28 THE NATURE OF THE ACTIVITIES OF MICROORGANISMS. 



looked upon the process as a purely chemical change; but event- 

 ually Pasteur and others proved that fermentation is a physiological 

 process, brought about only by the growth of yeast. 



For a long time there was no conception of more than one type of 

 fermentation. But even in the days of Schwann it was recognized 

 that there was another type of chemical changes which resembled 

 the yeast fermentation in some respects. This was the sort of 

 changes which occur in the digestion of food and which were known 

 even in those early days to be due to certain materials present in the 

 digestive fluids. As early as 1833 a substance called diastase was 

 known which could convert starch into sugar, and in 1836 pepsin, 

 causing the digestion of proteids in the stomach, was discovered. 

 Although these processes were realized to be different from the 

 fermentation produced by yeast, their general similarity led to their 

 being called fermentations, and the active substance in each case 

 was known as a ferment. 



It very soon appeared that these two types of fermentation were 

 different in some fundamental respects. Whereas alcoholic fermen- 

 tations, produced by yeast, can be stopped by certain chemicals 

 like glycerine, the other type of fermentation, due to digestive fer- 

 ments, cannot be stopped by such materials. Moreover, the 

 microscope shows that the second type of ferments does not contain 

 any living bodies like yeast. Hence, while yeast is a living ferment, 

 the digestive ferment cannot be regarded as riving. But these latter 

 ferments contain some substances which are very peculiar in their 

 nature. Like living organisms, they are destroyed by high heat, and 

 they act only at a moderate temperature. Unlike most simple 

 chemical changes, these fermentations do not occur at high tempera- 

 tures, but become impaired and stopped when the temperature 

 rises slightly above 100 F. It has been found possible to isolate 

 from the fermenting material (saliva, gastric juice, etc.) the ferment- 

 ing body. From the digestive juices a substance can be obtained in 

 the form of a powder which can be preserved indefinitely. It con- 

 tains no living cells, is not alive, and clearly does not belong to the 

 same class of bodies with the yeast plant. But it will cause the 

 fermentation to take place when added to a fermentable substance. 



