Bioaccessibility of Phenolic Compounds and Antioxidant Capacity of Pulai ( <i>Alstonia scholaris</i> ) Leaf Tea and Characterization of Bioactive Compounds Using Ultra-High-Performance Liquid Chromatography Coupled with High-Resolution Mass Spectrome

Drying is commonly used to extend the shelf life and use of herbal leaves, such as <i>Alstonia scholaris</i> (pulai) leaves. However, it can lead to the degradation of heat-sensitive antioxidant compounds. Therefore, the present study aimed to evaluate the changes in the total phenolic content (TPC), total flavonoid content (TFC), and antioxidant capacity of pulai leaf tea prepared from fresh and dried leaves following <i>in vitro</i> gastrointestinal digestion in order to predict the biofunctional changes of the leaves before and after drying. The results showed that dried pulai leaf tea (DPLT) had lower antioxidant capacity than fresh pulai leaf tea (FPLT) prior to digestion. This was supported by a corresponding decrease in the TPC and TFC of dried samples. After gastric digestion, FPLT and DPLT exhibited reductions in antioxidant capacity, TPC, and TFC. Interestingly, these values increased following the intestinal phase of digestion. The bioaccessibility index after gastrointestinal digestion, TPC, and antioxidant capacity [as measured by 2,2-diphenyl-1-picrylhydrazyl, ferric reducing antioxidant power, and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) assays] were higher in FPLT than in DPLT. The results of ultra-high-performance liquid chromatography coupled with high-resolution mass spectrometry showed that flavonoids were the dominant class of antioxidant compounds prior to digestion, different from those after digestion wherein substantial levels of antioxidant organic acids and amino acids were present.