INDUSTRIAL FERMENTATIONS 323 



which again undergoes dismutation ; the process continues 

 until all the alcohol has been converted to acetic acid. 

 Under normal aerobic conditions, however, this reaction 

 is much slower than the direct oxidation and contributes 

 but a small proportion of the yield. 



The activity of the acetic acid bacteria in bringing 

 about the oxidation of glucose to gluconic acid and keto- 

 gluconic acid, of glycerol to dihydroxyacetone, and 

 of secondary alcohol groups to keto groups has already 

 been described (see p. 241 et seq.). 



Methane and Hydrogen (Power Gas) Fermentation. — 

 In comparatively recent years the use of methane and 

 hydrogen produced by the fermentation of cellulose wastes 

 has developed considerably. The study of cellulose 

 fermentation has been largely the work of Omelianski. 

 There are three main types of cellulose degradation : — 



1. Anaerobic at 20° to 37° C. with the production of 

 methane and hydrogen. This fermentation is the result 

 of the action of tw^o organisms — one CI. fossicularum, 

 giving the hydrogen, and the other, CI. methanigenes, 

 forming methane. CI. fossicularum is a long slender 

 bacillus with terminal spores, which fails to grow on 

 ordinary media. Its chief products are acetic, lactic and 

 butyric acids, ethyl alcohol, carbon dioxide and hydrogen. 

 The methane fermentation also produces much fatty acid 

 but even more gas than the other, as much as 50 per cent, 

 of the decomposed cellulose appearing as carbon dioxide 

 and methane, the remaining 50 per cent, appearing 

 mainly as acetic acid. The organism concerned is 

 morphologically very similar to the hydrogen producing 

 type, but the two can be separated by repeated short 

 heatings at 75° C, by which the more rapidly developing 

 methane -bacillus is killed off, leaving the slowly develop- 

 ing hjTlrogen-producing organism in the resistant spore 

 stage. Conversely, the hydrogen-bacillus can be elimin- 

 ated by repeated transfers whilst the methane fermenta- 

 tion is at its most active, the slow-growing hydrogen- 



