Water relations of polyol accumulation by Zygosaccharomyces rouxii in continuous culture

Summary Glycerol and arabitol were the main polyols accumulated by Zygosaccharomyces rouxii in continuous culture but the intracellular and extracellular concentrations of the polyols varied with the dilution rate and osmoticum used to adjust the water activity (a_w) to 0.960. When the a_w was adjusted with NaCl, glycerol was the main polyol accumulated intracellularly whereas glycerol and arabitol were accumulated when polyethylene glycol (PEG) 400 was used. The extracellular glycerol and arabitol concentrations at 0.960 a_w (NaCl or PEG 400) were similar or decreased relative to cultures at 0.998 a_w. Compared to steady-state cultivation at 0.998 a_w, the yeast retained at 0.960 a_w (NaCl or PEG 400) a greater proportion of the total glycerol intracellularly against an increased concentration ratio without significantly greater production of glycerol. Arabitol was only significant in osmoregulation when cultivated at 0.960 a_w (PEG 400). The intracellular glycerol concentration was insufficient to balance the a_w across the membrane, but an equilibrium could be achieved under certain conditions if arabitol was also osmotically active. Offprint requests to: P. J. van Zyl