290 



MILK AND ITS HYGIENIC RELATIONS 



temperature for twenty minutes may be considered safe so far 

 as conveying infection from the micro-organisms tested is con- 

 cerned.' The micro-organisms included the Bacillus Tuberculosis. 



For the most part the investigations which have been carried 

 out on the thermal death-point of bacteria in milk have been 

 conducted entirely with a view to determining the lowest tempera- 

 ture to which it was necessary to heat milk in order to render it safe. 



The investigations, which have been described in detail in 

 Chap. XI, show that milk used for the feeding of infants may 

 be raised without disadvantage for a short, or even a prolonged 

 period, to a temperature which is approximately 100 C., or 212 F. 

 It is shown on p. 296 that this temperature is not reached when 

 milk is heated surrounded by a water-jacket. 



The conclusion which may be fairly made, is that milk when 

 used as a food in the home may be heated to 100 C. approximately 

 without any detrimental effect. Such heating, moreover, acts as 

 an efficient safeguard against the causation of disease by harmful 

 organisms accidentally present in the milk. 



The destruction of bacteria in the general milk supply assumes 

 a slightly different character from that in the home. 



In the former case economy of fuel, appearance of the cream- 

 line, etc., all tend to keep the temperature employed, as low as 

 may be consistent with a destruction of the main mass of bacteria. 



The bacteria which survive pasteurisation have been studied 

 in great detail by Ayers and Johnson (2) for the United States De- 

 partment of Agriculture. These authors show that the majority of 

 organisms are killed within the first half-hour of heating. Reduction 

 does continue when the milk is heated for longer periods, but this 

 reduction is proportionately small compared with that which 

 occurs in the earlier period. The greater number of bacteria are 

 destroyed in the first half-hour even at comparatively low tem- 

 peratures. This is shown both in the figure on the next page 

 and also in the tables appertaining to it, which are given below. 



The figures for the number of bacteria are given below, and 

 show the efficacy of the comparatively small difference of tem- 

 perature existing between 57-2 and 60 C. 



Table showing Bacterial Reduction during Pasteurisation for Three Hours at 

 54-4 C. (130 F.), 57-2 C. (135 F.), and 60 C. (140 F.). 



