CONDITIONS OF ALCOHOLIC FERMENTATION 79 



By experimenting with several flasks of this kind it can 

 be proved that fermentation goes on as well in darkness as 

 in light, and that it is quite independent of free oxygen. 

 Indeed the process does not go on if free oxygen i.e., 

 oxygen in the form of dissolved gas is present in the fluid ; 

 from which it would seem that Saccharomyces must be able 

 to obtain the oxygen, which like all other organisms it 

 requires for its metabolic processes, from the food supplied 

 to it. 



The process of fermentation goes on most actively 

 between 28 and 34C : at low temperature it is com- 

 paratively slow, and at 38C. multiplication ceases. 



If a small portion of yeast is boiled so as to kill the 

 cells, and then added to a flask of Pasteur's solution, no 

 fermentation takes place, from which it is proved that the de- 

 composition of sugar is effected by the living yeast-cells only. 

 There seems to be no doubt that the property of exciting 

 alcoholic fermentation is a function of the living protoplasm 

 of Saccharomyces. The yeast-plant is therefore known as 

 an organized ferment : when growing in a saccharine solu- 

 tion it not only performs the ordinary metabolic processes 

 necessary for its own existence, but induces decomposition 

 of the sugar present, this decomposition being unaccom- 

 panied by any corresponding change in the yeast-plant 

 itself. 



It is necessary to mention in this connection that there is 

 an important group of not-living bodies which produce 

 striking chemical changes in various substances with- 

 out themselves undergoing any change : these are distin- 

 guished as unorganized ferments. A well-known example is 

 pepsin, which is found in the gastric juice of the higher 

 animals, and has the function of converting proteids into 

 peptones (see p. 12) : its presence has been proved in 



