144 



PROVISION OF energy: fermentation 



the age of the culture, when the pR has fallen to about 4-5, 

 acetone and butyl alcohol begin to appear; their appearance 

 is associated with a corresponding disappearance of acetic 

 and butyric acids and a small rise in pH. It would seem that 

 the formation of acetone and butyl alcohol, involving the 

 formation of neutral substances from acids, is a neutralisa- 

 tion mechanism which is brought into play when the environ- 

 mental pH becomes strongly acid. Their formation can thus 

 be regarded as a mechanism in the same class as that of 



BUTYL ALCOHOL 



AGE OF CULTURE 



Fig. 11. Courseof fermentation of glucose by CZ. aceto^Mf^ZicMm 

 [after Davies and Stephenson, Biochem. J., 1941, 35, 1323]. 



acetylmethylcarbinol formation in Aerobacter aerogenes, and of 

 the production of amines from amino-acids by some strains 

 of Esch. coli. It would appear from Fig. 11 that the precursors 

 of acetone and butyl alcohol in the medium are acetic and 

 butyric acids respectively, and it has been shown that the 

 addition of acetate to the fermentation mixture results in a 

 marked increase in the production of acetone. At first it was 

 thought that a direct reduction of the acids by fermentation 

 hydrogen might take place, but it is fairly certain now that 

 this is not the case and that acetic and butyric acids undergo 



