Two-step high pressure solvent extraction of walnut (Juglans regia L.) husks: scCO2 + CO2/ethanol/H2O

Walnut husks, an abundant agroindustrial residue/waste rich in several bioactive phytochemicals, were subjected to a two-step high pressure solvent extraction methodology using supercritical CO2 (1st step) and a subcritical homogeneous liquid mixture of CO2/ethanol/H2O (0.69:0.28:0.03, molar fractions) (2nd step). Extractions were performed at 35 degrees C and 45 degrees C, and from similar to 10 MPa up to similar to 30 MPa. GC-MS analysis revealed that the most abundant compounds in CO2 extracts were alkanes, terpenes, oxygenated terpenes and alkaloids. The amounts of juglone plus 1,4-naphthoquinone in the 1st and 2nd step extracts were in the 0.33-0.88 mg/g and 0.65-2.77 mg/g range, respectively, and those of total phenols in 2nd step extracts were in the 71.5-116.8 mg GAE/g range. High antioxidant activities were obtained independently of juglone and 1,4-naphtoquinone contents, thus indicating the potential of the applied high pressure methodology to obtain selective extracts for valuable pharmaceutical, agricultural and food applications.