Alcoholic fermentation / by Arthur Harden.

  • Arthur Harden
Date:
1932
Catalogue details

Licence: In copyright

Credit: Alcoholic fermentation / by Arthur Harden. Source: Wellcome Collection.

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    amount of lactic acid used up by the juice is extremely small, even 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 dis- appears 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. Slator [1906 ; 1907 ; 1908, 1, 2] has based a criticism of this theory 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 was rendered extremely probable by Slator, who showed that lactic acid, even in concentrations insufficient to prevent the fermen- tation of glucose, is not fermented to any considerable extent. The final proof that lactic acid is neither formed nor fermented by pure yeast was 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]. As already mentioned the fermentation of added lactic acid by yeast preparations is irregular and very restricted in amount. At first sight these facts appear decisive against the validity of the lactic acid theory, and they were recognised as such by Buchner and Meisenheimer. Wohl, however, suggested that the non-fermentability of lactic acid by yeast was 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 mole- cule of the acid is in a condition of activation 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 if brought into the solution at a lower effective temperature. This “activation” theory of .chemical reaction is now generally accepted, so that the possibility of the intervention of lactic acid in alcoholic fermentation still remains. It is noteworthy in this con- nection that lactic acid restores the power of reducing methylene blue
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