A Substrate–Product Switch Mathematical Model for the Growth Kinetics of Ethanol Metabolism from Longan Solid Waste Using Candida tropicalis

A substrate–product switch model was proposed to describe ethanol fermentation from longan solid waste using Candida tropicalis at an initial glucose and xylose ratio of 2 to 1. The model incorporated multiple rate equations for cell growth, sugar uptake, and ethanol production along with ethanol consumption. It elucidated the following three-step mechanism: (I) sugar uptake, (II) sugar conversion, and (III) ethanol consumption concerning the effects of concentration factor (CF) and associated growth function. Optimal kinetic parameters were estimated and validated against experimental data. The identification of two critical xylose concentrations showed that ethanol consumption either preceded or coincided with xylose consumption cessation. The phenolics inhibitory effect of gallic acid, ellagic acid, pyrogallol, and catechol on cell growth and ethanol production was elucidated with relatively minimal effect. The highest ethanol concentration of 25.5 g/L was reached with corresponding ethanol mass yield and productivity of 0.30 g/g and 1.063 g/L/h, respectively. The proposed model and kinetics provide valuable insights for designing and optimizing ethanol fermentation, contributing to more sustainable and cost-effective ethanol production.