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Interfacial properties like shear behaviour of commercial milk proteins and food emulsifiers were studied, at both air-water and n-dodecane–water interfaces. Whey proteins form a protein network at interfaces. The film exhibits high elasticity, whatever the interface. On the contrary, micellar caseins and emulsifiers do not exhibit any viscoelastic behaviour at interfaces. Competitive adsorption of proteins with surfactants was clearly identified, both via rheological and tensiometrical measurements. The addition of an emulsifier to an elastic protein film, even at very small amounts, leads to a significant decrease of the interfacial elasticity, depending on the type of emulsifier and on the emulsifier/protein ratio. Water-soluble surfactants (like sucrose ester) are indeed more effective than oil-soluble surfactants in displacing protein molecules from our model interfaces. Most of our results on commercial samples do confirm but also extend what was already described in the literature with purified products. Our major contribution links these interfacial parameters to the stability of model foams. Foaming trials were performed by incorporating air in a sucrose syrup/water mixture, containing protein or emulsifier molecules (previously characterised) for stabilisation. In protein aerated systems, a high surface elasticity is required to prevent quick foam destabilisation: interfacial layers are quite deformable, their rupture and the bubbles' coalescence are limited. In the presence of low-molecular-weight surfactants, the formation and stabilisation mechanisms are quite different. Therefore mixed protein–surfactant systems offer a simple way for optimising foam ability and stability.

The rheological behaviour of xanthanguar gum systems has been investigated and compared to that of xanthanlocust bean gum mixtures using oscillatory shear and creep-recovery measurements. The total polysaccharide concentration was kept constant at 0.5% w/w, the xanthangalactomannan ratio ranged from 199 to 9010 and three ionic strengths were studied. As for xanthanlocust bean gum mixtures, strong synergistic phenomena were exhibited by xanthanguar gum systems. Clearly, the addition of xanthan gum even at a very low level to a guar gum solution induced a transition of the system from a macromolecular solution to a structured system displaying gel-like properties. The comparison between three guar gum samples with different molecular weights evidenced a strong effect of the molecular weight: the higher this parameter, the stronger the synergistic interaction. At a low xanthangalactomannan ratio (< 1090), xanthanguar gum mixtures, xanthanlocust bean gum mixtures (with comparable galactomannan molecular weights) resulted in similar viscoelastic behaviour. In contrast, at a higher xanthangalactomannan ratio, a stronger synergism was exhibited with locust bean gum, as expected. The rheological properties were greatly influenced by the presence of electrolyte. In the case of xanthanguar gum systems, the storage (G′) and the loss (G″) moduli were increased when electrolyte was present, in contrast to xanthanlocust bean gum mixtures which exhibited a reverse tendency. These results provide evidence that xanthan gum plays a major role in the rheological behaviour of the xanthangalactomannan systems even at a low concentration. Overall, similar results were found with xanthanguar or xanthanlocust bean gum mixtures. However, differences in the mechanism may exist according to the mannosegalactose ratio of the galactomannan and also to the xanthangalactomannan ratio and the ionic strength.

Dextran/locust bean gum (LBG) mixtures have been prepared and investigated with respect to their phase separation behaviour. These systems exhibited phase separation at 20 °C, the upper phase, itself biphasic, being enriched with locust bean gum but also containing dextran, whereas the lower phase contained only dextran. This lower phase was a liquid. The upper phase, which did not flow, was characterized by means of rheological dynamic measurements. Clearly, its behaviour was typical of a gel, the three-dimensional structure of which can be ascribed to self-association of LBG chains owing to the very high concentration of the galactomannan in this upper phase. The self-association of the galactomannan was confirmed by fluorescence microscopy carried out on mixtures containing fluorescein isothiocyanate (FITC)-labelled dextran. The rheological behaviour of a concentrated LBG solution was also investigated as a function of time, clearly showing progressive formation of a weak gel structure.

The aim of the present work was to investigate the role of whey protein denaturation on the acid induced gelation of casein. This was studied by determining the effect of whey protein denaturation both in the presence and absence of casein micelles. The study showed that milk gelation kinetics and gel properties are greatly influenced by the heat treatment sequence. When the whey proteins are denatured separately and subsequently added to casein micelles, acid-induced gelation occurs more rapidly and leads to gels with a more particulated microstructure than gels made from co-heated systems. The gels resulting from heat-treatment of a mixture of pre-denatured whey protein with casein micelles are heterogeneous in nature due to particulates formed from casein micelles which are complexed with denatured whey proteins and also from separate whey protein aggregates. Whey proteins thus offer an opportunity not only to control casein gelation but also to control the level of syneresis, which can occur.

Polarization microscopy has been used to investigate the structure of 50/50 xanthan/galactomannan (guar gum or locust bean gum) mixtures in aqueous solution, the total concentration ranging from 0.5 to 4%. By the use of polarized light microscopy birefringent areas resulting from the formation of cholesteric mesophases in xanthan gum was clearly seen as has previously been reported by several authors. In xanthan/galactomannan mixtures, we also observed birefringent areas. Moreover, these zones in the blend appeared more anisotropic than with xanthan gum alone. This suggests that xanthan molecules organize themselves as liquid crystalline mesophases in definite enriched xanthan areas resulting from a concentration of xanthan inside these birefringent zones. Upon heating, this anisotropy disappears at a temperature well below the helix-coil transition temperature of xanthan molecules. In fact, this loss of order of the mixed system occurs at the same temperature as the melting temperature of the gel, as assessed by the use of rheological measurements. Since the ordered helical structure of the xanthan molecules still exists beyond the melting temperature while anisotropy disappears, this suggests that the xanthan molecules are no longer concentrated in specific areas but more evenly distributed in the medium. Gel melting would, therefore, be the result of the disappearance of these xanthan enriched areas.