Ca2+-induced gelation of low methoxy pectin in the presence of oxidised starch. Part 1. Collapse of network structure

The effect of oxidised starch on the networks formed by low-methoxy pectin (2.0 wt%), on cooling (from 90 to 5°C) in the presence of Ca2+ (10¯100% stoichiometric) has been explored by rheological measurements under low-amplitude oscillatory shear, and by light microscopy. At low concentrations of Ca2+, incorporation of increasing concentrations of starch (across the range 0¯30 wt%) causes a progressive increase in modulus (Gʹ at 5°C), attributed to segregative interactions between the two polymers promoting conversion of pectin from the expanded coil conformation to a more compact associated form. Incorporation of starch at higher Ca2+ concentrations, however, causes large reductions in gel strength, which arise from sharp drops in Gʹ during cooling. The Ca2+ concentration at the transition from enhancement to depletion of network strength is independent of starch concentration, and increases with increasing degree of esterification of the pectin component (across the DE range 31.1¯ 55.8%). Reduction in gel strength is attributed to incipient precipitation of pectin into large aggregates within a supporting calcium pectinate network. This interpretation is supported by microscopy, which shows conversion from a homogeneous distribution of the two polymers to a grossly heterogeneous structure (length-scale ~10¯50 mu m) with increasing concentration of Ca2+, and by quantitative analysis of gel moduli reported in the following paper.