Combined effect of heat treatment and ionic strength on the functionality of whey proteins.

A 5% (wt/vol) whey protein isolate (WPI) dispersion (pH 6.5) with different concentrations of NaCl was submitted to dynamic heat treatment. Protein dispersions were characterized as to their rheological properties, particle sizes, morphology, denaturation temperatures, and protein surface hydrophobicity. At low ionic strength (<200 mmol/kg), gel elastic modulus increased and strongest gel stiffness was achieved. High salt concentrations lead to a weaker gel, whereas no gels at all were formed without salt. The gelation temperature was also influenced by ionic strength and an increase in denaturation temperature and thermal stability was also observed by using differential scanning calorimetry. Additionally, heat-induced changes in secondary structures upon salt augmentation were followed by Fourier transform infrared spectroscopy. Secondary structural elements estimations obtained from amide I assignments were correlated with those from amide III assignments. Upon salt increase, no differences in secondary structure were observed without heating, whereas upon heating and without salt increase, the Fourier transform infrared spectroscopy data revealed an increase in intermolecular β-sheets at the cost of β-turns and random coils, with no change in α-helical structures. However, NaCl addition along with dynamic heat treatment of WPI dispersion showed a stabilizing effect on the secondary structural elements of both amide I and amide III bands. Whey protein isolate dispersions in water were also characterized by transmission electron microscopy by a spherical shape with 2 populations (6 and 70 nm). Salt increase alone resulted in the formation of denser aggregates, whereas a transition from spherical/compact protein aggregates to linear ones was observed due to combined salt/heat effect. The important size of these edifices was confirmed by microscopy and light-scattering techniques. Moreover, protein surface hydrophobicity related to the number of hydrophobic sites available decreased significantly. Finally, experimental results demonstrated the strong interaction between ionic strength and dynamic thermal treatment on protein functional properties and their careful adjustment could enable the food industry to effectively use WPI as a gelling agent.