462 



MICROBIOLOGY OF MILK AND MILK PRODUCTS 



The work of other investigators seems to show that the so-called 

 germicidal action is felt by certain species and not by others as is indi- 

 cated by the following sample. 



This would seem to indicate that the decrease in number is due not so 

 much to a definite germicidal property possessed by the milk as to the 

 gradual dying out of certain species which for some reason do not find 

 the milk a suitable environment for development, while other types, 

 finding the milk suitable to their needs, develop uniformly from the 

 start. 



Rosenau and McCoy found that the germicidal properties of milk 

 were destroyed by boiling or by heating it above 80 and that lower 

 temperatures destroyed it for certain organisms. These workers also 

 found that there was marked agglutination of the organisms in raw milk 

 and conclude that this accounts for the decreased number of colonies 

 developing in plate cultures and that the germicidal action is therefore 

 more apparent than real. 



SECOND STAGE. PERIOD FROM END OF GERMICIDAL ACTION TO 

 TIME OF CURDLING. The period following immediately after the ger- 

 micidal action is characterized by the rapid development of the lactic 

 organisms. Under normal conditions this group develops much more 

 rapidly than any other type. Not only do they increase rapidly in 

 actual numbers but their percentage also rises rapidly. There may 

 be a continual increase in numbers in the other species, but their growth 

 is much less rapid than that of the Bad. lactis acidi type. As this period 

 advances certain of the miscellaneous types may cease to grow entirely. 

 During this time the gas-producing acid organisms of the B. coli and 

 Bact. lactis aerogenes type may develop more or less rapidly, but if the 

 milk is held at temperatures not much above 20, the Bact. lactis acidi 

 type will develop much more rapidly, so that by the time the milk be- 

 comes sour and curdles, this type will constitute 99 per cent approxi- 



