Thermal stability of Hibiscus sabdariffa L. anthocyanins in solution and in solid state: effects of copigmentation and glass transition

Kinetic studies on thermal stability of anthocyanins isolated from the dry calyces of Hibiscus sabdariffa L. (roselle) were carried out in aqueous solutions (55–98 °C), either as free or copigmented anthocyanins with chlorogenic acid, and in the dry state as free anthocyanins or co-lyophilized with an amorphous polysaccharide (pullulan) and stored in different relative humidity environments (water activities 0.33, 0.53, 0.75 and 0.84) at 40 °C. The rate constants for degradation were obtained from first-order reaction kinetic plots. The degradation kinetics of individual anthocyanin components in solution, as assessed by HPLC, followed an Arrhenius-type response with respect to temperature; activation energies, Ea, varied between 13.3 and 15.1 kcal/mol. Copigmentation of anthocyanins with chlorogenic acid did not seem to improve their stability in solution. In the dry state, the degradation rate constants increased with the water activity, particularly above 0.53. In the freeze-dried pullulan–anthocyanin mixtures, the polysaccharide matrix delayed colour degradation compared to the free anthocyanin preparations by 1.5–1.8 times. The degradation kinetics of anthocyanins did not show any dependence on the molecular mobility of the system, as it relates to the glass–rubber transition (Tg) detectable by calorimetry. Anthocyanin degradation occurred, even at sub-Tg temperatures of the amorphous matrices, whereas no changes in the rate constants were observed in the vicinity of the glass transition; the plot of (lnk)−1 against (T−Tg) was linear with all data fitting into a common line as predicted by the Williams–Landel–Ferry (WLF) equation. Both free and co-lyophilized with pullulan Hibiscus anthocyanins exhibited good antiradical activity throughout storage in all humidity environments studied, despite of a substantial loss in colour intensity.