312 BACTERIOLOGICAL CHEMISTRY 



A more recent development is the use of vegetable 

 wastes as a source of the necessary sugars for fermenta- 

 tion. Practically all vegetable waste materials, such as 

 straw, grass, maize cobs and husks, and sawdust contain 

 insoluble hemicelluloses, built up largely of pentosans. 

 The pentosans are broken down by a preliminary acid 

 treatment to give the soluble pentose sugars. The mash 

 so produced is fermented by yeast. 



Ethyl Alcohol-Acetone Fermentation. — Acetone and 

 ethyl alcohol are produced from maize, potatoes, molasses 

 and various vegetable waste materials by the action of 

 B. aceto-ethylicus, which is probably identical with the 

 starch fermenting organism, B. macerans. The fermenta- 

 tion proceeds best at a temperature between 40° and 

 43°C. and in presence of calcium carbonate, to prevent 

 development of excessive acidity, the mash after pre- 

 liminary acid treatment being brought to a ^^H value 

 about 8. The chief products are acetone and ethyl 

 alcohol corresponding to about 8 to 10 per cent, and 

 20 to 25 per cent., respectively, of the carbohydrate 

 fermented ; acetic, lactic and formic acids are also 

 formed together with hydrogen and carbon dioxide. 

 The solvents are separated by fractional distillation. 



Speakman suggested the following scheme as the 

 mechanism of fermentation. Glucose is broken down, 

 probably in a way similar to that in alcoholic fermentation, 

 to give pyruvic acid : — 



CfiHiaOe > 2CH3.CO.COOH + 4H 



The pyruvic acid is then transformed in three ways, 



(1) to give lactic acid : — 



CH3CO.COOH + 2H — > CH3CHOH.COOH 



(2) to give carbon dioxide and acetaldehyde, the latter 

 then giving alcohol : — 



+ 2H 



--^ CHX'HoOH 



