STRUCTURE OF WHEY-PROTEIN GELS, STUDIED BY PERMEABILITY, SCANNING ELECTRON-MICROSCOPY AND RHEOLOGY

The (micro)structure and rheological properties of heat-induced (20 h at 68.5 degrees 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 vari ed. The application of permeability measurements to characterize heat- set protein gels is new, and the permeability coefficient tamed out to be a sensitive parameter for testing gel structure. All WPI gels were identified as particulate, as revealed by SEM. Individual whey protei ns form 'building' particles which, in turn, are part of large lumps o r flocs. Gels became coarser with bigger pores when the NaCl concentra tion was increased. This observation was confirmed by a strong increas e in gel permeability. Simultaneously a marked decrease in the elastic modulus (G) of the WPI gels was seen on increasing the NaCl concentra tion. Power law relationships were found for the protein concentration dependence of the permeability coefficient (B-gel) and G. The exponen t 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 pre factor was (especially for Bgel) extremely salt concentration dependen t. Combination of gel permeability, rheological measurements and measu rements on aggregation kinetics revealed that gel structure did not ch ange much after gel formation, while gel rigidity continued to increas e, and that at the gel point only part of the protein in the dispersio n contributes to the gel network. The latter observation means that th e fractal concept cannot simply be applied to WPI gels. (C) 1998 Elsev ier Science Ltd. All rights reserved.