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The response of 31 ascomycetous yeasts to a reduction in water activity (a w) adjusted with d-glucose or NaCl was investigated. The growth of most yeasts was more tolerant to glucose than to NaCl at equivalent a w. Zygosaccharomyces rouxii was the most osmotolerant yeast examined. Natural abundance 13C-NMR spectroscopy and HPLC analyses of eight yeasts indicated that glycerol and arabitol or mannitol were accumulated intracellularly in response to a w. reduction. Pichia sorbitophila, Candida cacaoi, Candida magnoliae and Zygosaccharomyces bisporus responded to reduced a w by a decrease in specific growth rate and cell volume, and an accumulation of glycerol. The other polyol accumulated did not increase in concentration with a w reduction to the same degree as glycerol. A polyol concentration ratio (intra/extracellular) as high as 800-fold was attained across the membrane. Greater amounts of polyols were produced at equivalent a w values when adjusted with glucose than with NaCl. The ability to accumulate high concentrations of polyols appears to be the most important criterion in determining osmotolerance.
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