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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    then falls again to a value approximately equal to, but generally somewhat higher than, that which it originally had. Careful experi- ments have shown that during this period of enhanced fermentation the amounts of carbon dioxide and alcohol produced exceed those which would have been formed in the absence of added phosphate by a quantity almost exactly equivalent to the phosphate added in the ratio C02 or C2H60 : R'2HP04 [Harden and Young, 1906, 1 ]. This result is of fundamental importance, and the evidence on which it rests deserves some consideration. Quantitative experiments on this subject require certain preliminary precautions. The acid phosphates are too acid to permit of any extended fermentation and the phosphates of the formula R'2HP04 absorb a considerable volume of carbon dioxide with production of a bicarbonate, according to the reaction R2HP04 + H2C03 ^ RHCOg + RH2P04. The method which has been adopted, therefore, is to employ either a secondary phosphate saturated with carbon dioxide at the temperature of the experiment, or a mixture of five molecular pro- portions of the primary phosphate with one molecular proportion of a secondary phosphate (pK = 6-2) in which the amount of bicarbonate formed is negligible. In the former case it is necessary to ascertain how much of the carbon dioxide evolved is derived from the bicarbonate by a disturbance of the original equilibrium owing to the chemical changes which occur (p. 55). This is done by acidifying duplicate samples with hydrochloric or trichloroacetic acid before and after the fermentation and measuring the gas evolved in each case. Any necessary correction can then be made. The calculation of the extra amount of carbon dioxide evolved from yeast-juice containing sugar when a phosphate is added involves an estimation of the amount which would have been evolved in the absence of added phosphate, and this is a matter of some uncertainty. With some yeast preparations, especially dried yeast or zymin, the final steady rate of fermentation is often greater than the initial rate (owing to the greater concentration of hexosephosphate; see p. 71) [Boyland, 1929], and the practice has been adopted of ascertaining this final rate and then calculating the total evolution corresponding to it for the whole period from the time of the addition of the phosphate to the end of the observations. This amount deducted from the observed total leaves the extra amount of carbon dioxide formed, and it is this quantity which is equivalent to the phosphate added. This
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