Surface dilatational and foaming properties of whey protein and escin mixtures

Saponins are naturally occurring components in plants, which have promising interface stabilising properties. In this work, we will focus on the saponin type called escin, which is known to form very strong viscoelastic interfacial layers at the air-water interface. The ability to form stiff interfaces can be advantageous for stabilising foams. The interfacial properties of escin were studied in mixtures with whey protein isolate (WPI). Understanding multicomponent systems is crucial in designing food products, containing both proteins and saponins. Pure WPI and escin, and WPI-escin mixtures at 7:1, 3:1, 1:1, and 1:3 (w:w, WPI:escin) ratio were evaluated for their interfacial properties using large amplitude oscillatory dilatation (LAOD). The responses to the deformations were highly non-linear and could only be assessed by plotting Lissajous plots of the surface stress signal versus deformation. Escin appeared to co-adsorb with WPI, leading to mixed interfaces. A small addition of escin, such as in the 7:1 or 3:1 (WPI:escin) ratios, already resulted in substantially stiffer interfacial layers compared to pure WPI. At higher escin content (1:1 and 1:3), escin completely dictated the mechanical properties of the interfaces, which resulted in stiff and glassy-like systems. The stiff interfacial layers of escin resulted in about three times more stable foams compared to pure WPI. We also observed higher foam stability for mixtures with higher escin contents. Escin is a promising natural and plant-based foaming ingredient, which can be mixed with whey proteins to create stable aerated systems.