134 PROVISION OF energy: fermentation 



(see p. 127). However, lactic acid is still formed to some extent 

 from pyruvic acid, and it has been suggested that formic acid 

 will react with pyruvic acid to give lactic acid according to 

 the equation: 



CH3 . CO . COOH + H . COOH > CH3 . CHOH . COOH + COg 



(6) 



It is found experimentally that a molecule of COg is formed 

 for every molecule of lactic acid produced. As a result of 

 this, the fermentation gases contain more carbon dioxide 

 than hydrogen, the amount of carbon dioxide in excess of 

 the hydrogen being equivalent to the lactic acid formed. The 

 experimental evidence in support of reaction 6 is not, however, 

 convincing at present. It has also been shown that the amount 

 of lactic acid produced is increased by increasing the carbon 

 dioxide present during the fermentation. Fermentation 

 studies carried out in the presence of carbon dioxide containing 

 isotopic 0^3 (" heavy carbon ") have shown that C^^Og is 

 fixed during the fermentation, and that some of this fixed-COg 

 appears in the ■ — COOH group of lactic acid. 



In the case of the Gonococcus and S. faecalis it has been 

 shown that lactic acid can be formed from pyruvic acid by a 

 hydrolytic reaction (or dismutation) : 



2CH3 . CO . COOH + H2O = CH3 . CHOH . COOH + 



CH3.COOH + CO2. 



Succinic acid formation 



Succinic acid is formed to a variable extent during the 

 fermentation of either glucose or pyruvic acid by Esch. coli, 

 Aerobacter aerogenes, Propionibacteria, and other organisms. 

 For many years its formation constituted a puzzle, as it was 

 by no means obvious how a substance containing 4 carbon 

 atoms could be derived from a 6-carbon sugar or a 3-carbon 

 triose or pyruvic acid. Elsden of the Cambridge School 

 showed that succinic acid formation depends upon the presence 

 of carbon dioxide during fermentation, so that increasing the 

 CO2 tension results in increased succinic acid production or. 



