Controllable Viscoelastic Properties of Whey Protein-Based Emulsion Gels by Combined Cross-Linking with Calcium Ions and Cinnamaldehyde

Whey protein-based emulsion gels were fabricated by combined covalent cross-linking with cinnamaldehyde and ionic cross-linking with calcium ions. The structural and rheological properties of these emulsion gels were characterized by microscopy, macrorheometry, and microrheometry. Soft to hard emulsion gels could be fabricated by altering the Ca2+ level. However, water separation occurred in the emulsion gels at high calcium levels, indicating that an appropriate balance of protein cross-linking was essential for a good functional performance of the emulsion gels. At a constant calcium ion level, cross-linking with cinnamaldehyde softened the texture of the emulsion gels and reduced the level of water separation. Thermal treatment could be used to strengthen the interaction between the whey proteins and calcium ions, presumably due to protein unfolding and aggregation. Rheology demonstrated that cross-linking reactions occurred between the whey protein and cinnamaldehyde at the oil-water interface, which decreased the viscosity of the emulsion gels, but increased their viscoelasticity. Microstructural changes observed by fluorescence microscopy were in agreement with the rheology results. Scanning electronic microscopy showed that the microstructure of the emulsion gels was strongly impacted by the presence of cinnamaldehyde, which led to a more uniform and smaller pore size. The gastrointestinal fate of the emulsion gels was determined using a simulated gastrointestinal tract model. The oral and gastric processing of the emulsion gels was strongly influenced by cinnamaldehyde cross-linking, with less aggregation being observed. The information obtained in this study may facilitate the development of more innovative protein-based products with novel functional attributes for use in foods and other applications.