MICROWAVE-ASSISTED EXTRACTION CAN BE USED FOR SELECTIVE EXTRACTION OF ANTHOCYANINS FROM RED ONION

Anthocyanins are red to blue hydrophilic pigments widely distributed in the plant kingdom. The consumption of these bioactive compounds has been associated with the prevention of several chronic-degenerative diseases in humans. Solid-liquid extraction is the most common method applied for anthocyanin extraction; however, it is a time and solvent consuming method. Among the new extraction techniques, microwave-assisted extraction (MAE), which relies on fast heating generated by the microwaves to facilitated extraction, can be highlighted. However, the effect of MAE on the anthocyanin profile is not yet well-known. The effect of MAE on the anthocyanin profile of lyophilized red onion was studied by means of a factorial experimental design (23) in which the independent variables were HCl concentration of 0.3 and 0.7%, temperature of 90 and 110°C and volume of acidified methanol of 28 and 42 mL. In addition, a 4 point steepest ascent (SPA) was carried out with the same variables with the following levels: HCl concentration of 0.10 and 0.25%, temperature of 70 and 85°C and volume of acidified methanol of 45 and 54 mL. For extraction, the samples were subjected to a maximum power of 850 W for 1 min to allow the solution to reach the set temperature and after that; the temperature was maintained constant for 5 min at 350 W. The response variables were the concentrations of succinyl-derivatives, malonyl-derivatives and non-acylated anthocyanins, determined by HPLC-DAD-MS/MS. The anthocyanin profile was characterized by the presence of cyanidin and peonidin derivatives. The same eight anthocyanins were identified at different concentrations in the extracts obtained by the traditional extraction methods (static and with agitation) and MAE. Non-acylated anthocyanins were the major ones detected when traditional static extraction was used, while the content of acylated anthocyanins was higher in MAE and extraction with agitation. The extraction of total anthocyanins was more efficient by MAE than by the traditional methods. However, the independent variables affected the anthocyanin profiles in the levels evaluated in the factorial experimental design DCFC. Low concentration of HCl in methanol, low temperature and low solvent volume resulted in a high concentration of anthocyanins acylated with malonic acid; while a high concentration of HCl in methanol, high temperature and high volume resulted in a high concentration of anthocyanins acylated with succinic acid. Large volumes of acidified methanol exerted a positive effect on the extraction of total anthocyanins. The number of mono or disaccharides bond to the anthocyanin molecule did not result in preferential extraction of these compounds under the tested conditions. There was no evidence that the anthocyanins are degraded to low molecular weight compounds during MAE under the tested conditions.

Financial Support: CNPq, FAPESP (grant 2013/07914-8) and CAPES.