FERMENTATION 275 



find in it a trace of alcohol. The yeast has grown and 

 flourished, but it has almost ceased to act as a ferment. 

 And could every individual yeast-cell seize, without any 

 impediment, free oxygen from the surrounding liquid, it 

 is certain that it would cease to act as a ferment altogether. 



What, then, are the conditions under which the yeast- 

 plant must be placed so that it may display its character- 

 istic quality? Eeflection on the facts already referred to 

 suggests a reply, and rigid experiment confirms the sug- 

 gestion. Consider the Alpine cherries in their closed 

 vessel. Consider the beer in its barrel, with a single small 

 aperture open to the air, through which it is observed not 

 to imbibe oxygen, but to pour forth carbonic acid. 

 Whence come the volumes of oxygen necessary to the 

 production of this latter gas? The small quantity of at- 

 mospheric air dissolved in the wort and overlying it would 

 be totally incompetent to supply the necessary oxygen. In 

 no other way can the yeast-plant obtain the gas necessary 

 for its respiration than by wrenching it from surrounding 

 substances in which the oxygen exists, not free, but in a 

 state of combination. It decomposes the sugar of the 

 solution in which it grows, produces heat, breathes forth 

 carbonic acid gas, and one of the liquid products of the 

 decomposition is our familiar alcohol. The act of fer- 

 mentation, then, is a result of the effort of the little plant 

 to maintain its respiration by means of combined oxygen, 

 when its supply of free oxygen is cut off. As defined by 

 Pasteur, fermentation is life without air. 



But here the knowledge of that thorough investigator 

 comes to our aid to warn us against errors which have been 

 committed over and over again. It is not all yeast-cells 

 that can thus live without air and provoke fermentation. 



