Melatonin Content in Walnuts and Other Commercial Nuts. Influence of Cultivar, Ripening and Processing (Roasting)

Nuts are important components of a healthy diet since they provide nutritional value and bioactive components. Melatonin is a well-known molecule in plants, and its relevance in foodstuffs is increasing. This study investigated the presence of melatonin in nuts using chromatographic techniques and optimized extraction procedures according to the high oil content of nuts. Melatonin was detected in four walnut cultivars with levels similar to those previously reported. Moreover, the melatonin content in walnut seeds decreased sharply during the ripening process from the ripe green stage to the mature dehydrated fruit and increased after harvesting when the fruits were edible. A number of other commercial nuts were also measured, with melatonin contents varying markedly, and generally being lower than in walnuts. The presence of melatonin was lower in commercial roasted nuts than in raw nuts, with the exception of peanuts, where melatonin content increased with roasting. It seems that this industrial processing negatively alters the structure of this molecule and its availability, which should be taken into account when estimating its levels during nut consumption. Therefore, this study reveals new data on the presence of melatonin in walnut seeds in a natural format and its evolution with maturation as well as in other commercial nuts. We also highlight the importance that processing has on melatonin and other antioxidants in the nuts that reach the consumer.