Phenolic Acids and Their Derivatives in Tea: Chemical Diversity, Processing Changes, Sensory Contributions, and Health Implications—A Comprehensive Review

Phenolic acids are increasingly recognized as underexplored yet influential determinants of tea quality, sensory identity, and health functionality. This review integrates chemical, biochemical, sensory, and technological perspectives to construct the first comprehensive, cross-tea-type framework for understanding phenolic acid dynamics. We consolidate current knowledge on their structural diversity, biosynthetic regulation, and metabolic fluxes, and map how processing-specific transformations across the six major tea categories reshape free and bound phenolic acid pools. By linking these molecular changes to sensory outcomes—color formation via metal coordination and oxidation, aroma generation through thermal and enzymatic degradation pathways, and taste modulation driven by ortho-diphenolic structures—we highlight overlooked mechanistic connections. The review further synthesizes emerging evidence on phenolic acid–mediated biological activities, including antioxidant and anti-inflammatory effects, while evaluating recent advances in analytical chemistry and molecular biology that refine quantification and elucidate pathways. Comparative analysis reveals distinct phenolic acid trajectories: green tea preserves 85%–90% of total phenolics with elevated free forms; white tea maintains balanced free/bound ratios through gentle withering; and black and dark teas undergo extensive oxidation and polymerization, reducing free acids but generating complex pigments. By integrating these insights, we identify leverage points in processing—such as fixation temperature and fermentation control—that offer new opportunities to optimize both product quality and functional potential. The review concludes by outlining critical research gaps and proposing a forward-looking agenda to accelerate mechanistic, translational, and application-oriented studies on phenolic acids in tea.