Dual Mechanisms of pH-Dependent Tea Astringency: Binding and Metabolism of Polyphenols Revealed by In Vivo and In Vitro Oral Processing

pH critically influences the astringency of tea, yet the underlying mechanisms remain poorly understood. This study aimed to elucidate how pH modulates tea astringency through dual mechanisms (polyphenol–saliva binding and polyphenol oral metabolism) with comparable in vivo and in vitro oral processing (OP) models. After optimizing OP parameters (10 mL, 10 s), the effect of pH (3.0–6.0) on green tea infusion and representative polyphenols (epigallocatechin-3-O-gallate (EGCG), epigallocatechin (EGC), and gallic acid (GA)) was investigated by sensory evaluation, turbidimetry, and paper spray mass spectrometry (PS-MS). Astringency intensity was shown to increase as pH decreased from 6.0 to 3.5, with a slight decline at pH 3.0. Turbidimetry revealed that lower pH led to greater turbidity increases after OP, indicating enhanced polyphenol–saliva binding. PS-MS revealed that lower pH was associated with greater reductions in polyphenol concentrations and increased formation of metabolites, with changes consistent with the conversion of EGCG to EGC and GA. These pH-dependent trends were consistent between in vivo and in vitro OP and were validated with single polyphenol solutions. Collectively, these findings reveal that pH modulates tea astringency by simultaneously enhancing both binding and metabolism of polyphenols. The comparable in vivo and in vitro OP approach provides a robust platform for mechanistic studies of astringency in complex food systems.