382 MILK AND THE PUBLIC HEALTH chap. 



bacteria. The number of peptonisers in a good grade of com- 

 mercially pasteurised milk on the initial count and on succeed- 

 ing days was found to be approximately the same as in a clean 

 raw milk when kept under similar temperature conditions. 



Contrary to the views generally expressed, the authors 

 found that lactic acid bacilli were always present after 

 pasteurisation. They found that commercially pasteurised milk 

 always sours because of the development of certain lactic acid 

 bacilli, which, on account of their high thermal death-point, 

 survive pasteurisation, and perhaps in some cases because of 

 subsequent infection with acid-forming bacteria during cooling 

 and bottling. They found that when milk was heated for 30 

 minutes at 60° C, on an average about 4'8 per cent of the 

 total acid colonies resisted the heating, when heated to 65"6° C. 

 about 0"74 per cent survived. The authors consider that these 

 heat-resisting lactic acid bacilli play an important role in 

 pasteurised milk. They also found that the total bacterial 

 increase in an efficiently pasteurised milk and a clean raw 

 milk was about the same when the samples were kept under 

 similar temperature conditions. 



(3) While these experiments fail to support the view that 

 the bacteria surviving pasteurisation are more toxic in kind, 

 and produce harmful products in stored pasteurised milk, they 

 do not refute the possibility that if pathogenic bacteria gain 

 access to a milk after pasteurisation, they are likely to more 

 abundantly multiply in such milk than in ordinary raw milk. 

 Since the bacterial content is so much less this is certainly 

 likely to occur. This liability shows the great importance 

 of rapidly cooling pasteurised milk, and the need for prevent- 

 ing any bacterial contamination of milk after pasteurisation. 

 In illustration of this it may be mentioned that a number 

 of food -poisoning outbreaks from ice-cream, brawn, etc., 

 owe some, if not most, of their virulent effects to the fact 

 that the food-poisoning bacilli, added subsequent to the partial 

 cooking, have found themselves in an environment largely 

 freed from competitive bacteria and heated to a temperature 

 just suited to their growth. 



(4) A further objection to pasteurisation is that although 

 pasteurised milk will sour it sours much later, and so may 

 be, and in practice probably often is, kept for a number of 



