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Strengthening interfacial whey protein films by conjugation with gellan
Whey protein can be used as an emulsifier that forms nanometer-thick rigid layers at hydrophobic-hydrophilic interfaces in dispersed systems and provides stability against coalescence of the dispersed phase. If another surfactant is added, however, whey protein will be displaced competitively from the interface, leading to a loss of stability of the dispersed system. Gellan is a network-forming polysaccharide widely used in the food industry. Whey protein-gellan conjugates are expected to show enhanced resistance against surfactant-induced competitive displacement because gellan is considered to form additional networks at the interface. The objectives of this study were to conjugate whey protein either covalently or electrostatically with gellan and to investigate the effect of the different conjugation methods on interfacial structure and resistance to the surfactant-induced competitive displacement from the interface.
Whey protein was conjugated either covalently or electrostatically with gellan. The conjugate was dissolved in water and spread on the air-water interface formed on a Langmuir trough. Food-grade nonionic surfactant Tween 20 was then injected into the aqueous phase to induce competitive displacement. Langmuir-Blodgett interfacial films were sampled at pre-specified surface pressures by dipping a freshly cleaved mica sheet in and out through the interface. The interfacial films thus transferred on the mica surface were imaged using atomic force microscopy.
Both covalent and electrostatic conjugates formed close-packed interfacial films at the air-water interface. String-like structures of gellan attached to globular protein molecules were also evident. The thickness and surface pressure of the interfacial films were approximately 0.2-0.4 nm and 8-12 mN/m, respectively. Upon the addition of Tween 20, the surface pressure increased further due to the adsorption of the surfactant to the interface. Nanometer-sized surfactant domains first appeared at a surface pressure around 20 mN/m, and expanded their areas with increasing surface pressure. The thickness of protein domains increased with increasing surface pressure, consistent with the previously proposed orogenic displacement mechanism. At a certain surface pressure (e.g., 23 mN/m), the covalent conjugate occupied a larger interfacial area (83%) than both electrostatic conjugate (74%) and the WPI control (61%), The interfacial area occupied by the covalent conjugate displaced from interface less rapidly than electrostatic conjugate demonstrating a more resistant interfacial structure.
These results suggest that covalent conjugation of whey protein with gellan is a more effective approach than electrostatic conjugation in strengthening interfacial protein layers and enhancing their resistance against surfactant-induced competitive displacement from the interface.
Keywords: whey protein, gellan, interface