Effect of Drying Method on the Major Active Components, in Vitro Antioxidant Activity, α-Glucosidase Inhibitory Activity, Volatile Components, and Metabolites of Panax notoginseng Leaves

In order to find an excellent drying method for Panax notoginseng leaves (PNL), PNL were dried by using hot air drying (HAD), heat pump drying (HPD) or vacuum drying (VD). The contents of total saponins, polysaccharides, total phenols, total flavonoids, and monomer saponins, in vitro antioxidant activity and α-glucosidase inhibitory capacity of fresh and dried PNL were determined. By using gas chromatography-ion mobility spectrometry (GC-IMS) and non-targeted metabolomics, the volatile components and metabolites in fresh and dried PNL were identified and analyzed, respectively. The results showed that all three drying methods could significantly increase the contents of total saponins and polysaccharides in PNL, reduce the contents of total phenols and total flavonoids, but decrease the functional activities of PNL to varying degrees. Among the four samples, the HPD dried sample had the highest contents of total saponins and polysaccharides, the strongest 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and α-glucosidase inhibitory capacity, and moderately strong 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical scavenging capacity and ferric ion reducing antioxidant power (FRAP). High-performance liquid chromatography (HPLC) analysis showed that HPD treatment significantly reduced the contents of notoginsenosides Fc and Fe, while VD treatment showed the opposite effect. A total of 85 volatile components were detected, and the HPD dried sample showed the highest relative contents of alcohols and aldehydes, and the lowest relative content of ketones. In addition, there were significant differences between treatment groups. Furthermore, ultra-high performance liquid chromatography-quadrupole-orbitrap mass spectrometry (UPLC-Q-OT-MS) identified 659 metabolites in the fresh and HPD dried samples. In the positive and negative ion modes, 113 and 68 metabolites were significantly up-regulated, and 98 and 31 metabolites were significantly down-regulated, respectively. These differential metabolites predominately included lipids, lipid-like molecules, organic heterocyclic compounds, organic acids and their derivatives. Out of the 24 saponins in PNL, seven were significantly down-regulated and one was significantly up-regulated, which verified the HPLC results. Through the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of differential metabolites, it was found that the most likely metabolic pathways in HPD dried PNL included the biosynthesis of amino acids, 2-oxycarboxylic acid metabolism, and the biosynthesis of cofactors. In conclusion, HPD provided the best preservation of the chemical composition and functional activities of PNL, making it more suitable for drying PNL. Through multiple metabolite pathways, HPD could alter the composition of metabolites in PNL and affect its volatile components.