Corylus avellana (hazelnut) is one of the most popular tree nuts on a worldwide basis. The main products of C. avellana are kernels, a nutritious food, with a high content of healthy lipids, contained in a hard shell. In recent years, along with the ongoing research carried out on hazelnut kernels, a growing interest has been addressed to the hazelnut byproducts including hazelnut skin, hazelnut hard shell, and hazelnut green leafy cover as well as hazelnut tree leaf. These byproducts deriving from the roasting, cracking, shelling/hulling, and harvesting processes have been found as a source of “phytochemicals” with biological activity. The aim of this review is to provide a comprehensive and critical update on the chemistry and biological activity of specialized metabolites occurring in hazelnut kernels and byproducts. Phenolics are the most abundant phytochemicals not only in the kernels, but also in other processing byproducts. Attention has been also devoted to taxane derivatives isolated from C. avellana leaves. An overview on the biological activity, mainly antioxidant, antiproliferative, and antimicrobial along with less common biological effects, has been provided, contributing to highlight C. avellana as a source of bioactive phytochemicals with the potential to exert beneficial effects on human health. Finally, analytical techniques for the quali-quantitative analysis of specialized metabolites occurring in the different parts of C. avellana have been reviewed.
Introduction
Prunus dulcis Mill. is a plant cultivated for the production of its edible seeds, known as sweet almonds. In the last decades almond demand has highly increased due to their wide use in the industrial manufacturing of almond‐based foods; this has led to a huge production of waste materials, mainly corresponding to shells and husks.
Objective
To achieve a deeper understanding of the chemical composition of almond husks with the aim of exploring these byproducts as a source of bioactives to be used in nutraceutical and cosmetic formulations.
Methods
Methanol, ethanol and ethanol–water extracts of the almond husks were analysed by high‐resolution liquid chromatography electrospray ionisation Orbitrap mass spectrometry (HR‐LC‐ESI‐Orbitrap‐MS) in negative ion mode. Tandem mass spectrometry (MS/MS) data were acquired by using the Data‐Dependent Scan experiment, allowing the precursor ion to be selected as the most intense peak during LC‐MS analyses. Nuclear magnetic resonance (NMR) experiments were performed on a Bruker DRX‐600 spectrometer. Folin–Ciocalteu, DPPH• (2,2‐diphenyl‐1‐picrylhydrazyl) and TEAC (Trolox equivalent antioxidant capacity) assays were employed to determine the total phenolic content and the radical scavenging activity of the extracts.
Results
The LC‐MS/MS analysis of the methanol extract guided the isolation of phloroglucinol derivatives, flavonoids and terpenes. Eco‐friendly extraction methods showed to be selective in extracting flavonoids while the comparison of the LC‐MS profiles of the Italian cultivars Toritto and Avola showed significant differences, confirming how different growing conditions may influence the metabolome of a plant species.
Conclusion
This study led to a deeper insight into the chemical constituents of almond husks and showed how the eco‐friendly extraction resulted in an effective method to obtain extracts rich in antioxidant sources.
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