Influence of pH on mechanical relaxations in high solids lm-pectin preparations

The influence of pH on the mechanical relaxation of LM-pectin in the presence of co-solute has been investigated by means of differential scanning calorimetry, ζ-potential measurements and small deformation dynamic oscillation in shear. pH was found to affect the conformational properties of the polyelectrolyte altering its structural behaviour. Cooling scans in the vicinity of the glass transition region revealed a remarkable change in the viscoelastic functions as the polyelectrolyte rearranges from extended (neutral pH) to compact conformations (acidic pH). This conformational rearrangement was experimentally observed to result in early vitrification at neutral pH values where dissociation of galacturonic acid residues takes place. Time-temperature superposition of the mechanical shift factors and theoretical modeling utilizing WLF kinetics confirmed the accelerated kinetics of glass transition in the extended pectin conformation at neutral pH. Determination of the relaxation spectra of the samples using spectral analysis of the master curves revealed that the relaxation of macromolecules occurs within ~0.1 s regardless of the solvent pH

Solution viscosity and structural modification of pumpkin biopectin

Solutions of 'biopectin' obtained from pumpkin pulp by digestion with the multi-enzyme culture supernatant from Bacillus polymyxa (strain 88A) were prepared in 0.10 M NaCl and characterised by rotational viscosity measurements at 20 degrees C. The resulting double-logarithmic plot of specific viscosity versus the product of concentration (c) and intrinsic viscosity ([eta], from combined Huggins and Kraemer extrapolation to c = 0) showed a sharp increase in slope at c[eta] = approximate to 1 in comparison with the normal value of c[eta] approximate to 4 for disordered coils, suggesting a branched structure, possibly arising from Ca2+-mediated association of constituent chains. Pumpkin biopectin is non-gelling, in marked contrast to the pectin obtained from the same source by conventional extraction with acid, although the yield is more than doubled. Attempts to induce gelation (with 70 wt% solids at low pH or with stoichiometric Ca2+ at neutral pH) by removing the high content of divalent cations naturally present in the biopectin and by chemical deacetylation under acidic conditions (pH 1.2; 4 days; 40 degrees C) proved unsuccessful. Further research using enzymic deacetylation is suggested. (C) 2005 Elsevier Ltd. All rights reserved

Tailoring gel modulus using dispersed nanocrystalline hydroxyapatite

Mammalian cells are known to respond to the elastic modulus of the surface to which they adhere. Consequently, there is interest in developing strategies to control the elastic moduli of materials, including hydrogels. One way of controlling modulus in hydrogels is to introduce reinforcing agents such as inorganic materials, for example hydroxyapatite (HA). Although several authors have reported the reinforcement of hydrogels with ceramic particles, there have not been any studies to investigate the effect of size and crystallinity of HA particles on the mechanical properties of hydrogel. In this study, synthetic Calcium phosphate of two different crystallite sizes: one on the nano-scale (50 nm) and the other on the micro-scale (150 nm) have been used to manufacture HA/gellan gum (GG) composites. It was shown that while nano-scale HA (nHA) reinforced the hydrogel structure, the micro-scale HA (mHA) material acted to weaken it (2.5 wt% HA). Furthermore, it was found that by increasing the content of the nHA in the composite to 50 wt%, the yield strength and bulk modulus was increased by four- and nine-fold, respectively. The reinforcing effect of nHA was attributed to its higher association with the GG coil structure when compared with the mHA, which disrupted gel structur