MICRO-ORGANISMS: ACTION ON MILK. 445 



Those acting on proteins (a) curdling milk without acidity and not 



dissolving the curd ; 



(b) curdling milk without acidity and after- 



wards dissolving the curd ; 



(c) peptonising the proteins without curd- 



ling the milk ; 



(d) producing evil-smelling sulphur com- 



pounds. 



Those producing coloured substances. 



Those having no action on milk. 



We may also place in another class those which are pathogenic. 



Milk is a model food for micro-organisms, for it contains in 

 an assimilable form all those compounds which are necessary for 

 the sustenance of life. 



It has been shown by experiment that it is possible, though 

 not easy, to obtain milk which is quite free from micro-organisms. 

 It is necessary, however, to reject the first portions drawn, as 

 these contain micro-organisms which have found their way down 

 the duct of the teat. The last portions are practically sterile, 

 and it is highly probable the few organisms found are due to 

 accidental contamination of the milk during its passage from 

 the teat to the sterilised bottle into which it was drawn. Prac- 

 tically speaking, all the organisms found in milk fall in after 

 milking ; in certain diseases e.g., tuberculosis of the udder 

 the Bacillus of tuberculosis is not derived from external sources, 

 but passes from the diseased tissue into the milk. 



Generally speaking, micro-organisms only develop between 

 the temperatures of 4 C. (39 F.) and 50 C. (122 F.) ; each 

 organism has an optimum temperature i.e., one at which its 

 development and action are most rapid ; this varies from 12 C. 

 (53-6 F.) to 40 C. (104 F.) in different species; the optimum 

 temperature of pathogenic organisms and of most of those acting 

 on milk is about blood-heat. Among other conditions which 

 regulate their development are (1) the amount of acid present 

 in the milk thus most of the organisms which produce lactic 

 acid are paralysed in their functions when more than about 

 1 per cent, has been produced ; and (2) the presence or absence 

 of oxygen. Some organisms can do without oxygen, and are 

 called anaerobic ; others require it for their life processes, and 

 are designated aerobic. 



Growth of Bacteria in Pasteurised and Unpasteurised 

 Milk. L. A. Rogers has made experiments on the average 

 acidity of raw and pasteurised milk, and the number of bacteria 

 present at different times. 



The following are his results : 



