Effect of oxidised starch on high methoxy pectin–sucrose gels formed by rapid quenching

Abstract

The effect of progressive replacement of sucrose by oxidised starch in mixtures with high methoxy pectin (1.0 wt%; degree of esterification 70.3%; pH 3) has been characterised by low amplitude oscillatory measurements (1% strain) of storage modulus ( G′), loss modulus ( G″), loss tangent (tan δ) and complex dynamic viscosity ( η ∗) across the frequency range 0.1–100 rad s −1. Samples were prepared at 90°C, and measured 1 h after rapid quenching to 5°C. In the main series of experiments, the total concentration of cosolute (sucrose plus oxidised starch) was held constant at 65 wt%, and the starch concentration was varied between 0 and 50 wt% (i.e. 65–15 wt% sucrose). The observed moduli showed an initial sharp decrease (particularly evident at low frequency) and subsequent steady increase with increasing concentration of starch. Critical gel spectra were recorded at ∼9 wt% starch (56 wt% sucrose) and ∼41 wt% starch (24 wt% sucrose). The moduli of the mixed systems at low starch concentration, before the loss of gel structure, remained close to those for 1.0 wt% pectin at the same sucrose concentrations but in the absence of starch, and at high starch concentrations became close to those of the starch component in the absence of pectin. In the intermediate region, however, the overall moduli were substantially (up to ∼10×) higher than those of the individual constituents. The origin of this behaviour was explored by quantitative analysis of the results from a subsidiary series of experiments in which the sucrose concentration was held fixed at 50 wt% and the concentration of oxidised starch (in mixtures with 1.0 wt% pectin at pH 3) was varied between 0 and 15 wt%. The observed values of G′ and G″ at 0.1, 1.0, 10 and 100 rad s −1 could be matched, to within experimental error, by values calculated on the basis of excluded volume effects within a monophasic solution, with the volume occupied by 1 g of oxidised starch corresponding to ∼1.8 g of solvent and the volume occupied by 1 g of (partially associated) pectin corresponding to ∼20 g.