R·CO·COOH = R·CHO + CO2
and it can scarcely be doubted that this is the actual course of the reaction.
(3) The final conversion of the aldehyde into the corresponding alcohol is also a change which it has been proved can be effected by yeast [Neuberg and Rosenthal, [1913]] probably by the aid of the reductase which is one of the weapons in its armoury of enzymes.
Yeast is capable of producing many vigorous reducing actions and rapidly reduces methylene blue and sodium selenite. It is in all probability due to a reaction of this kind that the iso-amylaldehyde and isovaleraldehyde were reduced to the alcohols in Neuberg and Steenbock's experiments [[1913], [1914]], and that considerable quantities of ethyl alcohol are formed in the sugar free fermentation of pyruvic acid [Neuberg and Kerb, [1913, 1]] (see later p. [110] for a discussion of this question).
A further possibility exists that in some cases the aldehyde may [p094] be simultaneously oxidised and reduced or the molecule of one aldehyde reduced and that of another oxidised with production of the corresponding acid and alcohol by an "aldehydo-mutase," similar to that which has been observed by Parnas [[1910]] in many animal tissues. Finally the aldehyde may simply be converted into the corresponding acid by oxidation as appears to take place in the formation of succinic acid.
The intermediate production of an aldehyde would thus be consistent both with the production of alcohols and acids from amino-acids.
Fusel oil would be formed by the reduction of the aldehydes arising from the simple monobasic amino-acids, succinic acid would be produced by oxidation of the aldehyde derived from the dibasic glutamic acid.
In favour of this view is to be adduced the fact that aldehydes such as isobutyraldehyde and valeraldehyde have been found in crude spirit, whilst acetaldehyde is a regular product of alcoholic fermentation [see Ashdown and Hewitt, [1910]]. Benzaldehyde, moreover, has been actually detected as a product of the alcoholic fermentation of phenylaminoacetic acid, C6H5·CH(NH2)·COOH [Ehrlich, [1907, 1]]. Further, the aldehydes so produced would readily pass by oxidation into the corresponding fatty acids, small quantities of which are invariably produced in fermentation.
This view of the nature of the alcoholic fermentation of the amino-acids is undoubtedly to be preferred to that previously suggested by Ehrlich [[1906, 3]] according to which a hydroxy-acid is first formed and then either directly decomposed into an alcohol and carbon dioxide or into an aldehyde and formic acid, the aldehyde being reduced and the formic acid destroyed (see p. [115]).
| R·CH(NH2)·COOH → | R·CH( | OH)·COOH | |
| ↓ | or | ↓ | |
| R·CH2OH + CO2 | R·CHO + H·CO2H | ||
| ↓ | |||
| R·CH2OH | |||