Corn (Zea mays) is the third most important crop worldwide and might serves as a staple food, especially in the developing countries. Moreover, corn is essential because it contains naturally occurring melatonin [N-acetyl-3-(2-aminoethyl)-5-methoxyindole]. It has a number of important activities that include neurohormone and chronobiotic actions in biological systems. There are good evidences that melatonin provides effective treatments for cardiovascular diseases, sleep disorders and headache. Reliable determination of melatonin in regular food in human diet would help to evaluate its effect. Hence, a suitable extraction method is needed to determine melatonin level in corn grains.

In this research, a new ultrasound-assisted extraction (UAE) of melatonin has been optimized and validated. Central Composite Design (CCD) in conjunction with Response Surface Methodology (RSM) was applied based on six factors and three levels to evaluate the significances of studied factors prior to optimize the UAE conditions. The effects of solvent composition (25−75% methanol in water), extraction temperature (10−50oC), solvent-solid ratio (2.5:1−5:1), solvent pH (2−7), amplitude (30−70%) and cycle (0.2−0.7 s-1) on the extraction yields were taken into account. Subsequently, extraction kinetic (5−30 min) was studied to confirm the full recovery of phenolic compounds extracted from the matrix. The levels of melatonin in the UAE extracts were quantified by Ultrahigh Performance Liquid Chromatography using a fluoresce detection (UPLC-FD) in less than 4 min.

The significant (p<0.05) response surface models with high coefficients of determination values of 0.7096 were fitted for the experimental data. Solvent composition, solvent-sample ratio and quadratic effect of temperature were found to have a greatly significant effect on the response value (p<0.05). The optimal UAE condition was as follows: extraction time 5 min, ultrasound amplitude 45%, cycle 0.7 s-1, extraction temperature 50oC, 70:30 methanol:water as the extraction solvent at pH 6 and solvent-solid ratio 2.5:1. Complete validation of the method was performed. High recovery and high precision were found for the full analytical method.

The validated method was successfully applied to the analysis of phenolics content in four corn cultivars including black, yellow, red and white. Therefore, this particularly rapid ultrasound extraction method was demonstrated to be suitable for the extraction of melatonin from corn grain samples.