| dc.description.abstract | Rheological comparisons have been made between preparations of high methoxy pectin (DE approximate to 70%) gelled by acidification with D-glucono-delta-lactone (GDL) on holding for 16 h at 25 degrees C in the presence of 60 wt% sucrose, and otherwise identical preparations gelled by acidification with citric acid at high temperature and cooling from 90 to 25 degrees C at 1 degrees C/min. Two series of experiments were carried out for both methods of acidification. In the first series, the concentration of pectin (c) was held constant at 1.0 wt% and the final pH attained after holding (with GDL) or cooling (with citric acid) was varied from 3.75 to 2.25. In the second series, the final pH was held constant at 3.0 and c was varied from 0.25 to 2.00 wt%. All samples were then heated (1 degrees C/min) from 25 to 90 degrees C. Rheological changes on cooling/holding and heating were characterised by low-amplitude oscillatory measurements of storage modulus (C) and loss modulus (G '') at 1 rad s(-1) and 0.5% strain, and mechanical spectra were recorded at 25 degrees C. Selected samples, gelled with GDL, were also characterised by compression testing (at 25 degrees C), and a direct linear relationship was found between the logarithm of yield stress and log G'. The concentration-dependence of moduli for the samples acidified to pH 3.0 with GDL had the form typical of biopolymer gels, with logG' versus logc approaching a limiting slope of 2 as c was raised above the minimum critical gelling concentration (c(o) approximate to 0.3 wt%). Under all conditions of pH and pectin concentration studied, the values of G '' (at 25 degrees C) for the samples acidified with citric acid were higher than those of the corresponding GDL-induced networks. The values of G' were also higher, except at very low pH (below similar to 2.7 at c = 1.0 wt%) or very high concentrations of pectin. At pectin concentrations above similar to 1.5 wt%, the moduli of the samples gelled with citric acid (at pH 3.0) levelled out, or decreased slightly, with the values of C dropping below those of the GDL-induced networks towards the end of the concentration range studied (at c; 2 wt%). All samples acidified with citric acid showed gel-like response (G' > G '') at 90 degrees C, attributed to hydrophobic association. The downturn in moduli at 25 degrees C for high concentrations of pectin is attributed to formation and disruption of strong networks during mixing with citric acid at high temperature ("pregelation"). It is suggested, however, that "weak gels" formed at lower concentrations or at pH values above similar to 2.7 may enhance gel properties by preserving a continuous network as hydrophobic junctions dissociate on cooling and are replaced by hydrogen-bonded junctions, in contrast to random percolation during gelation with GDL at 25 degrees C. On re-heating from 25 to 90 degrees C, the reverse processes (dissociation of hydrogen-bonded structures and formation of hydrophobic associations) were evident in an initial reduction and subsequent increase in moduli, as observed in previous studies. Similar heating traces were obtained for samples acidified with GDL to pH values above similar to 3.0 (at c = 1.0 wt%) or with pectin concentrations below similar to 1.0 wt% (at pH 3.0). However, at higher concentrations or lower values of pH (i.e. conditions favourable to extensive intermolecular association) an abrupt decrease in C, with an accompanying maximum in G '', was observed on heating through the temperature range similar to 60-80 degrees C. This is attributed to excessive hydrophobic association, causing collapse of network structure. It is further suggested that, for samples acidified with citric acid, there is preferential association of chain sequences of high ester content into hydrophobic junctions at 90 degrees C, leaving sequences with a high content of unesterified carboxyl groups available to form long hydrogen-bonded junctions during cooling, and thus giving gels that are stronger and more resistant to network collapse. (C) 2008 Elsevier Ltd. All rights reserved. | en |
