Effect of redox potential on stationary-phase xylitol fermentations using Candida tropicalis

Abstract

Redox potential was used to develop a stationary-phase fermentation of Candida tropicalis that resulted in non-growth conditions with a limited decline in cell viability, a xylitol yield of 0.87 g g⁻¹ (95% of the theoretical value), and a high maximum specific production rate (0.67 g g⁻¹ h⁻¹). A redox potential of 100 mV was found to be optimum for xylitol production over the range 0–150 mV. A shift from ethanol to xylitol production occurred when the redox potential was reduced from 50 mV to 100 mV as cumulative ethanol (Y _ethanol) decreased from 0.34 g g⁻¹ to 0.025 g g⁻¹ and Y _xylitol increased from 0.15 g g⁻¹ to 0.87 g g⁻¹ (α=0.05). Reducing the redox potential to 150 mV did not improve the fermentation. Instead, the xylitol yield and productivity decreased to 0.63 g g⁻¹ and 0.58 g g⁻¹ h⁻¹ respectively and cell viability declined. The viable, stationary-phase fermentation could be used to develop a continuous fermentation process, significantly increasing volumetric productivity and reducing downstream separation costs, potentially by the use of a membrane cell-recycle reactor.