Ca2+-induced gelation of low-methoxy pectin in the presence of oxidised starch. Part 2. Quantitative analysis of moduli

The observed moduli (Gʹ; 10 rad s-1; 0.5% strain; 5°C) of the co-gels formed by mixtures of calcium pectinate (DE 34.2; stoichiometric Ca2+) and oxidised starch (0¯40 wt%) have been matched with good precision by an analysis based on the following postulates. (i) Thermodynamic incompatibility between the two polymers causes partial precipitation of pectin chains into aggregated bundles within a supporting calcium pectinate network. (ii) The extent of precipitation is controlled by a solubility product (K), defined as the starch concentration multiplied by the square of the concentration of unprecipitated pectin. (iii) The volume occupied by the precipitated pectin fraction is directly proportional to the amount precipitated (constant of proportionality, k). The system was regarded as comprising a continuous phase of oxidised starch and unprecipitated pectin surrounding a dispersed phase of precipitated pectin, and overall moduli were calculated using the Takayanagi isostress blending law. The root-meansquare difference between observed and calculated values of log Gʹ was minimised by varying the two adjustable parameters, k and K (which determine phase volumes and partition of pectin between the phases). The moduli of the individual components at their effective concentration within the appropriate phase were determined from standard calibration curves for calcium pectinate and oxidised starch alone. The overall modulus of the continuous phase was estimated by simple addition of the separate contributions from unprecipitated pectin and oxidised starch. The modulus of the dispersed particles of precipitated pectin was effectively infinite in comparison with that of the continuous phase, allowing a simplified form of the isostress equation to be used. Best agreement was obtained with K=0.83 (for concentrations in wt%) and with a value of k corresponding to a pectin concentration of ~16.4 wt% within the precipitated bundles (~5 g water per 1 g of pectin). Discrepancies between observed and fitted values of log Gʹ were not significantly beyond experimental error, suggesting that the concept of partial precipitation of calcium pectinate in response to segregative interactions with oxidised starch is a realistic interpretation of the reductions in co-gel moduli reported in the preceding paper.