Optimization of Chemical Pretreatments Using Response Surface Methodology for Second-Generation Ethanol Production from Coffee Husk Waste

Abstract

Within the strategies of substitution of energy from fossil fuels by renewable energies, the research is based on second-generation ethanol production (2G ethanol). One of the raw materials considered for this is residual biomass of the coffee industry, being the subject of study here. The cellulose contained in the coffee husk (coffee husk or coffee skin or coffee exocarp or pericarp) was maximized using pretreatment processes. In dilute acid hydrolysis (DAH), using a fixed 1:6 w:v solid to liquid ratio (SLR), process times (35, 45, 55 min) and H2SO4 concentrations (3, 4, 5% v/v) were evaluated, achieving 53.63% hemicellulose removal. A delignification process resulted in 58.82% lignin removal, evaluating the effect of process times (30, 35, 40 h) and SLR (1:8, 1:10, 1:12 w:v) at a fixed concentration of 8% v/v H2O2. A 115.59 g/L glucose concentration was obtained with an interaction of fixed concentrations of 4–6% w/w Cellic CTec3 enzyme and 6:1 to 1:12 v:w SLR. The fermentation process considered the composition variation of the culture medium (enriched culture C1 and non-enriched culture C2), generating ethanol at 48.19 and 29.02 g/L concentrations, respectively. Fermentation efficiency (ηf) was improved from 21.99 to 81.74% with the addition of inorganic nutrients (KH2PO4, (NH4)2SO4, and MgSO4·7H2O). These results confirmed that the optimization of the pretreatments in coffee husk waste favored the cellulose production and facilitated the enzymatic process to produce a high glucose concentration, revealing these residues as a carbon source promising for second-generation ethanol production.