Exploring the Potential of Coffee Husks as a Raw Material for Second-Generation Ethanol Production

Bioeconomy is a sustainable development strategy involving the production of high-value products using renewable resources and by-products instead of new raw materials to avoid waste. Second-generation ethanol is essential for producing high-value products from residues, and new sources of lignocellulosic biomass are crucial. Coffee is an important agricultural product: in Brazil, a major world producer, 3 million tons of coffee were produced in 2022. Coffee husks, a by-product of coffee, are a potential raw material for use in second-generation ethanol production. The overall purpose of this study was to evaluate the potential of this residue for ethanol production. A compositional analysis of coffee husks showed a high lignin content of 42%. The coffee husks were subjected to aqueous, acid, and alkali pretreatments, and the chemical composition of each fraction was determined. The lignin contents were high: 46%, 52%, and 42%, respectively. The production of yeast inhibitors, furfural, and hydroxymethylfurfural and also the production of reducing sugars in the liquid fraction were determined to verify the severity of the pretreatments. The pretreated material was saccharified to produce glucose. The saccharification process was optimized based on pH and temperature conditions to achieve maximum enzyme efficiency with conversion yield of 16.2%. The optimal conditions were pH 5.5 and a temperature range of 30–75°C. The second optimization process was carried out for the enzyme load and biomass concentration. The condition producing the highest glucose concentration was a biomass loading of 11–14% and an enzyme concentration of 20–25 FPU/g. The optimized conditions for saccharification produced 5 g/L of glucose. For biomass conversion yield, the 3.2% biomass and 25 FPU/g provided highest efficiency, 24.46%. Graphical Abstract