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The effects of bulk protein concentration, Cp, (0.01, 0.1, 1 wt%), pH (3, 4.7 and 7) and heat treatment (unheated or 95 °C for 30 min) on whey protein isolate (WPI) stabilized interfaces were examined. The interfacial pressure and shear rheology of WPI-stabilized sunflower oil-water (o/w) interfaces were characterized using a pendant drop tensiometer and a rheometer equipped with a Du Nouy ring. The rate of WPI adsorption was faster at higer Cp and pH 3. Heat-enhanced surface activity was more pronounced at pH 7 compared to pH 3 as a result of greater heat stability of WPI at acidic pH. The elastic modulus of WPI stabilized interfaces increased with Cp (≤0.1 wt%). A further increase in Cp (to 1 wt%) resulted in monolayer collapse and weaker films. Non-heated (NHT) WPI formed less elastic interfacial films at pH 3 than at pH7. Heat treatment enhanced the elastic behavior of interfacial films with longer relaxation times. This may be associated with the formation of intermolecular β-sheets. The knowledge gained on the nature of WPI-stabilized interfaces can be used to better understand the stability of dairy emulsions during subsequent processing, digestion or storage.

The current study investigated the effect of a neutral polysaccharide, konjac glucomannan, on the heat-induced gelation of whey protein isolate (WPI) at pH 7. Oscillatory rheology (1 rad/s; 0.5% strain), differential scanning calorimetry and confocal laser scanning microscopy were used to investigate the effect of addition of konjac in the range 0–0.5% w/w, on the thermal gelation properties of WPI. The minimum gelling concentration for WPI samples was 11% w/w; the concentration was therefore held constant at this value. Gelation of WPI was induced by heating the samples from 20 to 80 °C, holding at 80 °C for 30 min, cooling to 20 °C, and holding at 20 °C for a further 30 min. On incorporation of increasing concentrations of konjac the gelation time decreased, while the storage modulus ( G ′) of the mixed gel systems increased to ∼1450 Pa for 11% w/w WPI containing 0.5% w/w konjac gels, compared to 15 Pa for 11% w/w WPI gels (no konjac). This increase in gel strength was attributed to segregative interactions between denatured whey proteins and konjac glucomannan.

Mixtures of 0.5 wt% xanthan with 0.5 wt% konjac glucomannan (KGM) in water and in 30 mM KCl were prepared by mixing stock solutions of the individual polymers at 20 °C, and characterised rheologically by low-amplitude oscillatory measurements, and by creep–recovery experiments using applied stresses of 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2 and 102.4 Pa. Both mixtures gave gel-like mechanical spectra, and showed typical elastic response to stresses in the range 0.2–12.8 Pa, with recoverable strain increasing in direct proportion to applied stress. Fracture and flow occurred at higher stress (25.6 Pa for the mixture prepared in water; 102.4 Pa for the network formed in 30 mM KCl). On heating from 20 to 95 °C, both mixtures showed a typical melting process, seen as a sigmoidal reduction in G′ and G″, with G′ dropping below G″, and an accompanying endothermic transition in DSC (differential scanning calorimetry). Rheological and thermal transitions were observed on cooling to 20 °C and reheating to 95 °C, over the same temperature-range as those observed on initial heating. As anticipated, the networks formed after heating and cooling were stronger and more cohesive than those obtained on initial mixing at 20 °C. The order–disorder transition of 0.5 wt% xanthan in water, as characterised by DSC, was centred at ∼50 °C, within the temperature-range of the gelling/melting transition of the xanthan–KGM mixtures prepared in water. Conformational ordering of xanthan in 30 mM KCl, however, was complete by ∼70, 50 °C above the mixing temperature of 20 °C used in preparation of the mixtures with KGM. The transition enthalpy (ΔH) of the DSC endotherm observed on initial heating of the mixture prepared in 30 mM KCl was virtually identical to the values obtained on cooling and re-heating (6.11 J/g xanthan, in comparison with −6.00 J/g on cooling and 6.14 J/g in the second heating scan), indicating the same nature and extent of intermolecular association in mixtures prepared at 20 °C as in those cooled from high temperature. We therefore conclude that mixing at low temperature inhibits formation of a cohesive gel by disruption of network structure during mixing, rather than by a requirement for the xanthan molecules to be disordered before association can occur, as has been proposed previously.