Simultaneous analysis of melatonin, quercetin and resveratrol in grapes using liquid chromatography with fluorescence detection

Phenolic compounds ubiquitous in all edible plants are an essential part of the human diet and have great interest owing to its antioxidant properties and pharmacological activity. Polyphenol content of grapes is influenced by environmental and post-harvesting conditions, but also genotype plays an important role in the variation. In this study an analytical method was developed for the quantification of melatonin, quercetin and resveratrol using reversed-phase HPLC with fluorescence detection. 35 varieties of grapes were analyzed. The analysis of these components from grape skin was performed by a liquid-solid extraction with ethyl acetate solvent. Quantifiable amounts to these compounds were proved. Characters like grape type (wine, table, wild), color, density and skin thickness had shown significant differences. Melatonin concentrations oscillated between 17-304 nM and the higher values are associated to the wild varieties with blue-black color and thin skin. Quercetin was founded in values from 40 to 110 mM and highlighted in wild grapes with thin skin and dark color. Resveratrol showed quantities between 5-13 µM. Vinificacion grapes, associated to blue-dark colour and thick skin, were the richest in this compound. In all tested grapes highlighted Trincadeira Preta and Cuatendra varieties, which presented the highest levels to the studied compounds