A study published in the Journal of Food Science shows that the antioxidative potential and total phenolic content of hazelnuts are significantly reduced when the skin is removed.
The researchers studied six hazelnut cultivars to determine the effect of skin removal and roasting on individual and total phenolic content and on antioxidative potential. HPLC-MS identification of individual phenolics confirmed the presence of seven flavan-3-ols, three flavonols, two hydrobenzoic acids, and one dihydrochalcone.
The researchers found that flavonols were only detected in whole hazelnut kernels. The content of individual phenolics, with the exception of gallic acid, was always highest in whole unroasted hazelnuts and was significantly reduced after skin removal. Similarly, total phenolic content and antioxidative potential decreased when skin was removed. However, roasting only had a significant negative effect on individual phenolics but not on the total phenolic content and antioxidative potential of kernels. From a health promoting phytochemical composition of hazelnuts, the consumption of whole unroasted kernels with skins should be preferential to peeled kernels either roasted or unroasted.
In hazelnut skin, many phenolic compounds are located, which are not present in flesh and, therefore, the health properties of hazelnuts are strongly affected by skin removal. Thermal processing and roasting conditions used in this study had a lesser effect on the individual phenolic composition of the kernel and thus roasted and unroasted hazelnuts without skin contain comparable amounts of health-promoting compounds.