Extraction of Saponins from Xanthoceras sorbifolium Bunge leaves using Deep Eutectic Solvents and Their Inhibitory Activity against Postharvest Pathogenic Fungi

To investigate the impact of deep eutectic solvents (DES) on the extraction of saponins from Xanthoceras sorbifolium Bunge leaves (XLs) and their antifungal properties, a total of 43 DES were utilized for the extraction. Among these, choline chloride:lactic acid (1:1) exhibited the highest extraction efficiency and was therefore chosen as the extraction solvent. Optimization of the extraction procedure was carried out through one-way and Box-Behnken experiments. The extracts were characterized using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The mechanism of XLs saponins extraction by DES was investigated by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). Furthermore, the inhibitory effects of XLs saponins on six typical fungi that pose a threat to fruits and vegetables were evaluated. The results showed that under the conditions of DES concentration of 47.00%, liquid-solid ratio of 42.00 mL/g, and extraction temperature of 43.00 ℃, the extraction rate of XLs saponin using DES reached (10.22±0.28)%, representing a 38.8% increase compared to traditional solvents. It was an extraction method of XLs saponin with great potential for application. The saponins extracted by DES from XLs mainly include madecassic acid, saikogenin D, glycyrrhetinic acid, oleanolic acid, ginsenoside Rg3, maslinic acid, saikosaponin A and asiatic acid. DES was observed to disrupt cell integrity and cell wall structure by dissolving lignin and hemicellulose components, facilitating the release and solubilization of saponins. Additionally, XLs saponin demonstrated effective inhibition of spores of Aspergillus niger, Rhizopus oryzae and Aspergillus flavus of three out of the six typical post-harvest pathogenic fungi of fruits and vegetables. Among the pathogenic fungi studied, the spore germination of Rhizopus oryzae was inhibited with the best effect, the inhibition rate reached (78.76±2.56)%, and XLs saponin inhibited the growth of the fungus by suppression the antioxidant enzyme activity. These results suggest promising applications for XLs saponins in the postharvest preservation of fruits and vegetables.