Investigation of serotonin, free and protein-bound tryptophan in Turkish hazelnut varieties and effect of roasting on serotonin content

Hazelnuts and their skins are important sources of health-promoting compounds. In this study, serotonin, a neuroactive compound, and its precursor tryptophan in 14 hazelnuts, almost all hazelnut varieties in Turkey, in two consecutive harvest years were investigated. Serotonin content of hazelnut skins and the effect of roasting on the serotonin content of hazelnuts were also revealed for the first time. Serotonin and free tryptophan concentrations were determined after water extraction while an alkaline hydrolysis was performed for bound tryptophan before UPLC-MS/MS analysis. Free tryptophan ranged from 42 to 127 μg/g and bound tryptophan was between 3670 and 8904 μg/g in hazelnut varieties. Serotonin content of hazelnut varieties was ranged from 0.1 μg/g (Kalınkara) to 2.0 μg/g (Palaz). Significant differences (p 0.05) were observed after roasting at 150 °C for 30 min. [Display omitted] •Serotonin content of Turkish hazelnut varieties was evaluated for the first time.•Interestingly serotonin was found to concentrate on hazelnut skins.•Comparing to hazelnut, its skin was found to contain 4 times higher serotonin.•Roasting at 150 °C did not affect significantly the serotonin content of hazelnut varieties.•Turkish hazelnuts were found to contain high amounts of free and bound tryptophan.