Gelation of oil-in-water emulsions stabilized by heat-denatured and nanofibrillated whey proteins through ion bridging or citric acid-mediated cross-linking

This work addresses the fabrication and characterization of gels from emulsions stabilized by heat-denatured and nanofibrillated whey protein isolate (WPI) using citric acid and CaCl₂ as cold-gelling agents. Nano-thick fibrils and wormlike aggregates were produced by heating of WPI at pH 2.0 and 8.0, respectively. Fibrillated WPI, heat-denatured WPI, and their mixture at a concentration of 65 mg mL^-1 and pH 8.0 were used to stabilize oil-in-water emulsions. Emulsions stabilized by fibrillated WPI showed a higher creaming stability than samples stabilized by heat-denatured and mixed fibrillated/heat-denatured WPI. However, they were more polydisperse and had larger droplets. Gelation of emulsions by citric acid through the generation of covalent bonds was confirmed by OPA colorimetric method and FT-IR spectroscopy. Emulsion gels prepared by citric acid-mediated cross-linking were firmer than those formed by CaCl₂ and also showed a higher ability to hold water and retain oil. Scanning electron microscopy also demonstrated a more open and porous structure for ion-induced emulsion gels compared to citric acid-induced counterparts. Generally, the results indicated that the citric acid as a new gelling agent and different forms of WPI aggregates can be used in the formulation of edible emulsion gels with modifiable functional properties.