278 CITY MILK SUPPLY 



spores of moulds are certainly present and are often abundant in market 

 milk. Studies were made with pure cultures of a series of moulds including 

 several species of Aspergillus and of the mucors with the addition in some 

 experiments of Oidium (oospora) lactis and one strain of Fusarium. Ex- 

 periments were made to test the effect of temperature in the holder, and in 

 the flash processes of pasteurization, and by dry heat. The results agree 

 in general with the bacteriological studies of pasteurization. 



Very few of the spores survived heating in milk to 145F. for 30 min. 

 Certain moulds, notably Aspergillus fumigatus and A. flavus, do survive 

 but they occur only rarely in milk. Oidium lactis and the mucors which 

 are probably more important as milk-borne organisms than all the other 

 moulds, are destroyed at this temperature and exposure. So pasteuriza- 

 tion of milk at 145F. may be regarded as destroying mould spores com- 

 pletely enough to render them a negligible quantity. 



In using the flash process of pasteurization temperatures of 165F. to 

 175F. for 30 sec. are commonly employed. It was found that very few of 

 the moulds were killed at 145F. for 30 sec. but that nearly all were in that 

 time at 155F. and likewise that 30 sec. exposure at 165F. to 175F. 

 destroys practically all spores as they are found in milk, although a few 

 conidia in some species may occasionally survive. The observation 

 that the moulds were killed at temperatures of from 165F. to 175F. is 

 confirmed by the results of Webster of the Bureau of Chemistry showing 

 that commercial butter made from flash-pasteurized cream contained no 

 mould spores whereas cultures from commercial butter contained 20,000 

 to 60,000 per cubic centimeter. 



Multiplication of Bacteria in Pasteurized Milk. Certain investigators 

 were of the opinion that bacteria increased more rapidly in pasteurized 

 than in raw milk and that this constituted a serious objection to its use, 

 for in cooling, bottling and in the subsequent handling of it, the milk was 

 often rather heavily contaminated, but careful work by Ayres and Johnson 

 showed this criticism to be ill-founded for when the bacterial increment 

 in an efficiently pasteurized milk is compared with that in a clean raw 

 milk held at the same temperature, the two are found to be about the 

 same. 



Destruction of Toxins by Pasteurization. Others pointed out that 

 pasteurization probably does not destroy toxins that may develop as the 

 result of bacterial growth in milk. If such toxins exist they have not 

 been isolated, therefore the temperatures at which they are destroyed 

 are unknown. The true bacterial toxins are not destroyed by pasteuri- 

 zation by heating at 140F. for 20 min. but heating milk to this temper- 

 ature kills many germs and so makes the further formation of toxins in 

 it less probable. Of course, raw milk that supports a heavy growth of 

 germs may contain toxins and would be quite as injurious before 

 pasteurization as afterward. 



