Melatonin content in walnuts and other commercial nuts. Influence of cultivar, ripening and processing (roasting)

•Melatonin was efficiently extracted with a mixture of hexane-methanol-water.•Melatonin was detected in nuts by HPLC-FLD and confirmed with LC–MS.•Melatonin content in walnuts was higher than in the other nuts.•In commercial nuts melatonin was negatively affected by processing (roasting).•Melatonin decreased in walnut during ripening and increase after post-harvest.