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.

中文翻译：

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食品副产品中不可提取的多酚：有关回收，表征和潜在应用的最新知识

不可萃取的多酚（NEP）或结合的多酚是在常规水性有机溶剂萃取后保留在萃取残留物中的大部分多酚。它们包括附着在大分子上的高分子量聚合物多酚和低分子量酚类。当前的知识证明这些生物活性化合物具有很高的抗氧化，抗糖尿病和其他生物活性。植物性食品副产品（例如果皮，果渣和种子）具有大量NEP。这些有价值的化合物的回收被认为是回收食品副产品并减轻污染，不良生产实践以及残留管理带来的经济损失的有效方法。从植物副产品中提取NEP的当前挑战是采用适当的技术提高提取效率，选择适当的表征方法并探索在某些产品中使用的潜在功能。基于这种情况，本综述旨在总结提取程序和用于从植物副产品中回收NEP的技术。此外，它还描述了用于表征NEP的主要技术，并概述了它们在食品，药物，营养保健品和化妆品中的潜在应用。