As a grain of life, cultivated rice (Oryza sativa) serves as the foremost food for more than half of the world’s population. Moreover, rice is essential because it contains naturally occurring melatonin [N-acetyl-3-(2-aminoethyl)-5-methoxyindole]. Recent research has led to an increase in the production of melatonin in transgenic rice seeds and melatonin-rich rice plants have been successfully generated. Functional rice products have also become more widely appreciated in the current market.
Research into melatonin was started almost a century ago and this has continued to be an active field to the present day, with many results revealing the utility of this compound. Melatonin diminishes neuro-degenerative diseases, such as Parkinson’s and Alzheimer’s diseases, and it also acts as an anticancer agent. Melatonin effects are more interesting if the compound is taken regularly at a specific time. Therefore, it should be better if people can consume melatonin-rich foods regularly instead of taking it as a drug. Rice grains, potential source of melatonin, normally contains low concentration of this compound. Hence, it is of great interest to extract and quantify endogenous levels of melatonin accurately by selecting the most suitable analytical technique.
Recent studies have shown that Microwave-assisted Extraction (MAE), Ultrasound-assisted Extraction (UAE) and Pressurized Liquid Extraction (PLE) are important sample preparation techniques and these approaches have been used to increase the extraction efficiency for various bioactive compounds. However, these advanced extraction methods have not been developed for the extraction of melatonin from rice grain yet. Thus, the methods still need to be optimised and validated to reach reliable extraction methods. As the outcome of the study, the final method could serve as an analytical tool to evaluate the level of melatonin through the production processes of rice from farm to fork.
This research prompted us to compare the performances of various extraction methods for the extraction of melatonin from rice grains including MAE, UAE and PLE, to the best of our knowledge, this kind of study has never been performed. Of the three studied extraction techniques, one may be chosen depending on the necessity and fitness for purpose.
The developed and validated method for the extraction and determination of melatonin in rice grains reported in this study will enable to come up with reliable data on the quantity of melatonin in wide variety of rice grains, which is currently lacking due to unreliable analytical data. The proposed extraction method also provides opportunities to develop advanced techniques in rice processing. Results obtained from this specific study recommended that the rice production processes should be carefully controlled in order to attain a high level of melatonin in rice products by efficient drying and the optimum degree of milling. Additionally, bearing in mind the importance of rice science and technology, it is envisaged that this work will significantly support further studies to improve rice production techniques in the coming decades.
As other products of ready to eat food, little is known about the melatonin concentration in cooked rice. Yet, several differences on its level can be expected as compared to other rice-derived products, notably in relation to the treatment during food processing. It is, therefore, the proposed analytical method was also useful to asses the level of melatonin through a series of treatments. And finally, it is noteworthy that the recommended melatonin-rich rice products, based on the result applying the analytical method, could be chosen instead of the regular ones by the population interested in the properties associated to melatonin, without dramatic changes in their regular diets.