Factors that determine the fracture properties and microstructure of globular protein gels

Protein gel matrices are responsible for the texture of many foods. Therefore art understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and beta-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation (Li+, K+, Rb+, Cs+) chlorides, sodium sulfate or sodium phosphate at ionic strengths less than or equal to 0.1 mol/dm(3). This matrix has a well-defined network structure, and varies in stress and strain at fracture at different salt concentrations. At ionic strengths >0.1 mol/dm(3) the matrix becomes mixed. This network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The salt concentration required to change microstructure depends on the salt's position in the Hofmeister series. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations (10-20 mmol/dm(3)) of divalent cation (Ca2(+), Mg2(+), Ba2(+)) chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on beta-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide-amide interproton connectivities (determined by H-1 NMR) at 40-45 degrees C, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25-45 degrees C) which are much lower than the denaturation temperature (similar to 65 degrees C). Therefore, general (e.g. Hofmeister series) and protein-specific factors cart affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.