Evaluation of the Structure and Biological Activities of Condensed Tannins from Acanthus ilicifolius Linn and Their Effect on Fresh-Cut Fuji Apples

Condensed tannins (CTS) have been isolated and purified from leaves of Acanthus ilicifolius Linn. And their structures were investigated by three methods: 13C nuclear magnetic resonance (13C NMR), reversed-phase high-performance liquid chromatography (RP-HPLC), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The results showed that the CTS were a mixture of catechin/epicatechin, galatechin/epicatechin, and amphicin/epigalin, and that the polymer chain lengths were 3-mers to 14-mers. Antityrosinase activities and antioxidant activities of the CTS from A. ilicifolius leaves were further studied. The IC50 of the CTS on mushroom tyrosinase activity was determined to be 19.7 ± 0.13 μg/mL, and inhibition type analyses indicated that the CTS were mixed type inhibitors and their inhibition CTS was reversible. The CTS from A. ilicifolius leaves also exhibited potential antioxidant activity. The IC50 of DPPH and ABTS scavenging activities were 104 ± 0.894 μg/mL and 86 ± 0.616 μg/mL, respectively. And the FRAP value was 758.28 ± 2.42 mg AAE/g. In addition, we found that the CTS from A. ilicifolius leaves had an excellent effect on preserving the quality of fresh-cut apples by preventing apples from browning through reducing polyphenol oxidase activities in apples.