Although cocoa powder alkalization (Dutching) is a widely used industrial process to improve taste, dispersibility, and coloring of the final product, nevertheless knowledge about the compounds causing a change in coloring is fragmentary. By means of alkaline model reactions starting from the major cocoa polyphenol monomers, (+)-catechin or (-)-epicatechin, eight chromophores were derived from the first rearrangement product catechinic acid. LC-MS/MS analysis, one- and two dimensional-NMR, and electron paramagnetic resonance (EPR) spectroscopy led to the unequivocal identification of 6'-hydroxycatechinic acids (1a, 2a) and their radical states (1b, 2b), which were highlighted as main red chromophores. Six new dehydrocatechinic acid dimers (dehydrocatechinic acid-C-6'B/C-8D-(2 R,3 S)-catechin (3), dehydrocatechinic acid-C-6'B/C-6D-(2 R,3 S)-catechin (4, 5), dehydrocatechinic acid-C-6'B/C-8D-(2 R,3 R)-epicatechin (6), and dehydrocatechinic acid-C-6'B/C-6D-(2 R,3 R)-epicatechin (7, 8)) were also characterized as chromophores. 1-8 as well as their precursors were detected and quantified in alkalized cocoa powders via LC-MS/MS. With the increasing grade of alkalization, a decrease in catechin and epicatechin together with an increase in catechinic acid was observed. Compounds 1b, 2b, and 3-8 also showed a decrease in concentration by Dutching, which correlates to the accumulation of/to higher ordered chromophore oligomers and underlined the increase of the high molecular weight fraction. These findings give a first insight into the formation of structures causing the red coloring of cocoa, which offers the opportunity to optimize the alkalization process toward a better color design of cocoa powders.
Keywords: Dutching; catechinic acid; chromophore; cocoa powder; dehydrocatechinic acid dimer; epicatechin; model reaction; polyphenol; radical.