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The denaturation and aggregation of reagent-grade (Sigmaalpha-La), ion-exchange chromatography purified (IEXalpha-La), and a commercial-grade (Calpha-La) alpha-lactalbumin were studied with differential scanning calorimetry (DSC), polyacrylamide gel electrophoresis, and turbidity measurement. All three preparations had similar thermal denaturation temperatures with an average of 63.7 degrees C. Heating pure preparations of alpha-lactalbumin produced three non-native monomer species and three distinct dimer species. This phenomenon was not observed in Calpha-La. Turbidity development at 95 degrees C (tau95 degrees C) indicated that pure preparations rapidly aggregate at pH 7.0, and evidence suggests that hydrophobic interactions drove this phenomenon. The Calpha-La required 4 times the phosphate or excess Ca2+ concentrations to develop a similar tau95 degrees C to the pure preparations and displayed a complex pH-dependent tau95 degrees C behavior. Turbidity development dramatically decreased when the heating temperature was below 95 degrees C. A mechanism is provided, and the interrelationship between specific electrostatic interactions and hydrophobic attraction, in relation to the formation of disulfide-bonded products, is discussed.

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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 (γf), fracture stress (σf), and a measure of strain hardening (R0.3) were determined using a torsion method. Gel microstructure was evaluated using scanning electron microscopy (SEM) and gel permeability (Bgel). Overlaying σf and γf curves for pH and NaCl treatments demonstrated an overlap where gels of equal σf and γf could be formed by adjusting pH or NaCl concentration. The high fracture stress (σf∼ 23 kPa and γf∼ 1.86) pair conditions were pH 5.47 and 0.25 M NaCl, pH 7.0. The low fracture stress (σf∼ 13 kPa and γf∼ 1.90) pair conditions were pH 5.68 and 0.6 M NaCl, pH 7.0. The 0.25 M NaCl, pH 7 treatment demonstrated higher R0.3 values than the pH 5.47 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.

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