Bioactive compounds in different hazelnut varieties and their skins

• Bioactive profiles of skins of fourteen hazelnut varieties were revealed.
• Distribution of polyphenols in soluble, conjugated and bound forms was determined.
• Conjugated fraction was found to contain highest amounts of polyphenols.
• Hazelnut skin was rich in tocopherols, especially α-tocopherol.
• Total antioxidant capacity of hazelnut skin was hundred times higher than hazelnut.

Bioactive profiles of hazelnut skins belonging to fourteen hazelnut varieties were identified. Concentration of phenolic compounds, flavonoids and phenolic acids in soluble free, conjugated soluble and insoluble bound fractions together with their total concentrations were presented. In addition, tocopherol content and total antioxidant capacity of hazelnuts and their skins were revealed. Concentration of total phenolic compounds ranged between 51.9 and 203.1 mg gallic acid equivalent/g of skin among varieties, which is in accordance with the total antioxidant capacity. Total flavonoid content was almost 60% of the total phenolic compounds. Flavonoids and phenolic acids were found to be concentrated mostly in the conjugated soluble fraction. Tocopherol contents of hazelnut skins ranged from 226 to 593 μg/g, and α-tocopherol was the most abundant. Total antioxidant capacity was between 309 and 1375 μmol Trolox equivalent/g of hazelnut skins, which is more than 100 times higher than for hazelnuts without the skins.