PRESENT STATUS OF PASTEURIZATION OF MILK. 13 



thermal death point and, on account of the survival of a few cells, 

 survive the pasteurizing process. 



The colon test as an index of the efficiency of the process of pasteur- 

 ization is complicated by the ability of certain strains to survive a 

 temperature of 145° F. for 30 minutes and to develop rapidly when 

 the pasteurized milk is held under certain temperature conditions 

 which might be met during storage and delivery. Consequently the 

 presence of a few colon bacilli in pasteurized milk under ordinary 

 market conditions does not necessarily indicate that the milk was not 

 properly heated. The presence of a large number of colon bacilli 

 immediately after the heating process may, however, indicate 

 improper treatment of the milk. 



If milk is pasteurized at a temperature of 150° F. or above for 

 30 minutes, it is not to be expected that any colon bacilli will survive ; 

 consequently under such conditions the colon test for the effectiveness 

 of pasteurization may be of value. It must be remembered, however, 

 that a study of more cultures may reveal strains of colon bacilli that 

 are able to survive this or even a higher temperature. 



MODERN THEORIES OF PASTEURIZATION. 



Pasteurization at present is looked upon with favor by medical 

 men, sanitarians, dairymen, and consumers, but the art has not been 

 developed without opposition, and even now its value is not univer- 

 sally accepted. Most of the objections to pasteurized milk have been 

 based on theory or on experiments in which the milk was pasteurized 

 at high temperatures and in view of our modern theories are of no 

 great importance. 



One of the greatest objections to pasteurized milk has been that the 

 heating destroyed the lactic-acid bacteria and that putrefactive organ- 

 isms were left, which, when relieved from the restraining action of the 

 acid-forming bacteria, would develop, forming toxins and putrefac- 

 tive products (12). It was believed that the milk, because it was not 

 sour, would be consumed in that condition. This objection was based 

 on experiments in which milk was heated to temperatures near the 

 boiling point and can not be applied to milk pasteurized at low tem- 

 peratures. From the results of seven years' work in the Dairy Divi- 

 sion on commercial pasteurized milk it has been found that such milk 

 sours, as raw milk does, but that the souring is delayed. Pasteuriza- 

 tion for 30 minutes at temperatures of about 145° F., as is generally 

 practiced in this country, does not destroy all the lactic-acid organ- 

 isms, and those which survive play an important role in the souring 

 of commercially pasteurized milk. 



Another objection to pasteurized milk has been that bacteria grow 

 faster in it than in raw milk. In spite of several experiments which 



