16 BACTERIAL FERMENTATIONS 



main result of such studies was the demonstration that a 

 variety of compounds can be more or less quantitatively 

 converted to methane and carbon dioxide. 22 Equations 

 summarizing some of the observed reactions are given in 

 Table 5. The equations show that fermentations of acetate, 



TABLE 5 



Fermentation of Organic Compounds by Mixed Cultures 



CH3COOH — > CH 4 + C0 2 



4CH 3 CH 2 COOH + 2H 2 — > 7CH 4 + 5C0 2 



2CH3GH2CH2COOH + 2H 2 — > 5CH 4 + 3C0 2 



2CH 3 CH 2 OH — > 3CH 4 + C0 2 



CH 3 COCH 3 + H 2 — > 2CH 4 + C0 2 



propionate, butyrate, ethanol, or acetone give the same 

 products, the ratio of methane to carbon dioxide depending 

 on the oxidation state of the substrate. The remarkable 

 aspect of this result is that the nature of the products is 

 independent of the structure of the substrate. A plausible 

 explanation for the carbon dioxide was easily found: 

 carbon dioxide might be formed by the complete oxidation 

 of part of the substrate. But it was difficult to understand 

 why methane was always the other product. For one sub- 

 strate, acetate, a simple and reasonable explanation was 

 suggested, namely, that the acid is decarboxylated to form 

 carbon dioxide from the carboxyl group and methane from 

 the methyl group. 22 Unfortunately this explanation is of 

 no help with propionate which, according to the hypothesis, 

 should yield ethane on decarboxylation. Actually no 

 ethane could be detected by ordinary analytical methods. 

 No generally applicable chemical explanation for the 

 origin of methane was developed until 1934, when van Niel 



