410 THE BACTERIAL ASPECTS OF PASTEURIZATION 



90° C.) for two minutes, but is little influenced at i42°-i45° F. (6i°-63°C.) for thirty 

 minutes. At most the germicidal action is feeble and transitory and cannot take the 

 place of cleanliness and ice, but may be taken advantage of by prompt use of fresh 

 milk. It is true that bacteria start to multiply more quickly in heated milk than in 

 raw milk, provided the raw milk is fresh; it should be remembered, however, that 

 stale milk, even milk that is a day old, no longer possesses this restraining action. 

 The germicidal property is, therefore, ordinarily absent from market milk. 



Rosenau and McCoy' have shown that, judged by the number of colonies that 

 develop upon agar plates, the bacteria in milk first diminish, then increase, in number. 

 This occurs only in raw milk during the first eight or twelve hours after it is drawn. 

 Although the bacteria seemingly decrease in numbers, they never entirely disappear. 

 After the initial decrease there is a continuous and rapid increase, until the milk 

 contains almost infinite numbers in each cubic centimeter. The power of milk to re- 

 strain the development of bacteria lasts from six to twenty-four hours, depending upon 

 the temperature at which the milk is kept. When the milk is kept warm, 37° C, the 

 decrease is pronounced within the first eight or ten hours; after this the milk has en- 

 tirely lost its restraining action. When the milk is kept cool, 15° C, the decrease is 

 less marked but more prolonged. They further showed that at least part of this de- 

 crease is due to agglutination. Chambers^ has shown a true germicidal decrease un- 

 der certain conditions. He finds the action quite specific, some bacteria decreasing 

 while others increase. 



DISEASES SPREAD BY MILK 



Diseases known to be conveyed through milk are tuberculosis, typhoid and para- 

 typhoid fevers, diphtheria, scarlet fever, septic sore throat, Malta fever, foot-and- 

 mouth disease, and milk-sickness; also some of the summer complaints of children 

 and the diarrhea and dysenteric diseases of adults, which are often referable to in- 

 fected milk. Epidemic arthritic erythema and infantile paralysis have recently been 

 added to this list. In addition to the specific diseases, milk may be harmful from 

 other causes. 



As a rule, milk becomes infected from human sources, sometimes on the farm, 

 sometimes at the dairy, sometimes in transportation, and occasionally in the house- 

 hold. Sometimes the milk becomes infected as a result of disease of the animal, as in 

 the case of bovine tuberculosis, foot-and-mouth disease, streptococcal garget from 

 cows, and Malta fever from goats. 



When all the facts are brought together, they make a strong indictment against 

 raw milk. Thus Table I, giving the official record^ of milk-borne infections in Mas- 

 sachusetts, gives the facts from 1907 to 1923; the story is brought up to date in a 

 report just off the press. ^ Since the general adoption of pasteurization for the milk 

 supplies of the large cities in Massachusetts, milk-borne outbreaks occur character- 

 istically in the small unpasteurized supplies. 



' Rosenau, M. J., and McCoy, G. W.: U.S. Pub. Health Bull. 56, 1909. 

 ^ Chambers, W. H.: J. Bad., 5, 527. 1920. 



3 Kelley, E. R., and Webber, W. J.: Am. J. Pub. Health, 14, 963. 1924. 

 ■< Bigelow, G. H., and Forsbeck, F. C: Am. J. Pub. Health, 17, 1019. 1927. 



