Copigmentation effects and thermal degradation kinetics of purple sweet potato anthocyanins with metal ions and sugars

Abstract

The copigmentation effects of purple sweet potato anthocyanins (PSPAs) with metal ions and sugars were investigated in model solutions at pH 4. The thermal stability of PSPAs was also explored in the presence of sugars and 5-hydroxymethylfurfural (5-HMF). Copigmentation are characterized by hyperchromic effect and bathochromic shift. The hyperchromic effect of Fe3+ reached 25.15 % even at a very low concentration 0.005 mol L−1, while the values of the other metal ions at 0.05 mol L−1 were ranked in the following ascending order: K+ < Ca2+ < Mg2+ < Zn2+ < Cu2+ < Fe2+ < Al3+. The bathochromic shift was not observed in all sugar reaction solutions; glucose showed the highest values of hyperchromic effect at the concentration range 150–300 g L−1, followed by fructose and sucrose; konjac glucomannan showed the highest effect even at a much lower concentration among the macromolecular sugars. Especially, the konjac glucomannan exhibited a much better color enhancement than glucose. Small molecular sugars accelerated the thermal degradation of PSPAs, whereas macromolecular sugars showed a protective effect particularly at high temperatures. Moreover, 5-HMF resulted in the deterioration of the thermal stability of PSPAs and was suggested to be an important labile factor for copigmented PSPA solutions.