Cold gelation of globular proteins

Keywords : globular proteins, whey protein, ovalbumin, cold gelation, disulfide bonds, texture, gel hardness

Protein gelation in food products is important to obtain desirable sensory and textural properties. Cold gelation is a novel method to produce protein-based gels. It is a two step process in which first thermally induced protein aggregates are prepared by a heat treatment of a solution of native globular proteins at low ionic strength. After cooling, gelation of the dispersion of repulsive aggregates is induced in the second step by lowering the pH or by adding salt at ambient temperature. Cold-set gelation finds applications in food products with a delicate flavor and texture. In addition cold-set gels can be used as a thickening agent or in encapsulation of sensitive materials.

The purpose of the research described in this thesis was to investigate the molecular mechanisms of the acid-induced cold gelation process. Therefore it was studied (i) how different aggregate properties determined the final properties of cold-set whey protein gels, (ii) how structural and rheological properties of the gels scaled with the protein concentration, and (iii) how the final gel properties depended on the use of different cysteine-containing globular proteins.

The results demonstrated that reduction of the electrostatic repulsion initiated the formation of a randomly aggregated protein network by physical interactions. Surprisingly, additional covalent disulfide bonds were formed under the acid conditions. The disulfide bonds stabilized the initial network and increased the mechanical gel strength. The formation of disulfide bonds depended on the number and accessibility of thiol groups and disulfide bonds present in the various protein molecules. Therefore, the disulfide bonds are important control parameters that can be used to tune the texture of (cold-set) gels. In addition, the contour length of the linear-shaped aggregates prepared in the first step affected the mechanical gel strength of cold-set gels. For smaller aggregates percolation is preceded by the formation of clusters, yielding less effective contact points and therefore weaker gels compared to cold-set gels prepared from long fibrillar structures. Moreover, the length of the linear-shaped aggregates determined the appearance of the cold-set gels. Cold gelation is a relevant method for the application of globular proteins as an efficient structuring ingredient in food systems.