The production of ethanol was carried out in two steps: hydrolysis of the oil-free spend coffee ground followed by fermentation.
The oil-free spent coffee grounds were hydrolyzed with diluted sulfuric acid. Different concentrations of H2SO4 (0.4 mol L−1 and 0.8 mol L−1) were applied and studied to evaluate its performance. The oil-free SCG and H2SO4 solution were mixed as to form a slurry with a solid concentration of 20%. The slurry was homogenized for 5 min to ensure that the solids were dispersed uniformly. The hydrolysis was carried out in an autoclave for 15 min at 121 °C. After hydrolysis, the solid fraction was separated from the liquid through vacuum filtration (GAST Manufacturing model DOA-P704, Michigan, USA). The pH of the liquid fraction was adjusted to 5.0 ± 0.3 with a NaOH solution (1 mol L−1) and the amount of glucose, xylarabin (xylose plus and arabinose), 5-hyroxymethyl-2-furfural, furfural, lactic acid and acetic acid were analyzed. All experiments were conducted in triplicate. A treatment carried out without adding sulfuric acid was used as a control.