Thearubigins: Formation, structure, health benefit and sensory property

Thearubigins (TRs) is a type of heterogeneous mixture from black tea, usually accounts for 60–70% of dry substance of black tea infusion. It is an important tea pigment for food industry and also contributes much to the sensory properties and biological activities of black tea. TRs therefore have a practical application value for the development of tea processing, natural food colorants and healthy products. The review paper summarizes the current knowledge from the perspectives of formation mechanism, chemical structure, sensory characteristics and health benefits of TRs. Through the oxidation mechanism and products of flavan-3-ols, the primary characteristics and reaction pathways of catechin, gallocatechin and galloylated catechin are profiled. Furthermore, the sensory properties and biological activities of TRs are comprehensively discussed. Gallocatechins containing vic-trihydroxy are more liable to lose protons and produce o-quinone, and subsequently condense through B–B ring to form dehydrotheasinensin, theasinensin, dehydrotheasinensin AQ in a pathway, or form bicyclo[3.2.1]octane to produce theacitrins, theacitrinins and proepitheaflagallin in another pathway. Furthermore, the oxidation of a couple of catechol- and pyrogallol-type catechins produce theaflavins (TFs), theanaphthoquinone and theanaphththeaflavins. Catechin can undergo nucleophilic reaction to form polymers, while catechin gallate also forms polymers through condensing consecutive benzotropolone moieties. The ratio of TRs to TFs is a critical index for color and brightness of black tea infusion, thus regulating enzymatic oxidation of tea polyphenols is not only an effective strategy in high-quality black tea processing, but also provides more options for tea pigment producing in beverage industry.