Alcoholic Fermentation / Second Edition, 1914

by Arthur Harden

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.

The most weighty criticism of the theory is that of Slator [[1906], [1907]; [1908, 1], [2]], which is based on the consideration that if lactic acid be an intermediate product of alcoholic fermentation the reaction by which it is fermented must proceed at least as rapidly as that by which it is formed, in order to prevent accumulation of lactic acid. The fermentation of lactic acid by yeast should therefore proceed at least as rapidly as that of glucose. So far is that from being the case that it has been experimentally demonstrated that lactic acid is not fermented at all by living yeast. This conclusion was rendered extremely probable by Slator, who showed that lactic acid, even in concentrations insufficient to prevent the fermentation of glucose, is not fermented to any considerable extent. The final proof that lactic acid is neither formed nor fermented by pure yeast has been brought by Buchner and Meisenheimer in a series of very careful quantitative experiments carried out with a pure yeast and with strict precautions against bacterial contamination [[1909], [1910]].

At first sight this fact appears decisive against the validity of the lactic acid theory, and it is recognised as such by Buchner and Meisenheimer. Wohl has, however, suggested that the non-fermentability of lactic acid by yeast is not really conclusive [[1907, 1]; see also Franzen and Steppuhn, [1912, 1]]. The production of lactic acid from glucose is attended by the evolution of a considerable amount of heat (22 cal.), and it is possible that at the moment of production the molecule of the acid is in a condition of activity corresponding with a much higher temperature than the average temperature of the fermenting liquid. Under these circumstances the molecule would be much more susceptible of chemical change than at a later period when temperature equilibrium had been attained. It has, however, been pointed out by Tafel [[1907]], that such a decomposition of the lactic acid would occur at the very instant of formation of the molecule, so that no ground remains even on this view for assuming the actual existence of lactic acid as a definite intermediate product. It has also been suggested by Luther [[1907]] that an unknown isomeride of lactic acid is formed as an intermediate product and fermented, and that traces of lactic [p104] acid are formed by a secondary reaction from this, but no satisfactory evidence for this view is forthcoming. There still remains a doubt as to whether the living yeast-cell is permeable to lactic acid, a fact which would of course afford a very simple explanation of the non-fermentability of the acid. Apart from this, however, it is difficult, in face of the evidence just quoted, to believe that lactic acid is in reality an intermediate product in alcoholic fermentation.