156 PHYSICAL INFLUENCES. 



isms to the r61e they play in nature can be illustrated by a few examples. 

 Most molds cannot cause disease in man and warm-blooded animals 

 because their maximum temperature is below the body temperature. 

 Exceptions are some A spergilli and Mucors. Pathogenic microorganisms 

 must have their optimum temperature coincide with that of their host. 



Organic substances may undergo a different change at different 

 temperatures. The biochemical changes in soil may not be the same in 

 northern Canada and near the Gulf of Mexico. Even the warm and 

 cold season of the same climate is apt to change not only the rate of 

 decomposition but possibly the products. Perhaps the most striking 

 example in this respect is the decomposition of ordinary market milk 

 kept at different temperatures. Such milk contains a great variety of 

 microorganisms; at various temperatures different types will predominate, 

 while the remainder are retarded or inhibited by unfavorable temperature 

 conditions and by the products of the dominant type of bacteria. If 

 milk is kept at about the freezing-point, only a few organisms will develop 

 slowly, but after a certain time their number will increase to many million 

 cells per c.c. There is, however, no apparent change ; no acid or deteriora- 

 tion can be discovered by the taste though chemical analysis proves the 

 presence of hydrogen sulphide and ammonia. Between 15 and 25, 

 milk will sour in about thirty-six to forty-eight hours, giving a firm curd 

 of an agreeable flavor without whey or gas; later Oldium lactis destroying 

 the acid develops on the surface. Near body temperature the milk will 

 lopper in twenty-four hours, the curd is usually contracted, a large quan- 

 tity of whey is extruded, and much gas is produced by Bad. aerogenes and 

 B. coll. The odor is disagreeable and later butyric acid is produced; 

 eventually the lactic acid increases further by the action of Bad. bulgaricum 

 If kept above 50 the milk usually keeps for twenty-four hours, but after 

 that a decomposition by thermophile bacteria begins which is either an 

 acid fermentation followed by digestion or a complete putrefaction 

 depending upon the species of thermophile organism that happens to be 

 in the milk sample. Thus there is in the same substance, containing the 

 same organisms at the start, four entirely different types of decomposition 

 induced only by the difference of temperature. 



This indicates the importance of temperature regulation in the fer- 

 mentation industries. Even pure cultures may give different products 

 if working at different temperatures. Cream ripened with a pure culture 

 starter at too high a temperature will have a sharp acid flavor. The cold 



