502 CURRENT PROBLEMS ON YEASTS 



3. CH3— CO— COOH -> CO2+CH — COH Acetaldehyde 



4. CH3— CO— COH O 1 f CH3— CO— COOH Pyruvic acid 



+ 1 = 

 CH3— COH Hj [CH3— CH3OH Ethyl alcohol 



Some of these reactions have since been verified in the laboratory. The others 

 suggest future experiments. According to this theory, methylglyoxal, the first com- 

 pound with three carbon atoms, is formed from the sugar. It may be formed by the 

 removal of two molecules of carbon dioxide from the hexose molecule. This requires 

 the intermediate stage of methylglyoxal-aldol. The other equations rest on Canniz- 

 zaro* rearrangements on methylglyoxal. 



The evidence for the occurrence of the third reaction is aft"orded by results of 

 fermentations carried out in the presence of alkali bisulphites. The acetaldehyde as 

 it is formed is "fixed" by the sulphite and prevented from undergoing further change. 

 The bisulphite-aldehyde complex then accumulates in large quantities. In compari- 

 son with alcohol the aldehyde is an oxidation product, the formation of which must 

 be accompanied by a reduction product. Glycerol is such a product, and it accumu- 

 lates in amounts equivalent to the amount of aldehyde fixed by the bisulphite. This 

 gives what Neuberg called the "second form of fermentation," which may be ex- 

 pressed : 



C6H..06 ^ CH3COH+CH.OH— CHOH— CH,OH+COa 

 Glucose Acetal- Glycerol Carbon 



dehyde dioxide 



When fermentation is carried out in the presence of alkaline salts (other than 

 bisulphites), there is no fixation of acetaldehyde. It was said to undergo a dismuta- 

 tion into equimolecular amounts of ethyl alcohol and acetic acid by aldehydemutase. 

 Then, for each molecule of aldehyde, there would be produced a molecule of the re- 

 duction product, glycerol. Neuberg called this the "third form of fermentation." It 

 was expressed as follows: 



2C6H:a06+H.O -^ CH3COOH+C.H50H-F 2C3H8O3 +2CO. 

 Glucose Water Acetic Ethyl Glycerol Carbon 



acid alcohol dioxide 



Neuberg's theory has not been accepted by all investigators. Kluyver and 

 Donker,^ for instance, have based an explanation of the processes of fermentation 

 on Wieland's dehydrogenation theory. The protoplasm of the yeast plays the role 

 of the hydrogen transporting catalyst. The affinity for hydrogen varies with the 

 protoj)lasm. Following is the scheme which they proposed for the decomposition of 

 hexoses : 



■ .\ Cannizzaro rearrangement is a reaction between two aldehyde molecules in which one is 

 reduced and the other oxidized. 



^ Kluyver, A. J., and Donker, II. J. L.: Veislas- Akad. Wcteiisiimppeu Anislcrdam, 33, 8y5 914. 



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