Very recently, we described the formation of (+)-catechin- and (-)-epicatechin-derived polar chromophores by means of a cocoa alkalization process. In this study we focus on the generation of unpolar chromophores using model reactions via Dutching with softer alkaline conditions. UPLC-HR-ESI-TOF-MSe analysis and one-dimensional and two-dimensional NMR spectroscopy led to the unequivocal identification of dehydrodicatechin- and hydroxyxanthene-derived chromophores. The previously unknown 6- C-linked constitutional isomers of C-6'B/C-6D-(2 R,3 S)-dehydrodicatechin (3, 5) were first described, and furthermore, the dimers dehydrocatechin-C-6'B/C-8D-(2 S,3 S)-epicatechin (2) and dehydrocatechin-C-6'B/C-6D-(2 S,3 S)-epicatechin (4, 6) as well as the reddish-colored chromophores 8- C-xanthenocatechin (7), 8- C-xanthenoepicatechin (8), 6- C-xanthenocatechin (9), and 6- C-xanthenoepicatechin (10) were identified as new compounds. A LC-MS/MS method was developed to screen and quantify both classes of chromophores as well as their precursors in alkalized cocoa powders. The dehydrocatechin dimers showed degradation via the alkalization process; in contrast, the xanthenocatechins highlighted an increase in concentration with stronger alkalization, and, therefore, contribute to cocoa reddening. These results, together with those previously published, give a clear insight into the chemistry of polyphenol-derived chromophores generated by cocoa powder alkalization and enable a better understanding of chromophore formation mechanisms toward a more comprehensive color design of cocoa powders.
Keywords: alkalization; catechin; catechinic acid; chromophore; cocoa powder; dehydrocatechin dimer; dehydrodicatechin; epicatechin; model reaction; polyphenol.