Viscoelastic behaviour of beta-lactoglobulin gel structures

The viscoelastic properties of beta-lactoglobulin gels, made by dissolution of beta-lactoglobulin in distilled water, have been investigated by dynamic rheological measurements at pH 2.5-9.0. Gels formed at pH 4-6 were opaque and have been defined as aggregate or particle gels; at pH below or above this range gels were transparent and have been defined as fine-stranded gels. The aggregate gels were more strain sensitive during gelation and more frequency dependent than the fine-stranded gels. They also had a higher storage modulus than the fine-stranded gels at a constant beta-lactoglobulin concentration of 12% (w/w). The critical concentration for gel formation was lower for the aggregate gels than for the fine-stranded gels, and as low as 1% in the pH range 4.5-5.5. This indicates a very open structure. The onset of gelation of 12% beta-lactoglobulin gels at intermediate pH, when measured as G' = G '' at 1 Hz or as the development of a measurable G' takes place at temperatures far below the temperature of denaturation. This is not the case at higher or lower pH, where the onset of gelation is above the temperature of denaturation. The heating rate during gel formation was varied between 0.008 and 1 degrees C/min. The gelation temperature of the fine-stranded gels was found to increase with an increasing heating rate.