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

Abstract

The production of xylitol from concentrated synthetic xylose solutions (S(o) = 130-135 g/L) by Debaryomyces hansenii was investigated at different pH and temperature values. At optimum starting pH (pH(o) = 5.5), T = 24 degrees C, and relatively low starting biomass levels (0.5-0.6 g(x)/L), 88% of xylose was utilized for xylitol production, the rest being preferentially fermented to ethanol (10%). Under these conditions, nearly 70% of initial carbon was recovered as xylitol, corresponding to final xylitol concentration of 91.9 g(P)/L, product yield on substrate of 0.81 g(P)/g(S), and maximum volumetric and specific productivities of 1.86 g(P)/L x h and 1.43 g(P)/g(x) x h, respectively. At higher and lower pH(o) values, respiration also became important, consuming up to 32% of xylose, while negligible amounts were utilized for cell growth (0.8-1.8%). The same approach extended to the effect of temperature on the metabolism of this yeast at pH(o) = 5.5 and higher biomass levels (1.4-3.0 g(x)/L) revealed that, at temperatures ranging from 32-37 degrees C, xylose was nearly completely consumed to produce xylitol, reaching a maximum volumetric productivity of 4.67 g(P)/L x h at 35 degrees C. Similarly, both respiration and ethanol fermentation became significant either at higher or at lower temperatures. Finally, to elucidate the kinetic mechanisms of both xylitol production and thermal inactivation of the system, the related thermodynamic parameters were estimated from the experimental data with the Arrhenius model: activation enthalpy and entropy were 57.7 kJ/mol and -0.152 kJ/mol x K for xylitol production and 187.3 kJ/mol and 0.054 kJ/mol x K for thermal inactivation, respectively.

Mutation of Aspergillus oryzae for improved production of 3, 4-dihydroxy phenyl-L-alanine (L-DOPA) from L-tyrosine

“…As these values are consistent with the values for formation of a transition state with more rigid structure with respect to the reacting system, they further lead to suggest that the phenomenon limiting L-DOPA production could be an enzymatic reaction under all varying fermentation conditions as observed for xylitol production (7).…”

“…The activation entropy of L-DOPA formation (-0.076 kJ/mol K) is very low and compares favorably with that of glucose isomerase reaction (6) and other fermentation processes (3,7). As these values are consistent with the values for formation of a transition state with more rigid structure with respect to the reacting system, they further lead to suggest that the phenomenon limiting L-DOPA production could be an enzymatic reaction under all varying fermentation conditions as observed for xylitol production (7).…”

“…In these studies, qp values have been used to calculate activation enthalpy and entropy for product formation and its deactivation after Aiba et al (1) while other workers used Qp values to calculate these parameters (2,3,7). It was found that the values calculated by this model ere comparable with those obtained with latter method.…”

Hyper-production of a thermotolerant b-xylosidase by a deoxy-D-glucose and cycloheximide resistant mutant derivative of Kluyveromyces marxianus PPY 125

“…previously (Lawford and Rousseau, 1993;Converti and Dominguez, 2001) in shake-flasks. Thermodynamics of enzyme production was performed as described previously (Converti and Dominguez, 2001).…”

“…Thermodynamics of enzyme production was performed as described previously (Converti and Dominguez, 2001). Kinetics of enzyme was performed as described previously (Siddiqui et al 1997).…”