Chemical composition, phytochemical content, and antioxidant activity of hazelnut (Corylus avellana L.) skins from Oregon

Hazelnut pellicles (skins) from four Oregon hazelnut cultivars for two growing seasons (2021 and 2022) were separated from their kernels and were analyzed for proximate composition, dietary fiber, total phenolic content (TPC), condensed tannins (CT), and antioxidant activity (through reduction of the 2,2-diphenyl-1-picrylhydrazyl and the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical). Cultivar type affected protein, tocopherols, and antioxidant activity, whereas crop year influenced total fat and dietary fiber. Significant levels of α-tocopherols and γ-tocopherols were measured in hazelnut skins (74–266 μg/g and 32–107 μg/g, respectively) and in hazelnut kernels (83–130 μg/g and 5–13 μg/g, respectively). Hazelnut skins are a rich source of dietary fibers (64–70 g/100g) and antioxidant phenolic compounds (122–172 mg gallic acid equivalents (GAE)/g soluble; 1–20 mg GAE/g insoluble) with growing year impacting relative concentrations in each cultivar. Hazelnut skins were rich in (+)-catechin (49–130 mg/100 g), epigallocatechin gallate (EGCG) (21–35 mg/100 g), dimeric procyanidin B2 (7–21 mg/100 g), and quercetin 3-rhamnoside(12–21 mg/100 g). Overall, extracts of hazelnut skins were shown to contain significant amounts of Vitamin E and the major portion of antioxidants in whole hazelnuts. These findings will be beneficial in differentiating and developing new hazelnut cultivars based on nutrient quality and content.