The effect of aeration on xylose fermentation by Candida shehatae and Pachysolen tannophilus

Abstract

The ability of a Candida shehatae and a Pachysolen tannophilus strain to ferment D-xylose to ethanol was evaluated in defined and complex media under different levels of aeration. Aeration enhanced the ethanol productivity of both yeasts considerably. C. shehatae maintained a higher fermentation rate and ethanol yield than P. tannophilus over a wide range of aeration levels. Ethanol production by C. shehatae commenced during the early stage of the fermentation, whereas with P. tannophilus there was a considerable lag between the initiation of growth and ethanol production. Both yeasts produced appreciable quantities of xylitol late in the fermentation. P. tannophilus failed to grow under anoxic conditions, producing a maximum of only 0.5 g · l-1 ethanol. In comparison, C. shehatae exhibited limited growth in anoxic cultures, and produced ethanol much more rapidly. Under the condition of aeration where C. shehatae exhibited the highest ethanol productivity, the fermentation parameters were: maximum specific growth rate, 0.15 h-1; maximum volumetric and specific rates of ethanol production, 0.7 g (l · h)-1 and 0.34 g ethanol (g cells · h)-1 respectively; ethanol yield, 0.36 g (g xylose)-1. The best values obtained with P. tannophilus were: maximum specific growth rate, 0.14 h-1; maximum volumetric and specific rates of ethanol production, 0.22 g (l · h)-1 and 0.07 h-1 respectively; ethanol yield coefficient, 0.28. Because of its higher ethanol productivity at various levels of aeration, C. shehatae has a greater potential for ethanol production from xylose than P. tannophilus.