SAPROPHYTES 393 



the yeasts go on producing alcohol in the culture until they 

 themselves are killed by it. It has been found that yeasts can 

 stand from 10 to 18 per cent, of alcohol in the culture before 

 being killed by it while nearly all other organisms which grow in 

 such cultures are injured by from 4 to 10 per cent, of alcohol. 



445. Alcoholic Fermentation. Now that you have a more 

 or less clear idea of this process by which yeasts produce 

 alcohol and carbon dioxide from sugar, you are in need of a 

 name for the process. The name in common use is ALCOHOLIC 

 FERMENTATION. Formerly, just the word FERMENTATION 

 was given as the name of the process but since it has been found 

 that the sugar is broken up into the alcohol and carbon diox- 

 ide by an enzyme which is always held within the yeast cell, it 

 is seen that the process is essentially identical with the process 

 of digestion and other processes induced by enzymes. There- 

 fore, the word fermentation has come to be applied to a 

 number of processes which were formerly given different 

 names because they were thought to be essentially different 

 processes. Since we have learned that the processes of decay 

 induced by molds, bacteria, and other fungi are in large measure 

 only the digestion of the decaying material by the organism 

 concerned in the decay, we now speak of these processes as 

 fermentation. Even the digestion that takes place in our 

 own stomachs is often spoken of as fermentation, and the 

 enzymes which bring it about are spoken of as FERMENTS. 



446. Different Kinds of Fermentation. Different kinds of 

 fermentation are often named after the principal product that 

 results from the process. It is in this way that we call the 

 fermentation that yeasts produce alcoholic fermentation. In 

 a similar way, we have BUTYRIC FERMENTATION which is the 

 fermentation of milk sugar by certain bacteria and which pro- 

 duces BUTYRIC ACID. It is this fermentation which underlies 

 the production of the desirable flavors of butter and cheese. 

 Again, LACTIC FERMENTATION is the fermentation of milk 

 sugar by a different kind of bacteria which results in the for- 

 mation of LACTIC ACID, and thus in the souring of milk. When 



