Bioprocessing of coconut husk to produce second-generation bioethanol using Zymomonas mobilis ZM4

Coconut husk is an abundant lignocellulosic residue generated by the tropical coconut processing industry and remains largely underutilized despite its high carbohydrate content. However, its recalcitrant structure limits efficient conversion into fermentable sugars and bioethanol. In this study, an integrated strategy involving sequential pretreatment, enzymatic hydrolysis, and microbial fermentation was developed for efficient valorization of coconut husk into second-generation bioethanol. Sequential pretreatment using dilute acid, alkaline treatment, and bleaching significantly improved cellulose accessibility, as confirmed by Fourier transform infrared spectroscopy analysis. Enzymatic hydrolysis using a cellulase–hemicellulase cocktail released substantial fermentable sugars, yielding approximately 230 mg glucose and 161.25 mg xylose per gram of raw coconut husk. High-performance liquid chromatography analysis confirmed efficient saccharification, corresponding to sugar recovery efficiencies of approximately 50–60% following pretreatment and hydrolysis. Mass balance analysis indicated that enzymatic hydrolysis released 0.23 g glucose g⁻¹ biomass, of which Zymomonas mobilis converted approximately 83% into ethanol, resulting in a final yield of 0.0975 g ethanol g⁻¹ biomass. These findings demonstrate the feasibility of converting coconut husk into fermentable sugars and bioethanol and highlight its potential as a sustainable feedstock for second-generation biofuel production. The integrated bioconversion framework presented here provides insights for improving biomass utilization and supports the development of circular bioeconomy strategies for agricultural residue valorization.