Effect of medium osmolarity on the bioproduction of glycerol and ethanol by Hansenula anomala growing on glucose and ammonium

The osmotolerant yeast Hansenula anomala survives in media at low water activity resulting from increasing NaCl concentrations in the culture medium by producing compatible solutes. High salinity resulted in the use of a large part of the assimilated carbon substrate (glucose) for cell maintenance (28%), required for intracellular synthesis compounds and for osmotic cell regulation. The maintenance coefficient for non-growth-associated glucose consumption was found to be 0.38 mmol glucose g biomass⁻¹ h⁻¹. For decreasing water activity, there is a competition between the pathways leading to glycerol and ethanol production, until an experimental ethanol/total glycerol ratio reached a value 3.4 for 2 mol l⁻¹ NaCl (close to the theoretical value of 4)—illustrating the osmodependent channelling of carbon towards polyols production. This competition leads to a cessation of ethanol production during the stationary state before that of glycerol. Since osmotic adjustment occurred mainly during growth, glycerol production during stationary state can be clearly related to another mechanism other than osmotic: it was excreted by a fermentative mechanism to ensure energy for cell maintenance.