Theaflavins as characteristic polyphenols formed during black tea fermentation: Formation chemistry, functional...

Theaflavins are characteristic polyphenols formed during black tea fermentation. They play an important role in tea color and quality. With the increasing global consumption of black tea, interest in fermentation-derived polyphenols has grown. Recent studies have examined the chemical properties and reported bioactivities of theaflavins. However, current findings remain scattered and lack a unified food chemistry perspective. This review summarizes the formation chemistry of theaflavins during black tea fermentation. It compares the key factors affecting their composition and abundance, including catechin precursors, enzymatic oxidation, and fermentation conditions. Structural features of different theaflavin derivatives are discussed. Particular attention is given to galloylated theaflavins, which show higher reactivity in experimental systems. Reported bioactivities are briefly addressed to illustrate structure-activity relationships. Challenges related to production standardization, stability, and bioaccessibility are also highlighted.