Microstructural features of composite whey protein/polysaccharide gels characterized at different length scales

Abstract

Mixed biopolymer gels are often used to model semi-solid food products. Understanding of their functional properties requires knowledge about structural elements composing these systems at various length scales. This study has been focused on investigating the structural features of mixed cold-set gels consisting of whey protein isolate and different polysaccharides at different length scales by using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Whey protein cold-set gels were prepared at different concentrations to emulate stiffness of various semi-solid foods. Mixed gels contained different concentrations of gellan gum, high methyl pectin or locust bean gum. Results obtained with CLSM, at the micrometer length scale, indicated the homogeneous nature of the investigated gels. Results obtained with SEM, at the sub-micron length scale, indicated the presence of spherical protein aggregates. During the gel preparation (acidification), the presence of polysaccharides in the whey protein gels led to on initially segragative phase separation into a gelled protein phase and a polysaccharide/serum phase at a micrometer length scale. At the final pH of the gels (pH 4.8, i.e. below the pI of whey proteins), the negatively charged polysaccharides interacted with the protein phase and their spatial distribution was effected by charge density. Polysaccharides with a higher charge density were more homogeneously distributed within the protein phase. Neutral polysaccharide, locust bean gum, did not interact with the protein aggregates but was present in the serum phase. Using SEM, a new type of microstructure formed in the whey protein/polysaccharide gels was characterized. It composed of a protein continuous, porous network at the length scale of 100 μm, coexisting next to the pools of serum which contained spherical protein-rich domains. Heterogeneity of the structure strongly related to the macroscopic behavior of the gels under large deformation. Upon uniaxial compression these heterogeneous gels releases a large amount of serum. Combination of the results of two microscopic techniques, CLSM and SEM, appeared to offer unique possibilities to characterize the structural elements of whey protein/polysaccharide cold-set gels over a wide range of length scales.