Antioxidant capacity of phenols from apricot kernel coat extracted with deep eutectic solvent

A new green solvent (Deep Eutectic Solvents, DESs) was used to investigate the antioxidant ability and phenolic composition of apricot kernel coat extracts. In this study, four kinds of conventional solvents and five kinds of deep eutectic solvents were also selected with different concentrations, thereby determining the extraction efficiency of apricot kernel coat polyphenols. DPPH and ABTS free radical scavenging ability and Oxygen Radical Absorption Capacity (ORAC) were used to evaluate the antioxidant ability of apricot kernel coat extracts. Furthermore, a High-Performance Liquid Chromatography (HPLC) was utilized to compare the phenolic composition of apricot kernel coat extracts by conventional and deep eutectic solvents. The standards were also set as gallic acid, protocatechuic acid, 3,4-dihydroxy phenylacetic acid, chlorogenic acid, and propyl gallate. The results showed that the acidified methanol among the conventional solvents presented the best extraction efficiency, where the extraction yield of phenols was 0.55 mg/g. The deep eutectic solvents showed a great difference in the extraction efficiency among different formulations and concentrations. However, the deep eutectic solvents presented a better extraction efficiency, compared with conventional solvents, especially that choline chloride - oxalic acid (DESs2) and choline chloride - malic acid (DESs3) presented a better extraction efficiency for the phenolic compounds from apricot kernel coat. The extraction yields of apricot kernel coat phenolic compounds by DESs2 and DESs3 were 2.20 and 1.85 times that of acidified methanol, respectively. Extracts by acidified methanol showed the best antioxidant ability among the conventional solvents, whereas, the antioxidant ability of apricot kernel coat extracts by deep eutectic solvents was also better than that of conventional solvents. The extracts by DESs3 showed a better DPPH and ABTS radical scavenging ability, followed by DESs2. Specifically, the DPPH and ABTS radical scavenging abilities of apricot kernel coat extracts by DESs3 were 1.73 and 2.15 times higher than that of acidified methanol. The apricot kernel coat extracts by DESs2 and DESs3 showed better oxygen radical absorption capacity, increasing by 14.92 and 17.73 μmol/g, respectively, compared with the acidified methanol. The results of HPLC showed that there were at least nine kinds of phenols, including gallic acid, protocatechuic acid, 3,4-dihydroxy phenylacetic acid, chlorogenic acid, propyl gallate, and four phenolic compounds with no standards in the extracts from the apricot kernel coat. The phenolic composition of extracts by conventional solvents was complex, and the extraction yields of each phenolic compound were relatively low. There were eight kinds of phenols in the extracts by conventional solvents, while there was a much simpler phenolic composition of extracts by deep eutectic solvents. Extracts by DESs2 and DESs3 contained a large quantity of phenolic components with a retention time of 7.314 min, while this component was not found in extracts by conventional solvents. There was only a large amount of protocatechuic acid in the extracts by choline chloride - malic acid - proline (DESs5), where the extraction yield of protocatechuic acid was 0.19-0.47 mg/g under different concentrations. Consequently, DESs can be expected to serve as an efficient solvent for the effective extraction of phenolic compounds from apricot kernel coats.