LXVII.—Studies in fermentation. Part III. The rôle of diffusion in fermentation by yeast cells
Journal of the Chemical Society Transactions
WHEN a fermentable sugar is t&ransformed into alcohol and carbon dioxide by the action of living yeast suspended in an aqueous solution of the sugar, it is generally accepted that the chemical action takes place within the yeast cell, and that the sugar has to diffuse into the latter before it is attacked by the enzyme. A consideration of the influence of various factors on the rate of alcoholic fermentation shows that under ordinary conaitions diffusion supplies the yeast cells rapidly and
... lls rapidly and efficiently with sugar. The high temperature-coefficient and the constancy of the velocity with different concentrations of the sugar constitute practically conclusive evidence on this point. It is, however, conceivable that under certain conditions, such as with very dilute sugar solutions, or with very active yeast cells, diffusion could not take place rapidly enough t o supply the yeast cells with sugar to enable them to exert their maximum fermentative power. I n the present investigation the limiting conditions have been examined under which diffusion alone would no longer be capable of supplying the yeast cells efficiently with sugar, and under which the apparent velocity of the reaction would thus be influenced by convection currents produced either owing to the evclution of gas in the liquid, or by external m-eans. I n order to obtain an insight into the extent to which such convection currents (stirring) might conceivably affect the rate of fermentation by yeast cells, it is useful first to determine what are the maximum differences of concentration which might arise in a liquid in which no convection currents whatever take place, and in which uniformly distributed yeast cells are operative. It will be reaaily cpnceded that if we place a single yeast cell in the centre of a spherical vessel containing a sugar solution, differences of concentration between the surface of the cell and the boundary of the vessel will arise, which will at first increase and become the greater the further away the boundary of the vessel is. These differences of concentration will attain their greatest value if no diminution of the concentration in the outermost layer is allowed t o take place and if the yeast ceil has been operative for an indefinite period of time, so that the " permanent " state has been attained. I n the latter case the amount of sugar removed by the yeast cell per unit of time is the same as that flowing to it, owing to diffusion across any concentric spherical surface inside the sphere.