FACTORS THAT DETERMINE THE FRACTURE PROPERTIES AND MICROSTRUCTURE OF GLOBULAR PROTEIN GELS

Protein gel matrices are responsible for the texture of many foods. Th erefore art understanding of the chemical reactions and physical proce sses associated with fracture properties of gels provides a fundamenta l understanding of select mechanical properties associated with textur e. Globular proteins form thermally induced gels that are classified a s fine-stranded, mixed or particulate, based on the protein network ap pearance. The fundamental properties of true shear stress and true she ar strain at fracture, used to describe the physical properties of gel s, 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 pro perties. A fine-stranded matrix is formed when protein suspensions con tain monovalent cation (Li+, K+, Rb+, Cs+) chlorides, sodium sulfate o r 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 s tress and strain at fracture at different salt concentrations. At ioni c strengths >0.1 mol/dm(3) the matrix becomes mixed. This network appe ars as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher conc entrations of monovalent cation salts cause the formation of particula te gels, which are high in stress and strain at fracture. The salt con centration required to change microstructure depends on the salt's pos ition 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 salt s or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effe ct on beta-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide-amide interproton connectivities (d etermined by H-1 NMR) at 40-45 degrees C, increasing hydrophobicity an d intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to dena turation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25-45 degrees C) which are much low er than the denaturation temperature (similar to 65 degrees C). Theref ore, general (e.g. Hofmeister series) and protein-specific factors car t affect the dispersibility of proteins and thereby determine the micr ostructure and fracture properties of globular protein gels.