It follows from this, in the light of what has already been said (p. [121]), that the action of living yeast on sugar follows the same course as a typical enzyme reaction, although in this case, as in that of yeast-juice, no information is given as to the exact nature of this reaction.
Influence of the Concentration of Yeast.
It appears to be well established that, when changes in the quantity and constitution of the yeast employed are eliminated, the rate of fermentation is exactly proportional to the number of the yeast-cells present (Aberson, Slator). This result might be anticipated, as pointed out by Slator, from the fact that the fermentation takes place within the cell, each cell acting as an independent individual.
The diffusion of sugar into the yeast-cell which necessarily precedes the act of fermentation has been shown by Slator and Sand [[1910]] to occur at such a rate that the supply of sugar is always in excess of the amount which can be fermented by the cell.
Temperature Coefficient of Alcoholic Fermentation by Yeast.
The temperature coefficient of fermentation by living yeast has been carefully determined by Slator by measurements of the initial rates at a series of temperatures from 5° to 40° C. The coefficient is found to be of the same order as that for many chemical reactions, but to vary considerably with the temperature, a rise in temperature corresponding with a diminution in the coefficient. The following values were obtained for glucose; they are independent of the concentration of yeast and glucose, the class of yeast, and presence or absence of nutrient salts, and remain the same when inhibiting agents are present. Almost precisely the same ratios are obtained for fructose and mannose:—
| t. | V(t+5)/Vt. | V(t+10)/Vt. |
|---|---|---|
| 5 | 2·65 | 5·6 |
| 10 | 2·11 | 3·8 |
| 15 | 1·80 | 2·8 |
| 20 | 1·57 | 2·25 |
| 25 | 1·43 | 1·95 |
| 30 | 1·35 | 1·6 |
| 35 | 1·20 |
Aberson's result, K(t+10)/Kt = 2·72, which represents the mean coefficient for 10° between 12° and 33°, agrees well with this. [p130]
Action of Accelerating Agents on Living Yeast.
Slator [[1908, 1]] was unable to find any agent which greatly accelerated the rate of fermentation of living yeast. Small concentrations of various inhibiting agents which are often supposed to act in this way were quite ineffective, and phosphates, which produce such a striking change in yeast-juice, were almost without action (cp. p. [124]).