Non-Extractable Polyphenols from Food By-Products: Current Knowledge on Recovery, Characterisation, and Potential Applications

Abstract

Non-extractable polyphenols (NEPs), or bound polyphenols, are a significant fraction of polyphenols that are retained in the extraction residues after conventional aqueous organic solvent extraction. They include both high molecular weight polymeric polyphenols and low molecular weight phenolics attached to macromolecules. Current knowledge proved that these bioactive compounds possess high antioxidant, antidiabetic, and other biological activities. Plant-based food by-products, such as peels, pomace, and seeds, possess high amount of NEPs. The recovery of these valuable compounds is considered an effective way to recycle food by-products and mitigate pollution, bad manufacturing practice, and economic loss caused by the residues management. The current challenge to valorise NEPs from plant-based by-products is to increase the extraction efficiency with proper techniques, choose appropriate characterising methods, and explore potential functions to use in some products. Based on this scenario, the present review aims to summarise the extraction procedure and technologies applied to recover NEPs from plant-based by-products. Furthermore, it also describes the main techniques used for the characterisation of NEPs and outlines their potential food, pharmaceutical, nutraceutical, and cosmetic applications.

Polyphenol-Mediated Gut Microbiota Modulation: Toward Prebiotics and Further

“…(Poly)phenol polymers, which represent a large proportion of those ingested in the diet, are often intimately bound through hydrophobic and hydrogen bonds to plant cell wall matrix and therefore reach the colon in the form of (poly)phenolic fibers ( 235 , 236 ). A clear case of fiber-associated (poly)phenols are polymeric flavan-3-ols, whose fermentation leads to SCFA biosynthesis while releasing the so-called “non-extractable (poly)phenols” ( 237 , 238 ), thereby illustrating another example of trophic interactions in the gut environment. For instance, Mateos-Martin et al ( 239 ) studied the fate and metabolism of grape dietary fiber rich in non-extractible PAC (14.8%), in female Sprague–Dawley rats for 24 h. The grape residues obtained after extraction with 70% acetone were rich sources of non-extractible PAC polymers.…”

Changes in structural and chemical composition of insoluble dietary fibers bound phenolic complexes from grape pomace by alkaline hydrolysis treatment

“…Bound phenolics are also known as non-extractable polyphenols (NEP's), and alkaline hydrolysis is one of the technical modality to achieve effective recovery of bound phenolics (Dzah et al, 2020). The possible reason for higher TPC of SDF-derived bound phenolics might the possibility of induction of cleavage due to alkaline pH between ester bonds existing between cell wall and phenolic acids which further led to achieve increased mass transfer of phenolic compounds from grape pomace matrix (Ding et al, 2020). Naturally, it is evident that change in bound phenolic content may exert significant effect on antioxidant capacity measured through ABTS and FRAP assays.…”

Effect of the Olive Oil Extraction Process on the Formation of Complex Pectin–Polyphenols and Their Antioxidant and Antiproliferative Activities

“…Unlike extractable polyphenols, non-extractable polyphenols are not extractable by organic solvents. They are generally higher molecular weight polyphenols, with a complex structure that can form polymeric compounds or bind to carbohydrates or proteins [ 7 , 8 ]. Such interactions can modify the bioaccessibility, bioavailability and bioefficacy of polyphenols.…”