Optimization of the supercritical fluid coextraction of oil and diterpenes from spent coffee grounds using experimental design and response surface methodology

• Supercritical fluid extraction (SFE) of spent coffee grounds (SCG).
• Optimization of three operating conditions (P, T, %EtOH) using Box–Behnken design and response surface methodology.
• Soxhlet extraction (n-hexane) was performed and set as reference for SFE results.
• Optimized SFE led to total extraction yields 39–79% lower than Soxhlet results.
• Optimized SFE led to diterpenes concentrations 212–410% higher than Soxhlet results.

The reported work aimed at the optimization of operating conditions of the supercritical fluid extraction (SFE) of spent coffee grounds (SCG) using pure or modified CO2, with particular emphasis on oil enrichment with diterpenes like kahweol, cafestol and 16-O-methylcafestol. The analysis comprised the application of Box–Behnken design of experiments and response surface methodology, and involved three operating variables: pressure (140–190 bar), temperature (40–70 °C) and cosolvent (ethanol) addition (0–5 wt.%). The best conditions to maximize total extraction yield are 190 bar/55 °C/5 wt.% EtOH, leading to 11.97% (goil/100 gSCG). In terms of the concentration of diterpenic compounds in the supercritical extracts, the best operating conditions are 140 bar/40 °C/0 wt.% EtOH, providing 102.90 mg g−1oil. The measurement of extraction curves near optimized conditions (140 bar/55 °C/0 wt.% EtOH and 190 bar/55 °C/0 wt.% EtOH) confirmed the trends of the statistical analysis and revealed that SFE enhances diterpenes concentration by 212–410% at the expenses of reducing the extraction yield between 39% and 79% in comparison to n-hexane extraction.

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