404 THE BACTERIAL ASPECTS OF PASTEURIZATION 



They develop at 37° C. and are not the same as the true therm ophiles occasionally 

 found in pasteurized milk, although they have probably been mistaken for the ther- 

 mophiles on account of the smallness of the colonies and the resistance to heat. The 

 source of these bacteria has not been disclosed, although there is some evidence that 

 they come from the utensils which are not kept scrupulously clean or thoroughly 

 steamed. 



Heat-resisting bacteria do not grow very rapidly in milk, especially when the milk 

 is kept at lower temperatures. They do not seriously affect the keeping quality of 

 the milk at 60° F. or lower, and while they have no known pathogenic effect, they 

 indicate neglect somewhere along the journey of the milk as it passes from the cow 

 to the final container. There is very little difference in the percentage reduction and 

 the bacterial count between milk pasteurized at 140° and at 145° F. 



THERMAL DEATH-POINTS 



The time and temperature of pasteurization depend upon the time and tempera- 

 ture necessary to kill pathogenic micro-organisms that are a hazard to man if taken 

 in milk. We need not concern ourselves with the spores of tetanus which often get 

 into milk, for these are not harmful when taken by mouth. Fortunately, all the 

 milk-borne diseases that concern man are due to viruses that are relatively thermo- 

 labile and have no resistant spores. It has been rightly assumed that any tempera- 

 ture and time that will surely kill the tubercle bacillus will also kill the other micro- 

 organisms in question, because the tubercle bacillus, on account of its waxy substance, 

 is somewhat more resistant than the ordinary spore-free bacteria. Much work has 

 therefore been done to determine with precision the thermal death-points of the 

 tubercle bacillus. 



Confusion and misunderstanding have resulted from a misapprehension that mi- 

 cro-organisms have a thermal death-point. There are, in fact, an infinite series of 

 thermal death-points, depending upon the time of exposure. Together these points 

 make a curve, from which it is clear that the temperature is a function of the time. 

 This is well shown in Charts I and II. 



THERMAL DEATH-POINTS OF THE TUBERCLE BACILLUS 



By common consent, the killing of tubercle bacilli in milk has become the criterion 

 by which the public-health efficiency of the process is judged. Attention therefore 

 has been focused upon the temperature and time necessary to kill this micro-organ- 

 ism. 



Since the first report in 1879 by Galtier, eighty-nine separate reports of investi- 

 gations by bacteriologists in all parts of the world upon the thermal death-points of 

 the tubercle bacillus have appeared in the literature. A detailed and critical review 

 of all the work on this important question is admirably summarized by North and 

 Park' with a complete bibliography to date. 



It was found that only a few of the earlier studies were based upon methods which 

 are now considered reliable. Thus, five of the nine studies made before Galtier's 

 work in 1879 reported that actual boiling would not kill the "tubercular virus. "^ 



'North, C. E., and Park, W. H.: Am. J. Hyg., 7, i47- iQ^?- 

 ' Koch discovered the tubercle hacilhis in 1S82. 



