Aerobic in vitro studies of spontaneous anthocyanin degradation are not useful models for anthocyanin breakdown in the human colon

It was recently reported that degradation of anthocyanins in the anaerobic human colon occurs via both microbiota-dependent and spontaneous processes. However, the existing literature only describes the spontaneous degradation of anthocyanins in aerobic conditions. We investigated the loss of cyanidin-3-O-glucoside (Cy3Glc) over time under aerobic and anaerobic conditions. In anaerobic conditions, spontaneous breakdown products were consistent with Cy3Glc undergoing a classic pH-dependent transformation to colourless intermediates (i.e., cyanidin (Cy) hemiketal-glucoside, Cy chalcone-glucoside, Cy chalcone quinone-glucoside, 2,4,6-trihydroxyethenylbenzene-glucoside), but no other breakdown products. In contrast, under aerobic conditions, in addition to the pH-dependent intermediates, multiple other breakdown products were identified, including the previously reported protocatechuic acid and phloroglucinaldehyde, and several products of oxidation reactions reported for the first time (coumarin-glucoside, 2,4,6-trihydroxyphenylacetic acid, 2,4,6-trihydroxyphenylglyoxylic acid, and 3,4-dihydroxyphenylglyoxylic acid). These observations show degradation of anthocyanins is completely different in aerobic compared to anaerobic conditions, and useful information of the spontaneous degradation of anthocyanins cannot be obtained using aerobic conditions.