Rheological Behaviour of Biopolymer Gels in Relation to Structure
The rheological behaviour of biopolymer gels with different network structures was studied to elucidate how the structure of the gel influences its rheological properties. Non-destructive viscoelastic measurements and fracture techniques were used. The different types of structures studied were fine-stranded gels, particulate gels, inhomogeneous gels and mixed gels.
The biopolymer gels chosen as the model systems were one protein, b-lactoglobulin, two polysaccharides, k-carrageenan and locust bean gum. b-Lactoglobulin forms gels on heating and develops different structures depending on pH: a particulate network at intermediate pH (4-6) and a fine-stranded network below and above this interval. It can also form inhomogeneous networks depending on pH or heating rate. The two polysaccharides formed mixed fine-stranded gels of various structures depending on the ionic form of k-carrageenan and on type and concentration of cations.
The fine-stranded gels of b-lactoglobulin were compared with the particulate gels of the same biopolymer. The particulate gels were stiffer with a higher modulus, G', than the fine-stranded gels at equal concentrations and formed gels at lower concentrations. The gelation of the particulate gels started at temperatures below the denaturation temperature, Td, and proceeded in two steps with the second step coinciding with Td.
A method for measuring fracture properties of the gels in tension was developed. The fine-stranded b-lactoglobulin gels formed at low pH were brittle whereas those formed at high pH were rubber-like. The stiffness at large deformations, E, had two maxima, one at pH 4, where a mixed network was formed and one around pH 6 where an inhomogeneous network was formed. It was shown that b-lactoglobulin forms inhomogeneous networks at pH 5.3 and 7.5 if the gel is formed at slow heating. At pH 7.5 the inhomogeneities were formed as a result of microphase separation in the incipient network. G' of these gels showed a stronger frequency dependence at low frequencies than at high, caused by different relaxation times in the dense and loose regions of the network. The inhomogeneous particulate gels at pH 5.3 had a network structure containing some large pores where fracture started. The strand structure differed between the homogeneous and inhomogeneous particulate gels causing different storage moduli.
Mixed gels of the potassium form of k-carrageenan and locust bean gum showed synergistic effects in a KCl-concentration 0.1M KCl. The synergistic effects probably originated from a different effect at low KCl-concentration than at high KCl-concentration. The sodium and calcium forms did not show any synergistic effects, neither did the mixed gels of the potassium form in high KCl-concentration (0.2M).