Application of materials science approach to the structural properties of whey protein based composite gels influenced by change in pH

Despite the increasing appreciation that whey is a valuable resource of dairy industry, as opposed to a waste product of the cheese and casein industries, there have been drawbacks in its utilization as a functional ingredient in processed food products. Potential benefits include improvement in nutritional quality, imparting flavor and color due to the presence of lactose that takes part in Maillard reactions with the protein, as fat replacer in low-fat dairy products when co-gelled with polysaccharide, and as an ingredient in starch-based formulations (e.g. snacks and cereals). Therefore, the aim of this research is to apply the technique and concepts of the material science approach for the determination of the composition of individual phases in biphasic gels of whey protein in the presence of other polysaccharide such as agarose.

The structural properties and morphology of mixed gels made of aqueous preparations of agarose and whey protein were modified by changing thermal treatment and pH. The conformationally dissimilar polymers phase separated and this process was followed by small-deformation dynamic oscillation in shear, differential scanning calorimetry and scanning electron microscopy.

Experimental protocol encourages formation of a range of two-phase systems from continuous agarose matrices perforated by liquid-like whey protein inclusions to phase inverted preparations where a soft protein matrix suspends hard agarose-filler particles. These distinct morphologies have widely different mechanical moduli, which were followed by adapting a theoretical analysis (isostress-isostrain and Lewis-Nielsen blending laws) from the literature in synthetic block polymers and polyblends. Based on this framework of thought, reasonable predictions of the elastic moduli in the composite gels were made that led to patterns of solvent partition between the two polymeric networks. It was shown that proteins, in mixture with polysaccharide, exhibit favorable relative affinity (P-factor) for water molecules at a pH above their isoelectric point. This is an unexpected outcome that adds to the central finding of a single P value for the distribution of solvent between the continuous matrix and discontinuous inclusions of binary gels. It was thus proposed that phase continuity and solvent distribution in agarose/whey protein systems are under kinetic control that can be heavily governed by pH changes in the aqueous environment.