Factors Determining Fracture Stress and Strain of Fine-Stranded Whey Protein Gels

Whey proteins form translucent heat-induced gels at low and high pH that differ in fracture properties. Gels formed at pH 7.0 and 6.5 were strong (fracture stress of 59−92 kPa) and rubbery (fracture strain of 1.7−1.2). Gels formed at pH 3.0 and 2.5 were weak (fracture stress of 17−19 kPa) and brittle (fracture strain of 0.33−0.39). Altering disulfide bond formation changed gel texture. Addition of a disulfide reducing agent (dithiothreitol) at concentrations ≥ 25 mM, at pH 7.0, caused the gel to weaken (>46% reduction in fracture stress) and become brittle (fracture strain of 0.30−0.32). Increasing the number of disulfide bonds in gels formed at pH values ≤3.5 increased fracture strain to 2.0 while causing a minor increase in fracture stress. Formation of disulfide bonds appears to be essential to the deformability of heat-induced whey protein gels.