PUTREFACTIVE PROCESSES. 29 



Putrefactive processes. The decomposition changes of the 

 casein and albumen of milk by bacterial action are less notice- 

 able than the decomposition of milk sugar, but more important 

 from a hygienic point of view. Such alteration of nitrogenous 

 compounds is designated putrefaction, and in the process the 

 milk assumes an alkaline reaction. Among the direct products 

 of the bacteria, or decomposition products of nitrogenous com- 

 pounds produced by them, are some that produce toxic effects 

 in man. The changes under ordinary conditions are not such 

 as to call the attention of the consumer to the condition, as in 

 the case of soured milk. 



Organisms causing putrefaction. As in the case of the lactic 

 acid fermentation, a great number of species are capable of 

 inducing putrefaction. The organisms are frequently spore 

 bearers, and when cultivated on gelatin usually cause lique- 

 faction of that medium. The change is induced by enzymes 

 elaborated by the organisms and acting thereafter quite inde- 

 pendently of them. This action is- illustrated in gelatin by 

 the area of liquefaction surrounding the growth of the organ- 

 ism. In milk the action on the nitrogenous constituents may 

 or may not be preceded by a curdling of the milk by enzyme 

 action. Milk poorly cooled in hot weather will sometimes 

 curdle, showing a weakly acid or alkaline reaction, due to 

 enzyme action. The group of organisms of which Bacillus 

 subtilis is the type is a common representative of the putre- 

 factive organisms. 



Antagonism of lactic acid fertnentation. The active progress 

 of putrefactive changes does not occur under conditions favor- 

 ing the souring of milk, for the rapid multiplication of the 

 lactic acid organisms creates conditions unfavorable for putre- 

 faction (5). In this sense, souring may be considered as a 

 beneficent process, protecting milk against a worse decomposi- 

 tion. 



Conditions controlling putrefaction . Putrefactive organisms 

 are able to multiply at temperatures somewhat below 20° C. 

 more rapidly than the lactic acid organisms. Hence, milk 

 kept at low temperatures is more apt to undergo putrefaction 

 than souring. The point should be made that at such tempera- 



Digitized by Microsoft® 



