ACETOUS FERMENTATION 103 



vinegar. The change is accompanied by the absorption of oxygen, 

 one atom of which combines with two of hydrogen to form water, and 

 a substance remains termed aldehyde, further oxidation of which pro- 

 duces the acetic acid. We may express it chemically thus : — 



CjHgO ( + oxygen and the ferment) = CjH^O + HjO. 



Alcohol. Aldehyde. Water. 



The aldehyde becomes further oxidised : — 



q,H^O + = CgH^Og (acetic acid). 



This method of simply oxidising alcohol to obtain acetic acid may 

 be carried out chemically without any ferment. If slightly diluted 

 alcohol be dropped upon platinum black, the oxygen condensed in 

 that substance acts with energy upon the spirit, and union readily 

 occurring, acetic acid results. Here the whole business of the plati- 

 num sponge is to persuade the oxygen of the air and the hydrogen of 

 the alcohol to unite. In the ordinary manufacture this is accom- 

 plished by the vegetable cells of Mycoderma aceti. 



There are two chief methods adopted in the commercial manu- 

 facture of vinegar, both of which depend upon the presence of the 

 mycoderma. The method in vogue at Orleans when Pasteur (about 

 1862) commenced his studies of the vinegar organism, was to fill vats 

 nearly to the brim with a weak mixture of vinegar and wine. Where 

 the process is proceeding the surface is covered with a fragile pellicle, 

 " the mother of vinegar," which is produced by, and consists of, 

 certain micro-organisms whose function is to convey the oxygen of 

 the air to the liquor in the vats, thus oxidising the alcohol into 

 vinegar. This oxidation may be carried on even beyond the stage of 

 acetic acid (when no more alcohol remains to be oxidised), resulting 

 in carbonic acid gas, which escapes into the air. But as in the 

 alcoholic, so in the acetic, fermentation there comes a time when the 

 presence of an excess of the acid inhibits the further growth of the 

 organism. This point is approximately when the acetic acid has 

 reached a percentage as high as 14. But if the acid be removed, and 

 fresh alcohol added, the process recommences. 



The second method, sometimes called by the Germans the " quick 

 vinegar process," is to pour the weakened alcohol through a tall 

 cylinder filled with wood-shavings, having first added some warm 

 vinegar to the shavings. After a number of hours the resulting fluid 

 is charged with acetic acid. What has occurred ? Liebig maintained 

 that a chemical and mechanical change had brought about the change 

 from the alcohol put into the cylinder and the vinegar drawn off at 

 the exit tube. It was reserved for Pasteur to demonstrate by experi- 

 ment that the addition of the warm vinegar to the shavings was in 

 reahty an addition of a living micro-organism, which, forming a film 



