Degradation of anthocyanins and polymeric color formation during heat treatment of purple sweet potato extract at different pH.

Purple sweet potato anthocyanins are common natural pigments widely used in food industry, while they are often thermally processed in application. Degradation of anthocyanins, formation of polymers and color changes of purple sweet potato extract (PSPE) were investigated at 90 °C in the range of pH 3.0-pH 7.0. Data analysis indicated a first-order reaction for anthocyanins degradation in solutions with pH 3.0, 5.0 and 7.0 have half-lives of 10.27, 12.42 and 4.66 h, respectively. The polymeric color formation followed zero-order kinetics, progressively increasing with pH values. The color of PSPE were changed with heating time and pH value through visual observation and colorimetric characterization. Analysis by UV-Vis spectrophotometry and HPLC indicated that anthocyanins in solution with pH 3.0 changed from monomeric anthocyanin into new polymers during heat treatment. Degradation of anthocyanins was accompanied by an increase in polymeric color index, due to the formation of melanoidin pigments and condensation reactions.