CHOCHOCHO
│││
CH(OH)C(OH)CO
│H║│
CH(OH) − OH CH ⇌ CH2
│││
CH(OH)CH(OH)CH(OH)
│││
CH(OH)CH(OH)CH(OH)
│││
CH2(OH) CH2(OH) CH2(OH)
I.
Glucose.		II.III.
Methyl-
glyoxal
CHO COOHCO2
││────
CO+H2OCH(OH)CH2OH
│││
CH3CH3CH3
CHOCHOCHOCOOHCO2
││││─────
CH(OH)H C(OH) ⇌ CO + H2O CH(OH) CH2OH
│· ║│││
CH2(OH)− HO CH2CH3CH3CH3
IV.
Glyceral-
dehyde.		V.VI.
Methyl-
glyoxal.		VII.
Lactic
acid.		VIII.
Alcohol
and
carbon
dioxide.

[p102]

This scheme agrees well with the current ideas as to the formation of lactic acid from glucose under the influence of alkalis (p. [99]). It postulates the formation as intermediate products of no less than three compounds containing a chain of three carbon atoms—glyceraldehyde, methylglyoxal, and lactic acid.

The Lactic Acid Theory of Alcoholic Fermentation.

A practical interest was given to these various schemes by the fact that Buchner and Meisenheimer adduced experimental evidence in favour of the view that lactic acid is an intermediate product in the formation of alcohol and carbon dioxide from sugar by fermentation [[1904], [1905], [1906], [1909]].

These observers proved by a series of very careful analyses that yeast-juice frequently, but not invariably, contains small quantities of lactic acid, not exceeding 0·2 per cent. When yeast-juice is incubated alone or with sugar the amount of lactic acid may either increase or decrease. Moreover, lactic acid added to the juice is sometimes diminished and sometimes increased in quantity. On the whole it appears that the addition of a considerable quantity of sugar or of some lactic acid favours the disappearance of lactic acid. Juices of low fermenting power produce a diminution in the lactic acid present, those of high fermenting power an increase.

In all cases the amounts of lactic acid either produced or destroyed are very small in relation to the volume of the yeast-juice employed.

Throughout the whole series of experiments the greatest increase amounted to 0·47 per cent. on the juice employed, and the greatest decrease to 0·3 per cent. [See also Oppenheimer, [1914, 1].] Buchner and Meisenheimer at one time regarded these facts as strong evidence that lactic acid is an intermediate product of alcoholic fermentation. It was thought probable that the production of alcohol and carbon dioxide from sugar occurred in at least two stages and under the influence of two distinct enzymes. The first stage consisted in the conversion of sugar into lactic acid, and for the enzyme which brought about this decomposition was reserved the name zymase or yeast-zymase. The lactic acid was then broken down into alcohol and carbon dioxide by the second enzyme, lactacidase.

This theory, which is quite in harmony with the current ideas as to the mode of decomposition of sugars by alkalis, and is also consistent with Wohl's scheme of reactions, is open to adverse criticism from several points of view. In the first place, it is noticeable that the total amount of lactic acid used up by the juice is extremely small, even [p103] in the most favourable cases, relatively to the amount of the juice [Harden, [1905]], and it may be added to the sugar-fermenting power of the juice. Moreover, as pointed out by Buchner and Meisenheimer themselves [[1909]], no proof is afforded that the lactic acid which disappears is converted into alcohol and carbon dioxide. It is not even certain, although doubtless probable, that the lactic acid which occurs or is produced in the juice is really derived from sugar.