Structure-resolved tannin interactions in food systems: nutritional effects, digestion, and applications

Tannins are structurally diverse food polyphenols whose behavior cannot be predicted from total tannin content alone. This review uses tannin class, linkage chemistry, degree-of-polymerization metrics, galloylation and hydroxylation, conformational flexibility, and oxidation state as a common framework for interpreting interactions with food proteins, digestive enzymes, starch, plant cell-wall polysaccharides, minerals, and salivary constituents. These descriptors explain differences in digestibility, mineral bioaccessibility, astringency, interfacial behavior, and crosslinking. Many apparently contradictory findings arise because matrix composition, dose, processing sequence, and analytical endpoint change together. The same multivalent and oxidation-prone chemistry that creates nutritional or sensory liabilities can be exploited in preservation, delivery systems, food colloids, and active packaging, provided that benefits are balanced against binding, color, migration, and sensory costs. Digestive fate should be evaluated at the level of free tannins, soluble complexes, precipitated or cell-wall-bound fractions, and transformed products. A phase-resolved framework integrating INFOGEST digestion, Caco-2 testing, colonic fermentation, animal studies, and human interventions is proposed. Minimum methodological requirements are provided for structural characterization, complex preparation, mass balance, microbiota experiments, and cross-model validation. Structure-resolved reporting should improve comparability and support selective processing and rational food design.