Antioxidant and prooxidant activity of acid-hydrolyzed phenolic extracts of sugar beet leaves in oil-in-water emulsions

This study aimed to enhance the oxidative stability of soybean oil-in-water emulsions using acid-hydrolyzed and unhydrolyzed extracts obtained from sugar beet leaves. The optimum extraction process, which includes 8 min of ultrasonication followed by a 2-h acid hydrolysis, released new phenolics (e.g., catechin, myricetin, etc.) and increased the total phenolic content (TPC) from 586.24 ± 11.45 to 982.42 ± 6.61 μmol gallic acid equivalent (GAE)/L, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical inhibition from 46.63 ± 1.39 to 60.87 ± 1.12%. Acid hydrolysis increased the cupric chelating activity of the extracts while decreasing ferrous chelating activity and trans-ferulic acid concentration significantly (p < 0.05). The acid-hydrolyzed extract at a TPC of 100 μmol GAE/L prolonged the lag phase of hexanal accumulation in the emulsion from 0 to 8 days, while 400 μmol GAE/L TPC of unhydrolyzed extract increased the lag phase to 12 days. The results show that acid-hydrolyzed extracts in high concentrations may act as prooxidants.