FERMENTATION 107 



oxidation. Such processes may be effected without the inter- 

 vention of atmospheric oxygen and are commonly known as 

 fermentations. Alcoholic, lactic and butyric fermentation of 

 sugar are well-known examples ; in all the amount of energy 

 released is much smaller than that from complete oxidation ; 

 this is shown in the following equations : — ■ 



C 6 H 12 O s = 2C 2 H b O + 2CO a + 28K 

 Alcohol 



C,H lg 6 = 2CH3CHOHCOOH + 18K 

 Lactic acid 



C 6 H l2 6 = CH3CHXH0COOH + aC0 2 + 2H 2 + 15K 

 Buytric acid 



In illustration, the fermentation of sugar by yeast may be 

 briefly considered. 



As in normal oxidation of sugar, the fermentation process 

 is accompanied by the evolution of heat, although to a much 

 lesser extent, a gram molecule of maltose yielding 427 kg. 

 calories, taking Brown's * value of the heat of fermentation 

 of 1 gram of maltose as being 125 calories. The alcoholic 

 fermentation of sugar is therefore a wasteful process for 

 obtaining energy when compared with oxidation, since by 

 its means nearly thirty-one times as much sugar must be con- 

 sumed to obtain as much energy as is yielded by the direct 

 oxidation of sugar. The significance of this figure of the heat 

 of fermentation of maltose may be realized by Brown's ob- 

 servations that between the temperatures of 14 and 16 C. 

 the time required by a yeast cell to ferment its own weight of 

 sugar varies from eighteen and a half to nineteen and a half 

 hours and that the heat generated during one hour is sufficient 

 to raise the temperature of the cell by 15 or 16 C. From 

 such observations Brown estimates that at 30 C. yeast can 

 ferment its own weight of maltose in 2-2 hours and the poten- 

 tial rise in temperature in the cell in one hour will be 75-5° C. 

 under adiabatic conditions, figures indicative of an intense 

 metabolism and, apparently, a great waste of energy. Brown 

 suggests that the explanation for this waste is to be found 

 in the fact that brewers' yeast is a cultivated plant grown in 

 unnatural conditions. The wild yeasts lead a quiet life on the 

 * Brown : " Ann. Bot.," 1914, 28, 197. 



