Structure of whey protein gels, studied by permeability, scanning electron microscopy and rheology

Abstract

The (micro)structure and rheological properties of heat-induced (20 h at 68.5°C) whey protein isolate (WPI) gels were investigated. Gels were made at near-neutral pH, while protein concentration (35–89 g/l) and NaCl concentration (0.1–3 mol/dm 3) were systematically varied. The application of permeability measurements to characterize heat-set protein gels is new, and the permeability coefficient turned out to be a sensitive parameter for testing gel structure. All WPI gels were identified as particulate, as revealed by SEM. Individual whey proteins form ‘building’ particles which, in turn, are part of large lumps or flocs. Gels became coarser with bigger pores when the NaCl concentration was increased. This observation was confirmed by a strong increase in gel permeability. Simultaneously, a marked decrease in the elastic modulus ( G′) of the WPI gels was seen on increasing the NaCl concentration. Power law relationships were found for the protein concentration dependence of the permeability coefficient ( B gel) and G′. The exponent of these relationships appeared to be almost independent of the NaCl concentration (0.1–3 mol/dm 3) for both B gel and G′, whereas the prefactor was (especially for B gel) extremely salt concentration dependent. Combination of gel permeability, rheological measurements and measurements on aggregation kinetics revealed that gel structure did not change much after gel formation, while gel rigidity continued to increase, and that at the gel point only part of the protein in the dispersion contributes to the gel network. The latter observation means that the fractal concept cannot simply be applied to WPI gels.