Stability Analysis and Optimal Xylitol Harvesting by Debaryomyces hansenii on OPEFB Hydrolysate

Abstract

Although xylitol fermentation from lignocellulosic hydrolysates has received considerable modeling attention, optimal harvesting timing and duration remain insufficiently explored. This study determined the optimal harvesting schedule for xylitol produced by Debaryomyces hansenii from oil palm empty fruit bunch (OPEFB) hydrolysate using mathematical analysis and numerical simulation. An existing fourteen-metabolite model was reduced to a three-dimensional xylose–xylitol–xylulose cascade using kinetic parameters from the original model. The reduced system was examined through equilibrium and stability analysis, while the peak time and width of a Gaussian harvesting window were optimized using grid search and Nelder–Mead optimization in MATLAB. The reduction preserved the full model’s steady state exactly, and the analysis established a unique, globally stable positive equilibrium under active harvesting. With the window width adopted in the earlier model, advancing the harvesting peak from 72 h to approximately 17 h increased harvested xylitol by 66.9%. Under the parameter perturbations examined, optimal peak times ranged from 34 to 44 h. Even continuous harvesting recovered less than 20% of the supplied precursor, with oxidation by xylitol dehydrogenase limiting recovery. These findings demonstrate the value of reduced metabolic models for bioprocess optimization and identify harvesting timing as an important determinant of predicted xylitol recovery. The study provides model-based scheduling guidance that complements enzyme-level strategies, while highlighting the need to incorporate biomass growth and experimentally validate the proposed harvesting schedules.