268 MICRO-ORGANISMS AND FERMENTATION. 



decomposition of the nutritive liquid, but more particularly 

 upon the yeast species. Its production is favoured by a high 

 temperature of fermentation, and by a greater sugar concen- 

 tration, or in general by the use of a rich nutritive fluid. It is 

 almost impossible to give limits for the proportion of alcohol 

 and glycerine. In wine fermentations the formation of 

 glycerine varies usually from 2-5 to 14 per cent, of the amount 

 of alcohol produced, whereas in beer it represents only 1-65 to 

 4-3 per cent, of the alcohol. By fermenting saccharose with 

 zymase Buchner and Rapp obtained even smaller quantities 

 of glycerine. 



Succinic acid is another by-product which varies in quantity. 

 According to Rau, the quantity increases with increasing 

 temperature, and apparently the composition of the nutritive 

 fluid has no influence on the result. 



Lactic acid is always found as a by-product in fermentations 

 carried on in the absence of living cells. 



Prior's detailed researches prove that the different races 

 of yeasts produce very varying quantities of volatile and non- 

 volatile acids. He found that acetic acid was a constant 

 product of fermentation. 



Formic acid is produced, according to Rayman and Kruis, 

 by the oxidising action of yeast on the albuminoids of the- 

 nutritive liquid. 



Aldehyde (acetaldehyde) also occurs regularly and must be 

 regarded as an intermediate product between the fatty acids 

 and the alcohols. Rayman and Kruis proved that, especially 

 in the case of distillery yeasts, considerable quantities are 

 formed when free access of air is permitted, and the surface 

 of the fermented liquid is then covered with a film of yeast. 

 They assume that acetaldehyde is produced by oxidation of 

 ethyl alcohol. 



Methyl alcohol, often found in bacterial fermentations, may 

 also be developed during a yeast fermentation, especially by 

 the fermentation of glucosides present in fruit juices. It also 

 appears possible that propyl and butyl alcohol may be produced 

 in a normal alcoholic fermentation, the former from lactic 

 acid. Especial interest is attached to the presence of amyl 

 alcohol (isoamyl alcohol), which forms the main constituent 



