102 'NUTRITION AND METABOLISM. 



According tc this formula; ;ioo parts of dextrose should give 51.11 parts 

 of alcohol and 48.89 parts of "carbon ^dioxide. The actual yield comes 

 very close to these numbers, but does not reach them ; the largest amounts 

 found were 46-47.5 per cent of carbon dioxide and 47.5-48.67 per cent 

 of alcohol. Under the most favorable conditions, the total yield of 

 the products of fermentation was only 95 per cent of the theoretical 

 yield. 



Other products are formed besides the alcohol and carbon dioxide. 

 The amount of glycerin found in fermented liquids varies very much with 

 the conditions of fermentation; it reaches from 1.6 to 13.8 per cent 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. All of these com- 

 pounds have been regarded as products resulting from the regular fermen- 

 tation, but 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 succinic 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. 



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 

 C 12 H 22 11 +H 2 = 4 C 3 H 6 3 

 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 determina- 

 tion. Small discrepancies will also be found in the fermentation of urea 

 and in the nitrifying process, where small amounts of the nitrogenous 

 material are" used for the 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 at 

 all constant. In the butyric fermentation of dextrose by B. amylozyma, 



