 F. 

 where the exposure is made for ten minutes. Where ex- 

 posed for a briefer period of time, the temperature limit is 



