Factors affecting the efficiency of pressurized solvent extraction of oil from spent coffee grounds

Abstract

Spent coffee grounds (SCG) are the main residues of the coffee beverage industry, and a potentially valuable source of energy-dense lipids. The present study investigates the effect of temperature, pressure and duration on the pressurized solvent extraction of lipids from dried SCG to evaluate their impact on oil extraction efficiency, composition and energy content. Solvent extraction trials at elevated temperatures performed by an Accelerated solvent extraction (ASE) unit revealed a beneficial effect of increased temperature on the crude lipid extracting efficiency of polar ethanol. Nuclear magnetic resonance (NMR) analysis showed that extraction temperature had a significant impact on the proportions of the ASE-extracted SCG oil constituents, with increasing temperature leading to lower concentration of triglycerides and higher levels of 1,2-and 1,3-diglycerides. Experiments performed in a closed pressurized vessel allowed individual selection of pressure and temperature and showed that pressure had a significant impact on oil extracting efficiency that largely depended on process temperature. There was no systematic effect of extraction temperature and pressure on the higher heating value (HHV) of SCG lipid samples, which ranged between 38.53 MJ/kg and 39.12 MJ/kg, while extraction pressure was found to have negligible effect on the free fatty acid (FFA) content of lipids extracted by pressure vessel experiments. Prolongation of pressurized extraction duration above 10 minutes was found to be counterproductive in terms of oil recovery, and consecutive short extraction static cycles with fresh solvent removed higher amounts of SCG oil relative to a single longer cycle.