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Spent coffee grounds (SCG) are a potentially valuable source of lipids for sustainable production of biofuels. However, there are several feedstock properties and solvent extraction parameters that can impact on the oil yield and quality, potentially reducing the possible environmental benefits of deriving oils from this waste stream. This study presents results of laboratory and pilot plant scale experimental investigations into lipid recovery from spent coffee, determining the effects of solvent extraction variables including duration, SCG-to-solvent ratio and SCG residual moisture. SCG samples from both the industrial production of instant coffee and retail coffee shops were characterized in terms of moisture content, particle size distribution and oil content to identify the impact of these variables on the efficiency of lipid recovery by solvent extraction. The dry weight oil content of the instant SCG samples ranged from 24.2 to 30.4% w/w, while the retail SCG samples contained considerably lower amounts of lipids with their oil content ranging between 13.4 and 14.8% w/w. The highest oil yields were found at an extraction duration of 8 h, while a moisture content of ~2% w/w led to increased yields relative to completely dry samples. A pattern of increasing acidity with decreasing extraction duration was observed, suggesting preferential extraction of free fatty acids (FFA), with the fatty acid (FA) profile of the oil found to be similar to lipids commonly utilized for biofuel production.

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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.

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