l6o NUTRITION AND METABOLISM 



of the alcohol or from 0.8 to 6.9 per cent of the fermented sugar. A 

 small quantity of succinic acid is also formed, usually about 0.6 to 0.7 

 per cent of the fermented sugar. Traces 'of acetic acid and of lactic 

 acid seem to be normal products of the process of fermentation, and we 

 always find fusel oil. The latest investigations seem to indicate that 

 glycerin and succinic acid are produced by yeast cells even in the absence 

 of sugar. This discovery makes it probable that the glycerin and suc- 

 cinic acid are derived from the reserve substances of the yeast cells, 

 such as lecithin, and are not direct products of fermentation. This 

 accounts also for the variation of the proportion between alcohol and 

 glycerin. Fusel oil is now believed to be a waste product of' cell 

 construction. 



Similar are the experiences with the lactic fermentation which has 

 been studied almost as extensively as alcoholic fermentation. If it is 

 supposed that the formation of lactic acid follows the equation 



C12H22O11 + H2O = 4C3H6O3 

 342 18 360 



^ Lactose Lactic acid 



the actual yield of acid is found to be between 90 per cent and 98 per 

 cent of the theoretical. The other 2-10 per cent are either used for 

 cell-growth or for products which thus far have escaped chemical de- 

 termination. Small discrepancies wiU also be found in fermentation 

 of urea and in the nitrif3dng process, where small amounts of the 

 nitrogenous material are used for cell-growth. 



Another difficulty in finding the chemical equation of a microbial 

 fermentation is the fact that this process may change with the age of the 

 culture. In those fermentations where several gases, as carbon dioxide 

 and hydrogen, are produced, the relative proportion of the two is not 

 always constant. In the butyric fermentation of dextrose by B. 

 amylozyma, Perdrix tries to account for this change by assuming three 

 different phases of the process at various ages of the cultures, repre- 

 sented by the following equations: 



First stage: s6C6Hi206-(-42H20 = ii6H2-l- ii4CO2-f3oCH3C0OH + 



Dextrose Acetic acid 



36CH3CH2CH2COOH. 



Butyric acid 



Second stage: 46C6H12O6 -1- i8H20 = 112H2 -f 94CO2 + 1SCH3COOH + 

 38CH3CH2CH2COOH. 



