30 FERMENTATION, PUTREFACTION, AND DECAY 
vigorously combated for years by Liebig, who looked upon the 
process as a purely chemical change; but eventually 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 recog- 
nized that there was another type of chemical changes which re- 
sembled 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 fe.mentation 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 
fermentations, produced by yeast, can be stopped by certain 
chemicals like glycerine, the other type of fermentation, due to 
digestive ferments, cannot be stopped by such materials. More- 
over, 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 living. 
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 temperatures, but become impaired and stopped 
when the temperature rises slightly above 1oo°F. It has been 
found possible to isolate from the fermenting material (saliva, 
gastric juice, etc.) the fermenting body. From the digestive 
Juices a substance can be obtained in the form of a powder which 
