FERMENTATION PRODUCTS 123 



such detailed attention that our knowledge of the intermediate 

 stages is now almost as great as that of the related glycolysis 

 cycles in yeast and other cells. The key substance in many 

 fermentations is pyruvic acid, CHg . CO . COOH, which is formed 

 by the breakdown of the carbohydrate molecule and is then 

 attacked in various ways by different organisms to give a 

 variety of products. In this chapter we shall first trace the 

 course of the formation of pyruvic acid and then show how 

 various organisms produce their varied fermentation products 

 by further elaboration of this key substance. 



FERMENTATION OF GLUCOSE 

 Glucose is the carbohydrate whose fermentation has been 

 studied in greatest detail. Some organisms, including many 

 of the Streptococci and Lactobacilli, carry out a simple 

 fermentation of glucose with the production of lactic acid 

 in almost theoretical yield; such organisms are called homo- 

 lactic fermenters. The majority of heterotrophic organisms 

 produce a variety of products amongst which can be listed 

 the following: — 



CO2 Pyruvic acid. Acetone, 



CH3.CO.COOH CH3.CO.CH3 



Hj Butyric acid, iso-Propyl alcohol, 



CH3. CHg. CHg. COOH CH3.CHOH.CH3 



Formic acid, Ethyl alcohol. Succinic acid, 



H . COOH CH3 . CH2OH HOOC . CH2 . CH2 . COOH 



Acetic acid, n-Propyl alcohol, Acetylmethylcarbinol, 



CH3.COOH CH3.CH2.CH2OH CH3.CO.CHOH.CH3 



Propionic acid. Butyl alcohol, 2.3.Butylene glycol, 



CH3. CHg. COOH CH3.CH2.CH2.CH2OH CH3.CHOH.CHOH.CH3 



Lactic acid, Diacetyl, 



CHj.CHOH.COOH CH3.CO.CO.CH3 



Fermentation releases energy. From this point of view it 

 is a less efficient form of metabolism than oxidation, for the 

 complete oxidation of glucose yields considerably more energy 

 than any fermentation can do, as Hhe latter involves the 

 production of partially reduced substances : 



CgHigOe + 6O2 > 6CO2 + 6H2O + 674 Cals. 



CgHiaOe > 2CO2 + 2C2H5OH + 22 Cals. 



