Abstract
Largely produced agri-food byproducts represent a sustainable and easily available source of phenolic compounds, such as lignins and tannins, endowed with potent antioxidant properties. We report herein the characterization of the antioxidant properties of nine plant-derived byproducts. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing/antioxidant power (FRAP) assays indicated the superior activity of pomegranate peels and seeds, grape pomace and pecan nut shell. An increase in the antioxidant potency was observed for most of the waste materials following a hydrolytic treatment, with the exception of the condensed tannin-rich pecan nut shell and grape pomace. UV-Vis and HPLC investigation of the soluble fractions coupled with the results from IR analysis and chemical degradation approaches on the whole materials allowed to conclude that the improvement of the antioxidant properties was due not only to removal of non-active components (mainly carbohydrates), but also to structural modifications of the phenolic compounds. Parallel experiments run on natural and bioinspired model phenolic polymers suggested that these structural modifications positively impacted on the antioxidant properties of lignins and hydrolyzable tannins, whereas significant degradation of condensed tannin moieties occurred, likely responsible for the lowering of the reducing power observed for grape pomace and pecan nut shell. These results open new perspectives toward the exploitation and manipulation of agri-food byproducts for application as antioxidant additives in functional materials.
Highlights
The antioxidant properties of the selected agri-food byproducts were investigated by two widely used assays, i.e., the DPPH and ferric reducing/antioxidant power (FRAP) assay, following the “QUENCHER” method, which allows to measure the efficiency of electron transfer processes from a solid antioxidant [76,77,78]
A similar trend was observed in the FRAP assay, with banana and pineapple wastes characterized by a number of Trolox eqs circa one order of magnitude lower than those exhibited by pecan nut shell, pomegranate wastes and grape pomace, whereas the other agri-food byproducts showed an even lower reducing activity
Total Phenolic Content (TPC) values about three-fold lower than those exhibited by the untreated materials were observed for pomace and pecan nut shell, pointing to substantial chemical modifications induced by the hydrolytic hydrolyzed grape pomace and pecan nut shell, pointing to substantial chemical modifications induced by the hydrolytic treatment on these agri-food byproducts, responsible for the lowering of the antioxidant properties emerging from the DPPH and the FRAP assay
Summary
One-third of food produced for human consumption, which is about 1.3 billion tons per year, is lost or wasted as the result of several processes occurring along the supply chain [1,2]. The effects of the hydrolytic treatment have been interpreted in terms of removal of the polysaccharidic fraction and dehydration/aromatization of the phenolic components, making the OH functionalities more available for interaction with oxidized species [68] Based on these findings, we report the results of a systematic evaluation of the antioxidant properties of a series of plant-derived byproducts, selected among those produced in largest amounts by the agri-food industry. In order to interpret the different activation/deactivation effects observed, the main structural modifications induced on the phenolic components by the hydrolytic treatment were investigated by spectroscopic techniques and chemical degradation methods, in comparison with those occurring on model natural and bioinspired, synthetic phenolic polymers
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