38 PROTOPLASM AND THE CELL 



the negative side, Schwann showed that oxygen had nothing 

 to do with the process. By simply heating the air about fer- 

 mentable fluids which had been properly prepared (sterilized), 

 he found that no fermentation took place, whereas fermenta- 

 tion does occur in such fluids if they are exposed to the ordinary 

 atmospheric air. Schwann concluded that something in ordi- 

 nary air is destroyed by heating, and fermentation is prevented. 

 On the positive side Erxleben in 1818 suggested that the glob- 

 ules discovered by Leeuwenhoek might be the cause of fermen- 

 tation. This view was elaborated and confirmed by Cagniard 

 de Latour in 1835, and was finally conclusively proved by the 

 epoch-making experiments of Pasteur (18571863). 



Yeast was thus proved to be the agent of alcoholic fermenta- 

 tion by acting in some way on the sugar contents of nutrient 

 media. By this action about 95 per cent, of the sugar is broken 

 down into alcohol and CO2; about 4 per cent, is decomposed 

 with the formation of glycerine, succinic acid, and C0 2 ; and 

 about i per cent, is used by the yeast cells as food. The small 

 amount of acetic acid that is usually present is not due to the 

 activity of the yeast cells but to oxidation, through the agency 

 of bacteria, of the alcohol already formed. 



The approximate chemical reactions involved in the forma- 

 tion of alcohol and acetic acid are as follows: 



C 6 H 12 O 6 (sugar) + yeast = 2C 2 H 6 O (alcohol) + 2 CO 2 

 C 2 H C O (alcohol) + O 2 + bacteria = C 2 H 4 O 2 (acetic acid) + H 2 O. 



The acetic acid is finally oxidized to water and CO 2 through 

 the action of bacteria again. Thus 



C 2 H 4 O 2 + 2O 2 + bacteria = 2CO 2 + 2H 2 O 



The next step in the investigation of alcoholic fermentation 

 was to determine just what yeast does in effecting the transfor- 

 mation of sugar into alcohol. Biichner in 1897 was the first 

 to demonstrate that yeast cells contain a substance which 

 causes the same destruction of sugar that living yeast cells do. 

 This was accomplished by grinding yeast cells in diatomaceous 

 earth, then compressing the mass and obtaining a clear fluid 



