Thermal degradation of anthocyanins with particular reference to the 3-glycosides of cyanidin. I. In acidified aqueous solution at 100 °C

Abstract Several highly purified 3‐glycosides of cyanidin were degraded at 100 °C in weak aqueous hydrochloric acid at various pH values between 1 and 4 under both nitrogen and oxygen atmospheres and it was shown that the aglycone‐sugar bond is the most labile of the glycosidic bonds at pH 2 to 4. At pH 1.0, however, all the glycosidic bonds are susceptible to hydrolysis. Evidence is presented that phenolic aglycones, and glycosides of undetermined structure, are formed during anthocyanin degradation. An essentially colourless phenolic product, tentatively identified as the chalcone or α‐diketone of cyanidin, was observed during the breakdown of all the cyanidin‐3‐glycosides studied in the pH range 1 to 4. This compound is stable under anaerobic conditions but easily degraded in atmospheres of oxygen. The rate of sugar formation was found to be similar to the rate of red colour loss and to the rate of anthocyanin disappearance in the pH range 2 to 4 showing that glycosidic hydrolysis and not opening of the pyrylium ring is the rate‐determining step of the principal reaction leading to red colour loss. At pH 1.0 (anaerobic conditions), the rate of red colour loss lags behind the rate of anthocyanin degradation because the red, cationic form of cyanidin is fairly stable at this low pH. Complete hydrolysis of the anthocyanin takes place at pH 1.0 in contrast to the partial hydrolysis observed in the pH range 2 to 4. The qualitative and quantitative information reported in this paper led to the proposal of a mechanism for anthocyanin degradation and the red colour loss which accompanies it for the pH range 2 to 4.