Chestnut shell represents a rich source of polyphenols: preparation methods, antioxidant activity and composition analysis of extractable and non-extractable polyphenols

Chestnut shell, a by-product of chestnut processing, is rich in bioactive polyphenols, which can be divided into extractable polyphenols (EPs) and non-extractable phenols (NEPs). The EPs have arisen a lot of interests, while NEPs combined with cellulose in the residue are ignored. This work describes the development of a traditional ethanol extraction method to obtain extractable polyphenols (EPs), and the residue was treated by alkaline hydrolysis to prepare NEPs optimized by Box–Behnken experimental designs. The total phenolic contents and the scavenging effects of DPPH and ABTS + free radicals were measured. Using UPLC–ESI-MS/MS technology, combined with principal component analysis (PCA) and orthogonal partial least square discriminant analysis (OPLS-DA), qualitative and quantitative analysis and difference analysis of EPs and NEPs were carried out. Optimal extraction conditions to extract NEPs by alkaline hydrolysis method were obtained, and the content of polyphenols obtained from NEPs was significantly higher than that of EPs. EPs have the higher scavenging effects of DPPH and ABTS + free radicals than NEPs. A total of 466 polyphenol compounds were detected. Among them, EPs are rich in flavonoids, phenolic acids, tannins, lignans coumarins and quinones, while phenolic acids were dominant in NEPs, which provides a reference for the utilization of polyphenols from the chestnut shell. Both EPs and NEPs are abundant in phenolic composition, although there were significant differences in the compositional situation. Notably, NEPs are a widespread source of polyphenols but have not received much attention in practical applications. This study can provide a basis for the reuse of chestnut waste.