Heat-induced gelation (80 °C, 30 min or 85 °C, 60 min) of whey protein concentrate (WPC) solutions was studied using transmission electron microscopy (TEM), dynamic rheology and polyacrylamide gel electrophoresis (PAGE). The WPC solutions (150 g/kg, pH 6·9) were prepared by dispersing WPC powder in water (control), 10 g/kg sodium dodecyl sulphate (SDS) solution or 10 mM-dithiothreitol (DTT) solution. The WPC gels containing SDS were more translucent than the control gels, which were slightly more translucent than the gels containing DTT. TEM analyses showed that the SDS-gels had finer aggregate structure (≅10 nm) than the control gels (≅100 nm), whereas the DTT-gels had a more particulate structure (≅200 to 300 nm). Dynamic rheology measurements showed that the control WPC gels had storage modulus (G′) values (≅13500 Pa) that were ≅25 times higher than those of the SDS-gels (≅550 Pa) and less than half those of the DTT-gels after cooling. Compression tests showed that the DTT-gels were more rigid and more brittle than the control gels, whereas the SDS-gels were softer and more rubbery than either the control gels or the DTT-gels. PAGE analyses of WPC gel samples revealed that the control WPC solutions heated at 85 °C for 10 min contained both disulphide bonds and non-covalent linkages. In both the SDS-solutions and the DTT-solutions, the denatured whey protein molecules were in the form of monomers or small aggregates. It is likely that, on more extended heating, more disulphide linkages were formed in the SDS-gels whereas more hydrophobic aggregates were formed in the DTT-gels. These results demonstrate that the properties of heat-induced WPC gels are strongly influenced by non-covalent bonding. Intermolecular disulphide bonds appeared to give the rubbery nature of heat-induced WPC gels whereas non-covalent bonds their rigidity and brittle texture.

Gelation of acidified milk at pH[ges ]5, after heat treatments is a well known phenomenon, due to the precipitation of whey proteins, and especially β-lactoglobulin onto κ-casein (Sawyer, 1969). High heat treatments cause denaturation of whey proteins which associate with κ-casein through disulphide interchange reactions (Hill, 1989). Since their charge is reduced, the denatured proteins associated with casein micelles become susceptible to aggregation when milk is then acidified, which promotes enhanced protein–protein interactions (Lucey et al. 1997). The gelation phenomenon involves disulphide bonds (Hashizume & Sato, 1988; Goddard, 1996) which are responsible for the gel firmness (Goddard, 1996). However, other interactions between proteins can occur, such as hydrogen and hydrophobic bonds, especially at the initial stage of interactions (Haque et al. 1987; Haque & Kinsella, 1988; Jang & Swaisgood, 1990). It is therefore relevant to investigate a possible contribution of weak linkages to the gel structure and firmness.