396 ACETIC FERMENTATION. 



and acetic acid." W. VON KNIERIEM and AD. MAYER (I.) share 

 the credit of having convincingly proved, in 1873, that the oxida- 

 tion of alcohol by means of platinum-black cannot be classed 

 along with the fermentation set up by the "mother of vinegar." 

 Platinum-black oxidises both concentrated and dilute alcohol, 

 whereas, according to the experience of vinegar-makers, acetic 

 fermentation cannot proceed in presence of more than 14 per cent, 

 of alcohol. Moreover, with regard to temperature, highly im- 

 portant differences touching the very existence of the question 

 are observed. Thus, whereas acetic fermentation proceeds most 

 satisfactorily at about 35 C. and is arrested altogether at 40 C., 

 the energy of the oxidation effected by platinum-black (starting 

 at 35 C.) increases as the temperature rises, and may become so 

 violent that the alcohol ignites explosively and burns away to 

 water and carbon dioxide. Hence the composition of the (by no 

 means uniform) oxidation products thus formed differs greatly 

 from those obtained from acetic fermentation. 



This latter process, whose purely physiological nature was placed 

 beyond doubt by these investigations, was examined more minutely 

 by A. J. BROWN (III.) in 1886. Meanwhile Hansen's discovery of 

 the existence of at least two species of acetic acid bacteria con- 

 siderably enlarged the field of research, since thenceforward " acetic 

 fermentation" could no longer be spoken of without coupling 

 with it the name of the organism by which it was caused. The 

 species forming the subject of Brown's researches was obtained 

 by him from sour (acetic) beer, and was called Bacterium aceti, 

 though not identical with Hansen's species bearing the same name. 

 Pasteur's discovery that the "acetic acid bacteria" first convert 

 alcohol into acetic acid and then burn the latter to carbon dioxide 

 and water, was also made by Brown in connection with his B. aceti, 

 but he did not institute any closer examination (more particularly 

 in connection with the ratio of transformation) on this point, so 

 that this theoretically and practically most important question has 

 still to be investigated. 



Methyl alcohol, isobutyl alcohol, and amyl alcohol are not 

 attacked by Brown's B. aceti, but normal propyl alcohol is oxidised 

 to propionic acid. If the nutrient medium (yeast- water) contains 

 dextrose but no alcohol, then gluconic acid is formed, a fact already 

 established by BOUTROUX (I.) in 1880, in connection with another 

 species of bacterium (of questionable purity). Saccharose, lactose, 

 and starch remain unaltered, but mannite is converted into levu- 

 lose, which then remains unchanged. Dulcite is unaffected, whilst 

 glycol is converted into glycollic acid. The behaviour of Bacterium 

 xylinum is approximately the same as the organism just described. 

 The extensive mucinous envelopes, consisting of cellulose, are pro- 

 duced when the nutrient solution contains dextrose, levulose, or 

 mannite ; whilst, on the other hand, cane-sugar and starch are 

 useless for this purpose. 



