Production of bioethanol and aroma compounds from pretreated coffee shell and coffee silverskins with binary and ternary deep eutectic solvents

Coffee shells and silverskins, which are underutilized by-products of the coffee processing industry, offer the potential for value-added products. This study investigated the use of binary and ternary deep eutectic solvents (DESs), choline chloride/oxalic acid (Chcl/OA), and choline chloride/oxalic acid/glycerol (Chcl/OA/Gly), respectively, to pretreat these residues to enhance efficiencies of downstream processes. Optimization experiments identified the optimal pretreatment conditions for coffee shells at 173 min, 120 °C, and 13.99 % and 15.76 % solid loading for binary and ternary DES, respectively. For coffee silverskins, 180 min, 120 °C, and 10 % and 10.21 % solid loadings were found to be optimal. Ternary DES pretreatment enriched the cellulose content and removed hemicellulose and lignin. Enzymatic hydrolysis after pretreatment yielded a two-fold increase in glucose for coffee shells (205.55 mg/g raw biomass) with binary DESs and for coffee silverskins (179.14 mg/g raw biomass) with ternary DESs. Fermentation with Saccharomyces cerevisiae and Kluyveromyces marxianus increased ethanol yields by up to 75 % for coffee shells and 77 % for coffee silverskins. The maximum yields of ethanol production of 33.29 g/L and 32.05 g/L, respectively, were obtained from coffee shells and silverskins, respectively, by using K. marxianus. Volatile aroma compounds, including phenylethyl alcohol and ethyl acetate, were produced during fermentation, indicating potential applications in food and cosmetics. The ternary DES demonstrated superior performance in the recovery of the cellulose content and improvement of ethanol yield, warranting further research to optimize volatile compound production for industrial use.