It has also been shown by Euler and Johansson [[1913]] that in the fermentation of a mixture of equivalent amounts of phosphate and glucose, the whole of the glucose had disappeared when the whole of the phosphate had become esterified.
Cycle of Changes Undergone by Phosphate in Alcoholic Fermentation.
According to equation (1) the free phosphate present is used up in the reaction, and the question at once arises whether there is any source from which a constant supply of free phosphate can be elaborated in the juice, or whether some other change occurs which results in the formation of carbon dioxide and alcohol in the absence of free phosphate. The experimental evidence points in the direction of the former of these alternatives, but the question is a very difficult one to decide with absolute certainty.
When a mixture of a phosphate with yeast-juice and sugar is examined at intervals and the amount of free phosphate estimated, the following stages are observed:—
1. During the initial period of accelerated fermentation following the addition of the phosphate, the concentration of free phosphate rapidly diminishes.
2. At the close of this period, the amount of free phosphate [p053] present is very low, and, as long as active fermentation continues, no marked increase in it occurs.
3. As alcoholic fermentation slackens and finally ceases, a marked and rapid rise in the amount of free phosphate occurs at the expense of the hexosephosphate, which steadily diminishes in amount, and this change is brought about by an enzyme in the juice and ceases if the liquid be boiled.
This last reaction may be represented by the equation
(2) C6H10O4(PO4R2)2 + 2 H2O = C6H12O6 + 2 PO4HR2.
In the light of this equation, together with equation No. 1, given above, all these facts can be simply and easily understood.