ALCOHOLIC FERMENTATION 279 



the alcohol, methylheptanol ; the process is not restricted 

 to organic compounds, but inorganic substances, too, 

 can be reduced in this way ; sodium thiosulphate, for 

 example, yields hydrogen sulphide and sodium sulphite, 

 and elementary sulphur gives hydrogen sulphide. 



The fact that yields of optically active alcohols of 

 considerably more than 50 per cent, can be obtained 

 from optically inactive starting material indicates that 

 true reduction occurs and not a Cannizzaro reaction, 



2R.CH0 + H2O > R.CH2OH + R.COOH, 



which would give a maximum possible yield of 50 per 

 cent. For instance, aldol, CH3.CHOH.CH2.CHO, gives 

 63-5 per cent, of optically active 1 : 3-butylene glycol, 

 CH3.CHOH.CH2.CH2OH. 



The hydrogen used in these redactions is in all 

 probability that produced during the formation of the 

 pyruvic acid from the three-carbon intermediate, since in 

 one case at least, when methylheptanone is reduced to 

 methylheptanol, an amount of acetaldehyde can be iso- 

 lated corresponding to the amount of methylheptanol 

 produced. Moreover, these reductions occur only during 

 active fermentation and do not take place in the presence 

 of yeast suspensions alone, which indicates that the reduc- 

 tions are coupled with the normal fermentation reactions. 



These reductions are frequently called ph}i:ochemical 

 reductions, yeast being regarded as a plant. 



Fusel Oil. — Besides the glycerol, alcohol and carbon 

 dioxide, which are the main products of the action of 

 yeast on sugar solutions, small quantities of other products 

 are also formed. The chief among these are the higher 

 alcohols constituting the high boiling fraction, fusel oil, 

 which represents 0-1 to 0-7 per cent, of the products. It 

 was originally thought that the higher alcohols originated 

 as by-products of the sugar breakdown, but it was proved 

 ]>y Ehrlich that their real source is the amino -acids 

 derived from the medium or from the autolysis of yeast 



]9 



