Effects of high pressure extraction on the extraction yield, phenolic compounds, antioxidant and anti-tyrosinase activity of Djulis hull.

The hulls of Djulis () are a type of agricultural waste. Using 70% ethanol as the extraction solvent, this study compared the extraction yields of high-pressure-assisted extraction (HPE) and conventional oscillation extraction (CE) for Djulis hulls (DH). The total phenolic and flavonoid contents, and antioxidant, anti-inflammatory and anti-tyrosinase activities were also compared. Our findings indicated that 600 MPa/5 min of HPE resulted in higher total phenolic (567-642 mg GAE/g) and flavonoid (47.2-57.2 mg QU/g) concentrations; gallic acid (44.5-53.2 μg/g) and rutin (26.8-34.2 μg/g) were the main phenolic and flavonoid compounds. When the extraction pressure was greater than 450 MPa, HPE extracts showed stronger antioxidant capacity and anti-tyrosinase activity than CE extracts. In a LPS-induced RAW 264.7 cell model of inflammation, HPE extracts had significant inhibitory effects on the cumulative concentrations of nitric oxide and prostaglandin E. These results indicate that HPE had a better extraction yield, and required a shorter time for the extraction of functional ingredients from DH. Hence, DH could be a potential source for natural antioxidants for the food and biotechnology industries.