Free, soluble-bound and insoluble-bound phenolics and their bioactivity in raspberry pomace

The free, soluble-bound (esterified and glycosylated) and insoluble-bound phenolics from the dry raspberry pomace without seeds (RP) and raspberry seeds (RS) were obtained by acidic and alkali treatment. The total phenolic contents of different phenolic fractions from RP and RS by the Folin method followed the order: free > esterified ≈ insoluble-bound > glycosylated. Then 18 phenolics from RP and RS were quantified by HPLC-MS. Free phenolics had the highest total tested phenolic contents, while glycosylated phenolics were the lowest. Their antioxidant activity was increased by the increasing phenolic contents of different fractions. Free phenolics in raspberry pomace had the highest antioxidant activity by DPPH assay. RS-esterified phenolics had the hightest tyrosinase inhibitory activity (TIA) by enzyme activity analysis, which was equivalent to the TIA of 1.57 mM vitamin C. Among 9 typical phenolics, quercetin, chlorogenic acid, gallic acid and p-hydroxybenzoic acid on the TIA showed obvious promoting effects, while catechin, epicatechin, and caffeic acid exhibited a significant background effect. Therefore, p-hydroxybenzoic acid and gallic acid in RS-esterified phenolics were of great significance for the TIA. These results are beneficial to the comprehensive evaluation and utilization of the phenolic compounds in raspberry pomace.