Maillard and Hydrolytic Reactions in Subcritical Water Extraction of Bioactive Compounds from Licorice

Nowadays, subcritical water extraction (SWE) techniques are extensively investigated worldwide, while the thermal reactions that inevitably occur under subcritical water conditions are rarely studied. In order to investigate the behaviors of the different reactions during SWE of bioactive compounds from licorice, the Maillard reaction process was accessed via their products and the hydrolytic reaction was analyzed according to the kinetic parameters. In addition, the contents of total phenolics and flavonoids in the extracts obtained at the different temperatures were determined and total antioxidant capacities were evaluated by HPLC-ABTS⁺. The results showed that flavonoids and phenolics from licorice as well as new compounds generated via the Maillard reaction contributed to the antioxidant activity of the extracts. The fluorescence, color and absorbance of the extracts showed that the degree of the Maillard reaction increased with the rise of the extraction temperature. The kinetics of extraction for glycyrrhizic acid showed that it was firstly extracted by diffusion, and then was hydrolyzed into glycyrrhetinic acid 3-O-mono-β-D-glucuronide and glycyrrhetinic acid following a first-order mechanism. These findings could provide deep insights into the SWE process and a new method for producing glycyrrhetinic acid 3-O-mono-β-D-glucuronide and glycyrrhetinic acid.