Hydroxyl radicals scavenging activity and corresponding mechanism of polyphenols in black pepper

Spices are widely used in meat processing industry for their unique flavor, the contained polyphenols could scavenge free radicals produced from lipid oxidation, and prolong the shelf life of meat products, and the polyphenols play a key role in these common applications. However, the multi-polyphenol species with various contents in spice powder or related products make it difficult to explain their effects in the food industry, and hinder further investigation. In this paper, polyphenol extracts in black pepper, cinnamon, clove, nutmeg, and fennel were prepared and their hydroxyl radical scavenging ability was determined by the Fe³⁺-salicylate-H₂O₂ system method. The polyphenol extract with better scavenging ability was selected, and the inclusive monomers were quantified with UHPLC-MS/MS for screening better scavengers. Then the O-H bond dissociation energies and spin density difference were calculated to explore the relationship between polyphenol structure, polyphenol content change, and the hydroxyl radicals scavenging capacity using the density functional theory. Results showed that the hydroxyl radicals scavenging percentage of black pepper polyphenol extract was 23.26 ± 1.45%. The 3-hydroxybenzoic acid, gallocatechin-3-O-gallate, 3,4′-dihydroxypropiophenone-3-β-D-glucoside, ethyl 4'-hydroxy-3'-methoxycinnamate, and sinapic acid contents decreased by 82.8%, 33.1%, 15.8%, 9.4%, 5.9% in the presence of hydroxyl radicals, with the corresponding dissociation energy of 0.4624 Eh, 0.4652 Eh, 0.4698 Eh, 0.4705 Eh and 0.4655 Eh, spin density difference of 1.3971, 1.1254, 1.0893, 1.1091 and 1.0426. Among these, 4-hydroxybenzoic acid had the best hydroxyl radical scavenging activity, for lower bond dissociation energy difference and larger spin density value difference of 7-OH site.