To examine the effect of digestive attributes such as digestive enzymes and pH on changes in phenolic compound content and antioxidant activity during digestion, the bioavailability of green tea infusion was investigated using a simulated in vitro gastrointestinal digestion model. The total polyphenol content (TPC) decreased to 65%–70% throughout the mimicked normal digestion (MD) compared to the initial value. The total flavonoid content (TFC) decreased to approximately 25% after starting the gastric stage (pH 1.2); however, it regained to approximately 60% in the intestinal stage (pH 6.8). The mimicked digestive condition without digestive enzymes (WOE), which followed only the pH conditions of MD, showed significantly lower TPC and TFC values than MD. The percentage of antioxidant activity based on the initial values indexed by DPPH, ABTS, and FRAP gradually declined from approximately 60% at the gastric stage to approximately 40% at the final digestion stage. Meanwhile, the percentage of residual MIC was around 50% at the gastric stage. However, it gradually increased at the intestinal stage. The significantly lower antioxidant activity showed for WOE than MD throughout the simulated digestion. This study demonstrated that digestive enzymes and pH play a crucial role in the bioavailability of green tea infusion.

To examine the effect of mechanical processing for plant-based materials on antioxidant properties during digestion, relationships between the size of pulverized plant tissue and changes in antioxidant activities during simulated in vitro digestion were investigated. The citrus peel tissue was pulverized and classified into four powder fractions followed their particle size distributions as 125 − 180 μm, 180 − 355 μm, 355 − 500 μm and 500 − 710 μm. These fractions were regarded as a plant tissue model with various degrees of cell damages. Powder samples were collected during the digestion and measured their particle sizes and colors. Besides, total phenolic content and antioxidant activities of digested fluid were also evaluated. The results showed that the powder color of smaller particles were significantly changed than the bigger ones. The bio-properties of digested fluid for the bigger particle sizes comparatively retained more than smaller ones at the simulated gastric stage. It also showed mostly stable during simulated gastric or small intestinal digestion stage. This suggested that the release of bioactive compounds from plant tissues during digestion could be related to their structural attributes such as degrees of cell damages which could be affected by processing methods and conditions.