Use of iso‐osmolality plots to characterize yeast fermentative performance

Abstract

Yeast fermentation data (ethanol product, P, vs glucose substrate, S) can be plotted on a graph showing lines of equivalent osmolality or water activity. Comparison of such plots for different strains and for single strains under different environmental conditions shows the conditions for optimum performance. With these plots the product/substrate yield coefficient (Yp/s) can be shown to vary with time, contrary to its usually assumed constant value. Under conditions of optimum environment and nutrition, five of the yeast strains tested ceased growing at a characteristic value of solution osmolality, indicating that the inhibitory effects of ethanol and glucose, to a first approximation, are the result of their molar contribution to water activity rather than due to any specific cellular effect. In one yeast strain this characteristic behaviour was not affected by temperature variations between 30° and 36°C.

Intracellular ethanol — accumulation and exit from yeast and other cells

“…In simple terms the osmolality is the effective molarity of the solution after correction for non-ideal solution behaviour, i.e. for osmol/kg read mol/litre of the solute were it ideal [12][13]. Osmolality (~m) and A,o are related by an equation of the form: ~m = -55.51 x loge(A,~ ).…”

“…Yeast of the genus Saccharomyces cease growth at values of solution osmolality between 3 to 6 osmol/kg, i.e. around 3 to 6 mol/litre [12][13].…”

“…For solution concentrations of below 1 osmol/kg the intracellular osmolality of baker's and brewer's yeast is around 1 osmol/kg [12][13] and so the accumulation of any significant amount of ethanol would be in addition to this value. The accumulation of just 50 g/1 of ethanol inside any yeast cell would therefore result in a total intracellular osmolality of 2.1 osmol/kg which would give approximately 50% inhibition of growth rate [14].…”

Intracellular ethanol — accumulation and exit from yeast and other cells

“…A related term, that of solution osmolality which has the units of osmol/kg solvent, can be used in place of A,o. around 3 to 6 mol/litre [12][13]. for osmol/kg read mol/litre of the solute were it ideal [12][13].…”

“…AH (1) where, Pc = permeability coefficient for concentration driven diffusion, P~ = permeability coefficient for osmotic driven diffusion, AC = concentration gradient for the solute, /117 = gradient in osmotic pressure (X all solutes). The osmolality of a mixture of solutes is simply calculated by addition of the individual components [12][13]. As pointed out above, osmolality represents a molar concentration after correction for non-ideal solution behaviour from which osmotic pressure and water activity are both calculated.…”