Assessing Process Conditions on Xylose Fermentation in Spathaspora passalidarum: Effects of pH, Substrate-to-Inoculum Ratio, Temperature, and Initial Ethanol Concentration.

Bioethanol represents a clean and renewable alternative to fossil fuels, offering a significant reduction in environmental impact. Second-generation ethanol (2G) is produced using lignocellulosic biomass, which presents additional challenges due to the presence of hemicellulose. The pentose sugars within hemicellulose cannot be efficiently metabolized by conventional yeast strains like Saccharomyces cerevisiae. Consequently, the yeast Spathaspora passalidarum has emerged as a promising candidate for mixed fermentation processes, given its ability to utilize xylose. This study presents an in-depth metabolic, stoichiometric, and kinetic analysis of the fermentation performance of Sp. passalidarum NRRL Y-27907 in mixed glucose and xylose cultures. Emphasis was placed on examining variables from a novel perspective compared to existing literature. Specifically, the impacts of initial inoculum-substrate ratios, substrate composition, pH, temperature, and ethanol sensitivity were analyzed using a mathematical bioprocess approach. Sp. passalidarum NRRL Y-27907 exhibited sequential sugar consumption, with xylose being utilized only after glucose was exhausted. Ethanol yields in mixed cultures were comparable to those in individual-sugar cultures. The best fermentative performance was observed at 30 °C, with 25 g/L of xylose and an inoculum of 0.50 g/L. The strain exhibited significant robustness at pH 4.0 and was notably affected by initial ethanol concentrations up to 20 g/L. These findings provide crucial insights into the metabolic and fermentative behavior of Sp. passalidarum NRRL Y-27907, offering valuable information for the design of consolidated bioprocesses from lignocellulosic materials.