Globular protein gelation - theory and experiment

Heat-set globular protein gel networks are discussed in relation to protein charge and screening. Homogeneous 'fine-stranded' gels form when electrost atic repulsion is high, with a transition to microphase-separated structure s as repulsion falls. Fine-stranded networks showing density fluctuations o ccur close to this transition and probably arise from a combination of kine tic trapping and a drive towards only limited phase separation. For the mos t uniform structures (pH far from pl, and low salt content) network buildin g appears to involve three main stages: initial protein unfolding, linear f ibrillar aggregation, and random cross-linking of the fibrils. A mean field model is described which incorporates these features and includes the poss ibility of cooperative linear aggregation (nucleation and growth). Applicat ion of this to cure data for acid beta -lactoglobulin gels was only partial ly successful, however, a higher order (n = 4) nucleation process being req uired to explain gel time-concentration data, while only a lower second ord er process could reproduce the shapes of the cure curves. As uniform gels g ive way to phase-separated structures network building becomes still more c omplex. Here solution demixing of unfolded monomers, and/or the initial agg regates, must be included in the model. This seems beyond the current mean- field approach, and simulation is likely to be required. This is true, even for the more homogeneous structures, when gel properties of interest exten d from the linear elastic, to the time-dependent, and non-linear. (C) 2001 Elsevier Science Ltd. All rights reserved.