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

The effect of progressive replacement of sucrose by oxidised starch in mixt ures with high methoxy pectin (1.0 wt%; degree of esterification 70.3%; pH 3) has been characterised by low amplitude oscillatory measurements (1% str ain) of storage modulus (G'), loss modulus (G "), loss tangent (tan delta) and complex dynamic viscosity (eta*) across the frequency range 0.1-100 rad s(-1). Samples were prepared at 90 degrees C, and measured 1 h after rapid quenching to 5 degrees C. In the main series of experiments, the total con centration 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 (p articularly evident at low frequency) and subsequent steady increase with i ncreasing concentration of starch. Critical gel spectra were recorded at si milar to 9 wt% starch (56 wt% sucrose) and similar to 41 wt% starch (24 wt% sucrose). The moduli of the mixed systems at low starch concentration, bef ore the loss of gel structure, remained close to those for 1.0 wt% pectin a t 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 similar to 10x) higher than those of the individ ual constituents. The origin of this behaviour was explored by quantitative analysis of the results from a subsidiary series of experiments in which t he 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 betwee n 0 and 15 wt%. The observed values of G' and G " at 0.1, 1.01 10 and 100 r ad s(-1) could be matched, to within experimental error, by values calculat ed on the basis of excluded volume effects within a monophasic solution, wi th the volume occupied by 1 g of oxidised starch corresponding to similar t o 1.8 g of solvent and the volume occupied by 1 g of (partially associated) pectin corresponding to similar to 20 g. (C) 2000 Elsevier Science Ltd. Al l rights reserved.