CARBON METABOLISM 



245 



CO 



I 



HOCH - 



HCOH 



I 

 HOCH 



CH2OH 



(Z-sorbose) 



COOH 



Jo 



I 



HOCH - 

 HCOH 



HOCH 



I 



(2-keto-Z-gulouic 

 acid) 



CO 



I ' 



HOC 



O 



CH 



I 

 HC 



HOCH 



CH2OH 



(Z-ascorbic acid) 



Acid Fermentation by Bacteria. — The acids most 

 usually found as a result of bacterial fermentation 

 are formic, acetic and lactic acids, but propionic, 

 succinic and butyric acids with some others are also 

 found with certain organisms and under appropriate 

 conditions. Various theories to account for their pro- 

 duction have been proposed, and a number of inter- 

 mediate compounds suggested. The chief among the 

 latter are acetaldehyde, pyruvic acid and methyl- 

 glyoxal. 



Acetaldehyde, like aldehydes in general, forms a 

 water-insoluble compound with sulphites or bisulphites. 

 If, then, bacterial fermentation is allowed to proceed in 

 the presence of sulphite (bisulphites are usually poisonous 

 to bacteria) any acetaldehyde formed as an intermediate 

 will be " trapped " as the insoluble compound and will 

 play no further part in the process but will accumulate. 

 In this way the formation of acetaldehyde has been 

 detected during the fermentation of glucose, mannitol 

 and glycerol by members of the Esch. coli group ; among 

 the products of all organisms giving a positive Voges- 

 Proskauer reaction ; in acetic acid fermentation ; in 

 acetone fermentation ; in the fermentation of sucrose by 

 Aerobacter aerogenes and in the fermentation of pentoses 

 by B. aceto-ethylicus. Acetaldehyde appears, therefore, 

 to be a very general, if not universal, intermediate in 

 ba(;terial fermentations. 



