Recovery of oils and antioxidants from olive stones

The olive oil industry generates enormous amounts of olive stones each year, which have the potential to be used as a biofuel but have high oil content, which negatively impacts the combustion process. In addition, olive stones contain high-value antioxidants, and their exploitation can provide additional revenues for the biofuel industry. In this work, we report the effect of different extraction solvents on the extraction of antioxidants and their activity. In addition, in vitro gastrointestinal digestion was used to evaluate the content and antioxidant activity of the olive stone extracts after gastrointestinal digestion. The extracts obtained by aqueous ethanol solvent (50% vol) exhibited the highest antioxidant activity with the DPPH IC50 of 1.27 mg mL⁻¹ and ferric reducing antioxidant power (FRAP) of 6.33 mg AAE g⁻¹. After in vitro digestion composed of gastric and intestinal processes, the antioxidant activity of olive stones decreased: DPPH IC50 value increased three times (a higher value of IC50 indicates lower antioxidant activity) and FRAP decreased almost five times with respect to the values obtained for original extracts. Furthermore, both phenomenological and shrinking core models were used to fit experimental oil extraction kinetics data and showed good agreement. Thermodynamic analysis showed that the extraction process is endothermic and irreversible while spontaneous and thermodynamically favourable for all conditions except for oil extraction from olive stones of 3.10 mm particle size at 20 °C. The calculated value for temperature coefficient is in good agreement with the previously reported values for the oil extraction from similar biomass.