120 PASTEURIZATION OF MILK. 



heated milk, and the medium in which they are present, en- 

 tirely preclude comparisons with ordinary milk showing a 

 similar count. 



Prescott (10) found in a series of 32 observations, from 

 samples containing mostly less than a million, that a machine 

 at an average temperature approximately 164°F. attained an 

 average percentage efficiency of 97.1% . The bacteria remain- 

 ing in the milk averaged 14,000 per cc. and the average 

 temperature of the pasteurized milk was 40°C. Very bad 

 milk, three to nine million, showed the high efficiency of 

 99.1%, with an average of 55,090 per cc. left alive. In regu- 

 lar practice the conditions for highest efficiency were not 

 always maintained. Russell (15) has summarized the results 

 of a number of such tests made by hirpself and others. 



Continuous-flow machines popular. The commercial interests 

 have refused to adopt generally the discontinuous or tank type 

 of pasteurizing machine for market milk, on account of 

 expense and time consumed, although it is used for cream. 

 The rapid, cheaply operated, continuous flow machine, run- 

 ning at 140° to 165°F., is a factor in the market milk business 

 to-day. Lederle (8) estimates the expense of treating milk by 

 this process to be from one-tenth to one-quarter of one cent 

 per quart. However much bacteriologists may prefer the use 

 of 140''F. for twenty minutes, the other practice is deeply 

 rooted. It has been estimated that one-quarter of the milk 

 sold in New York is pasteurized and about one-third of that in 

 Boston (12). 



Under the circumstances, it seems that a fruitful field for 

 persuasive educational effort is furnished by those instances 

 where the machine is so run that the milk is exposed too short 

 a time at a temperature below 160°F. It is entirely possible 

 to expose milk to 160"F. for a minute or even two, without 

 injuring the cream line or flavor, although it probably does 

 impair certain vital characteristics. Effort should be made to 

 have the machines run at 160°F. with the flow of milk slow 

 enough to permit exposure to the heat for at least one minute. 

 Better yet, the employment of machines heating to 140°F. for 

 twenty or thirty minutes should be encouraged. 



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