Effect of temperature on the microaerophilic metabolism of Pachysolen tannophilus

Xylitol production by Pachysolen tannophilus from detoxified hemicellulose hydrolysate was investigated under microaerophilic conditions at temperature ranging from 20 to 40°C. A carbon balance previously proposed to study the influence of pH was used in this work to evaluate the amounts of carbon source (xylose) utilised in competitive metabolic ways: reductive production of xylitol, ethanol fermentation and respiration. At pH = 5.5 more than 83% of xylose was reduced to xylitol at 25 < T < 30°C, whereas respiration became the main process at low temperature (71.1% at 20°C). At high temperature, on the other hand, all three processes took place at comparable rate, consuming at 40°C nearly the same percentage of carbon source (33–35%). Finally, the maximum values of volumetric productivity calculated at variable temperature were used to estimate the main thermodynamic parameters of both xylitol production (Δh∗ = 105.4 kJ mol−1; Δs∗ = −13.2 J mol−1 K−1) and thermal deactivation (Δh∗D = 210.5 kJ mol−1; Δs∗D = 3.63 J mol−1 K−1).