498 CHEMISTRY OF ALCOHOLIC FERMENTATION. 



conditions were unfavourable to the yeast. According to LIEBER- 

 MANN (IV.) and KITICSAN (I.), traces also occur in normal wines, 

 though, as observed by DUCLAUX (I.), it disappears readily in 

 presence of yeast THOMAS (II.) mentions the occurrence of 

 formic acid in aqueous infusions of malt culms ; nevertheless it is 

 also formed in fermentation, especially when a large surface of the 

 liquid is exposed and nitrogen compounds are present. Of these 

 latter, urea, either alone or in association with ammoniun bicar- 

 bonate, is best adapted to increase the output of formic acid, 

 which can be still further augmented by the addition of calcium 

 carbonate. During cell-less fermentation, BUCHNER and MEISEN- 

 HEIMER (I.) found traces of a volatile acid very similar to formic acid. 

 The frequent occurrence of the latter among the products of bac- 

 terial fermentation has already been mentioned on p. 181 of vol. i. 

 Acetic acid was recognised at an early date as a by-product of 

 alcoholic fermentation, by LAVOISIER (II.). DUCLAUX (XXVII.) 

 and BECHAMP (XIII.) also found acetic acid constantly, though 

 only as traces ; and these observations were confirmed by 

 SCHUTZEJNBERGER (II.). Owing to the wipespread occurrence of 

 acetic bacteria, the acid can only be regarded as a by-product of 

 alcoholic fermentation when yeast, free from bacteria, is employed ; 

 just as was remarked with regard to lactic acid. KRUIS and 

 RAYMAN (II.) failed to detect acetic acid at all, though they made 

 it the object of special attention ; whereas THOMAS (II.) obtained 

 it regularly, though merely as traces. BUCHNER and MEISEN- 

 HEIMER (IV. and II.) found it invariably in fermentations with 

 expressed yeast juice free from bacteria ; and it is therefore 

 certain that the acid is a normal by-product of alcoholic fer- 

 mentation. The yeast juice, prior to fermentation, contained 

 0.004-0.010 per cent, of acetic acid, and afterwards 0.08-0.33 

 per cent., so that considerable quantities were produced during 

 the operation. The specimens of yeast juice that fermented 

 lactic acid furnished considerable quantities of acetic acid, the 

 converse being the case with such of them as produced lactic acid 

 in abundance. The name glucacetase has been applied to the 

 yeast enzyme that is assumed to split up glucose into three 

 molecules of acetic acid. The hypothesis advanced by BIOURGE (I.) 

 to account for the formation of acetic acid is that the volatile 

 acids must be regarded as the result of assimilation processes, and 

 not of the decomposition of sugar, yeast itself yielding a con- 

 siderable amount of acid on distillation. According to PRIOR (V.), 

 however, it is more feasible to suppose and this is indicated by 

 the experiments of GILTAY and ABERSON (II.) that the oxygen 

 consumed in fermentation oxidises a portion of the alcohol in the 

 interior of the yeast cell. The influence of oxygen on fermenta- 

 tion a point to which we shall revert shortly also plays a part 

 in this case. It cannot, however, be credited as the sole cause of 

 the production of acetic acid during fermentation, the experiments 



