Growth of Osmotolerant Yeasts at Different Water Activity Values

The influence of water activity (a) on growth (lag-phase, mean generation time and cell yield) of osmotolerant yeasts was determined by culturing 7 strains in broths at 10 different a values in the range of 0.998 to 0.626 and by counting the Colony Forming Unit (CFU) per ml. Broths were adjusted to the desired a by means of fructose. None of the tested strains could grow at a 0.701 and 30°C. During 60 d of incubation at this a and temperature, slight reductions of the initial CFU/ml counts were noted. By incubation at a <0.701 these reductions were larger. Six strains of Zygosaccharomyces rouxii grew at a 0.760, whilst a strain of Zygosaccharomyces bailii could not grow at a <0.858. For six strains the optimum a for growth was in the range of 0.958 to 0.998. A single strain of Z. rouxii showed optimum a for growth in the range of 0.913-0.958. Therefore, it was appropriate to redefine it as "osmophilic". Because of high dehydration, cells actively grown at a 0.837 were approximately 30% smaller than cells actively grown at a 0.998.