154 MANUAL OF BACTERIOLOGY. 



containing the tubercle bacillus it is necessary to heat the milk 

 for 3 hour at 55 C. or 30 minutes at 70 C., or from 3 to 5 

 minutes at 100 C. Bang found that 2 minutes, heating at 

 00 C. caused a marked diminution in the virulence of the 

 tubercle bacilli in milk. Morgenroth found that a very short 

 heating at 100 C. sufficed to diminish the virulence 

 materially. 



Rosenau * found that in milk the tubercle bacillus loses its 

 virulence and infective power, i. e., is killed, by exposure to 

 60 C. for twenty minutes. 



The investigations of Park and Holt| show that in New 

 York City the number of bacteria in milk is much smaller in 

 winter than in summer, and has little effect on the health of 

 infants during cold weather; but that in warm weather with 

 milk of average quality the infants who received sterilized 

 milk throve on the average much better than those who re- 

 ceived raw milk. 



The process called pasteurization is designed, not to sterilize 

 the milk completely, but to destroy the vegetative forms 

 of bacteria, and to destroy the ordinary pathogenic bacteria 

 with which the milk might possibly be contaminated. J The 

 milk is subjected to a temperature of only about 70 to 75 C. 

 This temperature is less likely to produce alteration in the 

 milk than sterilization by steam at 100 C. According to Free- 

 man, milk which had been pasteurized at 75 C. and distrib- 

 uted among the poor people of New York City, whose homes 

 were not supplied with ice, usually kept very well even in the 

 summer time. 



The number of bacteria in milk may be reduced consider- 

 ably by the use of the centrifuge (separator). 



*Bull. No. 42, Hyg. Lab., U. S. Pub. Health and Mar. Hosp. Serv., Wash. 

 pp. 1-85. Jan., 1908. 



\ Archives of Pediatrics. December, 1903. 



|Theobald Smith. The Thermal Death-point of Tubercle Bacilli in Milk. etc. 

 Journal of Experimental Medicine. Vol. IV., p. 217. 



