Exploration of color formation and critical colored components during Keemun black teas processing by coloromics

Black tea has attracted widely attention due to its quality characteristics, especially the bright red color, but the intrinsic colored contributors and formation pathway during Keemun black tea processing remain poorly characterized. In the present study, a total of 141 critical substances were by identified coloromics approach to involve in two main pathways: (i) the formation of various catechin oxidation products and derivatives that mostly appeared in red hue resulting in the accumulation of red color; and (ii) the degradation of acylated and non-acylated flavonoid multi-glycosides that mainly exhibited pale yellow hue that attributed to the loss of yellow color. Epigallocatechin gallate and epicatechin gallate were the pivotal color-forming catechol and pyrogallol precursors for the formation of theaflavins, theaflavates, theacitrins, theaflavic acids, theaflagallins, theadibenzotropolones, theasinensins, oolongtheanins and their hydroxylated and ortho-quinone derivatives, through oxidative coupling (BB ring and BD ring) and hydroxylation by polyphenol oxidase and peroxidase. Most of these oxidative products rapidly formed during the rolling process and were involved in thearubigins (TRs) production, which both promoted the appearance of red hue after the rolling process. In addition, the occurrence of theaflavic acid derivative and theadibenzotropolone derivative confirmed the importance of gallic acid (GA) in color formation of black tea. These results emphasized the colored substances in actual Keemun black tea and their roles in color information, degradation and TRs production. Furthermore, the content reduction of oxidation products, as well as the loss of TRs and color during drying deserve further investigation.