Effects of peeling and/or toasting on the presence of tocopherols and phenolic compounds in four Italian hazelnut cultivars

Hazelnuts are a well-known source of different healthy molecules. However, only few studies have investigated deeply their amounts considering simultaneously the contribution of the cultivar, the pellicle and the effect of roasting. For such purpose, peeled/unpeeled and raw/toasted samples of Nocchione, Tonda di Giffoni, Tonda Gentile delle Langhe and Tonda Gentile Romana hazelnuts were investigated as regards to their fatty acid composition, tocopherols and total phenolic compounds. Our results indicate that all four cultivars contain a high fraction of mono- and poly-unsaturated fatty acids, about 110-210 mg/kg of tocopherols and, when unpeeled, 1250-2100 mg/kg of phenolic compounds. In particular, unpeeled and toasted Tonda Gentile delle Langhe hazelnuts contain more than 2 g/kg dry weight of hydrophilic phenolics and more than 200 mg/kg dry weight of tocopherols. The study confirms that the highest concentration of bioactive compounds is contained in hazelnut's pellicle. Accordingly, a principal component analysis (PCA) demonstrates that removal of the pellicle is associated with reduced amounts of phenolic compounds and alpha- and gamma-tocopherols. The PCA also indicates that beta-tocopherol, together with total fat, are the variables that most characterize the cultivar. Toasting, on the other hand, induces the oxidation of monounsaturated fatty acids, but does not influence the presence of tocopherols and has a positive impact on the presence of phenolic compounds whose concentration significantly increased regardless of kernel's pellicle.