Electrostatic effects on physical properties of particulate whey protein isolate gels

Physical properties of particulate whey protein isolate gels formed under varying electrostatic conditions were investigated using large strain rheological and microstructural techniques. The two treatment ranges evaluated were adjusting pH (5.2-5.8) with no added NaCl and adjusting the NaCl (0.2-0.6 M) at pH 7. Gels (10% protein w/v) were formed by heating at 80C for 30 min. The large strain properties of fracture strain (gamma (f)), fracture stress (alpha (f)), and a measure of strain hardening (R-0.3) were determined using a torsion method. Gel microstructure was evaluated using scanning electron microscopy (SEM) and gel permeability (B-gel). Overlaying sigma (f) and gamma (f) curves for pH and NaCl treatments demonstrated an overlap where gels of equal sigma (f) and gamma (f) could be formed by adjusting pH or NaCl concentration. The high fracture stress (sigma (f) approximate to 23 kPa and gamma (f) approximate to 1.86) pair conditions were pH 5.47 and 0.25 M NaCl, pH 7.0. The low fracture stress (sigma (f) = 13 kPa and gamma (f) = 1.90) pair conditions were pH 5.68 and 0.6 M NaCl, pH 7.0. The 0.25 M Na Cl, pH 7 treatment demonstrated higher R-0.3 values than the pH5.47 treatment. When the sulfhydryl blocker n-ethylmaleimide was added at 2 mM to the 0.25 M NaCl, pH 7 gel treatment, its rheological behavior was NSD (p > 0.05) to the pH 5.47 gel treatment, indicating disulfide bond formation regulated strain hardening. Altering surface charge or counterions, and disulfide bonding, was required to produce gels with similar large strain rheological properties. An increase in gel permeability coincided with an increase in pore size as observed by SEM, independent of rheological properties. This demonstrated that at the length scales investigated, microstructure was not linked to changes in large strain rheological properties.