THE PASTEURIZATION OF MILK 507 



Koske, and Hertel, however, have made a series of experiments with 

 milk from cows with udder and other forms of tuberculosis and have 

 subjected the milk to temperatures of 85, 90, 95, and 100 C. by 

 the continuous and discontinuous method in apparatuses of various 

 construction. Some of the tests furnished the very remarkable result 

 that at any of the above temperatures tubercle bacilli in the milk 

 from cows with udder tuberculosis were not destroyed and were able to 

 produce tuberculosis by subsequent inoculations into guinea-pigs and 

 in feeding to young pigs. Such positive results were obtained in four 

 series of experiments in which the continuous method and 100 C* 

 were used, and in several experiments in which the discontinuous 

 method, temperatures of 98 C., and time exposures from sixty to 

 one hundred and five seconds were employed. The milk samples 

 used in these tests were very bad and some of the resisting tubercle 

 bacilli were evidently inclosed in casein or pus coagula. Yet it 

 cannot be denied that under certain conditions tubercle bacilli in 

 milk will survive exposures to 100 C. and to 98 C. for over one 

 hundred seconds. Foster and DeMan, using milk from tubercular 

 udders, found that tubercle bacilli were killed at the following tem- 

 peratures and periods : 



At 55,C. (131 F.) after four hours. 



At*60 t C. (140 F.) after one hour. 



At 65 C. (149 F.) after fifteen minutes. 



At 70 C. (158 F.) after ten minutes. 



At 80 C. (176 F.) after five minutes. 



At 90 C. (194 F.) after two minutes. 



At 95 C. (203 F.) after one minute. 



That commercial pasteurization, as frequently practised in this 

 country, does not kill all tubercle bacilli in milk has been proved a 

 number of times by guinea-pig inoculation of pasteurized milk. 



The heating of the milk in pasteurization produces certain changes. 

 When milk is heated by the continuous method to 85 C., but cooled 

 rapidly, little change in taste and smell is produced, though a trace of 

 "boiled taste" may be noticeable. This is, as a rule, more marked in the 

 longer heating at lower temperatures. The same is true of cream and 

 skimmed milk after pasteurization. The formation of cream occurs 

 somewhat more slowly in heated than in non-heated milk, but the 

 yield in butter fat is somewhat larger in the former, as already stated. 

 The greatest effect of the heating of the milk in pasteurization is 

 undoubtedly exerted upon the enzymes of the milk. Even if they 

 are not all completely destroyed their action is undoubtedly much 

 weakened and modified so that from a purely physiological standpoint 

 pasteurized milk is of inferior value in the nutrition of infants 

 when compared with first-class raw milk. Tests have been devised 

 particularly for one of the groups of enzymes in milk, i. e., the per- 

 oxydases which are oxidizing ferments and transfer the oxygen in 

 metabolic processes of the organism. These tests can be used to 

 discover whether milk has been heated to a certain temperature. 



