Properties of Fibrillar Protein Assemblies and their Percolating Networks

Properties of Fibrillar Protein Assemblies and their Percolating Networks. PhD thesis, Wageningen University, The Netherlands Keywords: bovine serum albumin, complex fluids, excluded volume, fibrils, gels, innovation, b-lactoglobulin, ovalbumin, percolation, proteins, rheology, rheo-optics, self-assembly, structure function relations. Abstract The objective of this thesis was to explore the assembly of food proteins into fibrils, and to describe the resulting percolating systems at rest and under shear flow, in terms of mesoscopic fibril properties. The effect of ionic strength on the percolation concentration for three different food proteins, namely b-lactoglobulin, bovine serum albumin and ovalbumin is described. The dependence of ionic strength on the percolation concentration was explained using an adjusted random contact model, in which the percolation concentration is related to the average number of contacts per particle, and the excluded volume of the rod. Also the contour length, persistence length, and bending rigidity for these three protein assemblies were determined, as well as the phase behaviour of b-lactoglobulin at low pH. A new multistep Ca2+-induced cold gelation process is described to prepare b-lactoglobulin gels at very low protein concentrations (0.07%). The behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the critical percolation concentration, was probed with the use of rheo-optical measurements and Fourier transform rheology. Also the effect of shear flow on the critical percolation concentration for solutions of fibrillar protein assemblies was investigated. Results of viscosity measurements were analysed using percolation theory, where the effect of shear flow was taken into account. The experimental results were compared with our theoretical calculations for the percolation concentration versus shear, based on a random contact model for rodlike particles, making use of a shear dependent excluded volume per fibril. In conclusion conditions leading to gel formation, in terms of mesoscopic fibril properties, under non-flow conditions have been discussed. The observed critical gelation concentration was explained in terms of an excluded volume per fibril (at zero shear). The influence of shear flow on this critical gelation concentration was also described. Here, the critical percolation concentration versus shear flow could again be expressed in terms of an excluded volume per fibril, in this case as a function of shear