FERMENTATION. 43 



of fermentation which we now recognize. But even in the 

 days of Schwann it was recognized that there was another 

 type of chemical change which resembled the yeast fermenta- 

 tion 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 diastase was known which 

 could convert starch into sugar, and in 1836 pepsin, causing 

 the digestion of proteids in gastric digestion, was discovered. 

 Although these processes were seen 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. It was found 

 that whereas the fermentations akin to that produced by yeast 

 could be stopped by various chemicals, like glycerine, the 

 other type of fermentation could not be stopped by such ma- 

 terials. It was found that the second type of ferments did not 

 contain any living bodies which could be detected by micro- 

 scopic study, and they hence could not be regarded as living. 

 Nevertheless these ferments contained some substances which 

 were very peculiar in their nature. Like living organisms they 

 were destroyed by high heat and they acted only at a mod- 

 erate temperature. Unlike simple chemical changes these fer- 

 mentations do not occur at high temperatures but become im- 

 paired and stopped when the temperature rises slightly above 

 1 00 F. Eventually it was found possible to isolate from the 

 fermenting material (saliva, gastric juice, etc.) the fermenting 

 body in a tolerably pure form. It was then found to be a 

 substance which could be obtained in the form of a powder 

 and preserved indefinitely. It contained no living cells, was 

 not alive and clearly did not belong to the same class of bodies 

 with the yeast plant. 



