Network formation and viscoelastic properties of commercial soy protein dispersions: effect of heat treatment, pH and calcium ions

Heat treatments, varying levels of protein, ionic strength and pH, in addition to the molecular interactions among food processing, which affect certain physico-chemical properties in resulting gels of commercial soy isolates (> 90% protein) are discussed. Calorimetric measurements were used to describe a partially denatured molecular order and conclude that temperature and the heating rate of temperature are expected to correlate strongly to the macromolecular conformational changes.

Structural characterisation with small deformation rheological measurements (elastic modulus, tan δ, creep/compliance) was performed for a series of soy protein with the temperature treatments ranging from 30 to 90°C, in an attempt to examine the mechanisms involved. Aggregate gels were observed at lower temperatures (up to 60 ° C preheat treatment) with maximum network rigidity, which is associated with the different degrees of ‘pregelation’ stages during the production process. ‘Disaggregation’ at higher temperatures treatments weakened the molecular association and produced more elastic, fine-stranded gels.

Essentially, disulphide linkages on heated networks and mainly non-covalent physical forces of attraction in their cooled counterparts, contributed to the nature of gels' behaviour, while the involvement of hydrophobic interaction was negligible. Lower concentrations were more sensitive to temperature factor.

Similar conclusions on the structure and rigidity were also observed from long-time creep experiments. Both covalent and non-covalent topological interactions, which are involved in the heated (90 °C) network stabilisation, contributed proportionally with concentration to the viscoelastic constants from a six-element mechanical model.

Heated protein aqueous preparations with urea, a structure perturbing agent, produced entropically elastic networks, with substantial homogeneity, but still significant enthalpic contribution was observed. Textural changes due to sensitivity on ionic strength and pH were investigated for different soy protein concentrations, using texture analysis compression measurements.

The presence of salt strengthened the interaggregate forces, promoted aggregation and, therefore, stabilised the structure of partially denatured soy protein gels.