The effect of the water activity of the milieu on rates of glucose uptake by the osmophilic yeasts Saccharomyces rouxii and Debaryomyces hansenii.

Rates of glucose uptake in baker's yeast and in the osmophilic yeasts D. hansenii and S. rouxii were investigated at different values of water activity of the milieu, as regulated either by glycerol or sodium chloride. In both cases, D. hansenii could maintain relatively higher rates of glucose uptake. At lower values of water activity, sodium chloride exerted an inhibitory effect on rates of glucose uptake by S. rouxii, while in the presence of glycerol, rates of glucose uptake shown by S. rouxii resembled those shown by D. hansenii. Rates of glucose uptake by baker's yeast were drastically affected at lower values of water activity in the presence of either solute. Lower values of water activity exerted a stimulatory effect on catalase activity of both S. rouxii and D. hansenii. However, activities of baker's yeast with regard to catalase and invertase were moderately affected under such conditions. Results presented may lead to the presumption that osmophilic yeasts, at least partly, have solved the problem of osmotic tolerance over nonosmotolerant strains by possessing a high capacity for maintaining higher rates of glucose uptake, in spite of the adverse external concentration of solute.