Influence of temperature and pH on xylitol production from xylose by Debaryomyces hansenii UFV-170

The effects of temperature and starting pH on xylose-to-xylitol bioconversion by the new yeast isolate Debaryomyces hansenii UFV-170 were investigated. The optimum pH resulted to be in the range 4–8, being the maximum production of xylitol (Pm) 37.5–41.8 g l−1, volumetric productivity (QP) 0.70–1.0 g l−1 h−1, specific productivity (qP) 0.19–0.30 g g−1 h−1 and xylitol yield on consumed xylose (YP/S) 0.70–0.76 g g−1. The percentage of xylose consumed for xylitol production progressively increased with pH, whereas those associated to both biomass growth and catabolic reaction through the TCA cycle decreased, reaching nearly constant values at pH 4.0. The optimum temperature range for xylitol production (Topt) was 30–35 °C, under which conditions the yeast exhibited YP/S  =  0.74–0.77 g g−1, QP = 0.96–1.1 g l−1 h−1 and qP = 0.26–0.40 g g−1 h−1, whereas the process was hardly affected either at lower (10–20 °C) or higher (40–45 °C) temperatures. At low temperature (15 °C), the largest percentages of xylose were consumed by the catabolic reaction through the TCA cycle (46%) and for biomass production (43%), while xylitol formation became the most significant activity at 20 °C, further increased up to 30–35 °C and then decreased over 40 °C. The results collected at variable temperature were finally used for estimation of the main thermodynamic parameters of the system: the activation enthalpy and entropy of xylitol formation were 96.1 kJ mol−1 and −0.008 kJ mol−1 K−1, while the standard enthalpy and entropy of thermal inactivation were 294 kJ mol−1 and 0.89 kJ mol−1 K−1, respectively.