Supercritical fluid extraction from spent coffee grounds and coffee husks: Antioxidant activity and effect of operational variables on extract composition

The present study describes the chemical composition and the antioxidant activity of spent coffee grounds and coffee husks extracts, obtained by supercritical fluid extraction (SFE) with CO2 and with CO2 and co-solvent. In order to evaluate the high pressure method in terms of process yield, extract composition and antioxidant activity, low pressure methods, such as ultrasound (UE) and soxhlet (SOX) with different organic solvents, were also applied to obtain the extracts. The conditions for the SFE were: temperatures of 313.15 K, 323.15 K and 333.15 K and pressures from 100 bar to 300 bar. The SFE kinetics and the mathematical modeling of the overall extraction curves (OEC) were also investigated. The extracts obtained by LPE (low pressure extraction) with ethanol showed the best results for the global extraction yield (X0) when compared to SFE results. The best extraction yield was 15 ± 2% for spent coffee grounds with ethanol and 3.1 ± 04% for coffee husks. The antioxidant potential was evaluated by DPPH method, ABTS method and Folin–Ciocalteau method. The best antioxidant activity was showed by coffee husk extracts obtained by LPE. The quantification and the identification of the extracts were accomplished using HPLC analysis. The main compounds identified were caffeine and chlorogenic acid for the supercritical extracts from coffee husks.