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Harvey, H. G. (1956) Studies in the physical chemistry of pectins. Doctoral thesis, Sir John Cass College.
The physico-chemical properties of pectins are reviewed in the light of modern knowledge of polyelectrolytes.
In experimental studies, metal-free pectin solutions of known methyl-and acidic-polygalacturonate characteristics have been prepared by use of a mixture of anionic and cationic exchange resins, and employed in systematic investigation of their dissociation behaviour, their ionic equilibria at the sol-gel transformation and the rheological properties of their gels, when containing high concentrations of sugars and when buffered by bivalent and univalent metal salt-acid systems.
It is shown how the concentration of the various components present in ionic equilibria at any pH value may be evaluated and used, subject to allowance for association of bivalent metal ions at the surface of the pectin polyion, in the interpretation of pectin solubility and gelling phenomena.
Critical minimum concentrations of ionized and unionized pectin free acid groups are shown to determine respectively the sol-gel transformation for calcium and magnesium buffering.
The rheological behaviour was studied by means of shear deformation of the gels under constant loading and unloading and under varying loads at constant rates of loading, and by combination of both methods. Viscous flow, ordinary and time-dependent elasticity occurred and also considerable elastic recovery after complete breakdown of resistance to shear. These phenomena are shown to be consistent with a gel structure composed of a network of linear polymeric chains, cross-linked at intervals and containing entrapped liquid.
The influence of bivalent metal salts on overall gel strength was markedly affected by their concentration relative to the pectin, by pH value and by sugars concentration and the behaviour due to magnesium often differed from that of calcium. Some possible causes of this complex behaviour are advanced.
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