APPLICATIONS OF LACTIC ACID FERMENTATION 115 



The table first shows the behaviour of the uninoculated milk on 

 standing ; at 20 C. it was practically unaltered after twenty-eight 

 hours, which is explained by the slow growth of the heat-resisting 

 bacteria at the ordinary temperature, whereas at 40 C. it was 

 affected fairly quickly. The sudden interruption of the increase 

 in acidity on heating the milk above 75 C. is explained by the fact 

 that the" heat-resisting lactic acid bacteria will survive heating to 

 75 C., but not to 80 C. ; microscopic examination confirmed this 

 view,'as streptococci were found in great numbers in the milk heated 

 to 75C., but none were found in that heated to 80 C. or more 1 . From 

 the results already considered, we may draw the conclusion that 

 it is impossible to be sure of the exclusion of foreign lactic acid 

 bacteria from the starter unless the milk has been heated to at least 

 80 C. Microscopic examination also shows the milk pasteurised 

 at 75 C. to contain most hay bacilli after standing, 75 C. being the 

 lowest temperature at which the bactericidal substances of the 

 milk are completely destroyed. These points are particularly 

 well illustrated in the case of the milk which was kept at 64 C. ; in 

 this milk only those thermophile bacteria grew which were not in 

 the slightest degree affected by the pasteurisation, but which, on 

 the other hand, seem to be fairly sensitive towards the bactericidal 

 substances in the milk. This explains the somewhat paradoxical 

 result that under these conditions the milk which has been heated 

 to 75 C. or over is that which is most rapidly affected. 



The conditions are still more complicated in the inoculated milk. 

 In the raw as well as in the low temperature pasteurised milk the 

 lactic acid fermentation is inhibited by the bactericidal substances 

 but encouraged by the lactic acid bacteria of the milk itself, and 

 in the milk heated up to 70, 75 and 80 C., a sharp struggle for 

 existence takes place between the lactic acid and the hay bacteria, 

 which often results favourably for the latter at the outset. Finally, 

 it must be borne in mind that the higher the temperature to which 

 the milk has been heated, the less oxygen will it contain. As the 

 different species of lactic acid bacteria are differently affected by 

 the conditions under consideration, no definite rule for pasteurisa- 

 tion can be laid down on this head. Thus it was found that 

 Streptococcus cremoris and the nearly related Sc. thermophilus 

 develop most slowly in the milk heated to 75 C. (and the latter also 

 in the milk heated to 70 C.), while Sc. lactis shows a minimum 

 capacity for souring (though not very pronounced) in the milk 

 which has been pasteurised at low temperatures and retained its 

 bactericidal constituents. The point of special interest is that the 

 vigorous lactic acid culture (starter) is only slightly affected by the 



1 Living micrococci are, however, still found in milk pasteurised at 80 C. ; 

 they grow best at 20 C. 



