Interfacial properties of air/water interfaces stabilized by oligofructose palmitic acid esters in the presence of whey protein isolate

To study the applicability of oligofructose palmitic acid esters (OF-C16) as novel surfactants in food systems, the functional properties of OF-C16 were studied in the presence of whey protein isolate (WPI). Surface tension measurements, surface dilatational rheology, foam stability tests and Brewster angle microscopy were used to study the competitive adsorption of WPI and OF-C16 and the displacement of WPI by OF-C16. Pure WPI stabilized interfaces had a moderate surface tension (48 mN/m) and a dilatational modulus of 90 mN/m, while pure OF-C16 stabilized interfaces had a low surface tension (30 mN/m) and a dilatational modulus of 50 mN/m. The stabilization mechanisms of WPI (elastic network formation) and of OF-C16 (surface solidification) are very different, and the combined adsorption of these two components led to a structure with a much lower dilatational modulus. At the lowest WPI concentrations (0.5% and 1%), the equilibrium surface tension was similar to a pure OF-C16 stabilized interface, pointing to a low WPI surface concentration. However, apparently still sufficient WPI had adsorbed either at or just below the interface, to prevent the OF-C16 from solidifying. Despite the low moduli, the foam stability for the mixed systems was high. The interfaces were probably stabilized by the Gibbs–Marangoni mechanism. In contrast, at the highest WPI concentration (2%), the equilibrium surface concentration of WPI was sufficiently high to decrease the interfacial mobility of OF-C16, which decreased the Gibbs–Marangoni effect and resulted in decreased foam stability. Finally, OF-C16 could also displace a fully developed WPI network from the interface.