Role of cosolutes in gelation of high-methoxy pectin. Part 2. Anomalous behaviour of fructose: calorimetric evidence of site-binding

Gels of high-methoxy pectin (DE 70; 1.0 wt%; pH 3.0) in the presence of fructose at concentrations of 50, 55, 60 and 65 wt% showed an intense endotherm followed immediately by an intense exotherm on heating. These transitions occurred over approximately the same temperature-range as initial gelation on cooling (characterised by low-amplitude oscillatory measurements of G′ and G″) and increased in magnitude with increasing concentration of fructose. The displacement of both transitions, and particularly the exotherm, to progressively higher temperature as the rate of heating was increased was much greater than anticipated from simple thermal lag, indicating that the underlying structural changes are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered (threefold helix) and disordered state; the endotherm is attributed to helix melting and displacement of fructose; the subsequent exotherm is attributed to re-attachment of fructose to disordered chains, and the slow kinetics of this process to the conformational mobility of disordered pectin. On cooling over the same temperature range, a single exotherm was observed; the absence of detectable splitting is attributed to rapid re-attachment of fructose to conformationally-rigid helices. The magnitude of this endotherm (ΔH≈20 J/g) is close to the value found for cosolutes that show no evidence of site-binding, and to the net change in enthalpy for the endothermic and exothermic processes observed on heating, suggesting that the values of ΔH for displacement and re-attachment of fructose are essentially equal and opposite, with the net change coming from formation or melting of threefold helices. A smaller thermal process at higher temperature (endothermic on heating and exothermic on cooling) is attributed to hydrophobic association, which was also seen as an increase in G′ and G″ on heating in two consecutive cycles of temperature change.