On the link between surface rheology and foam disproportionation in mixed hydrophobin HFBII and whey protein systems

Here we study the surface dilational properties of spread and adsorbed layers of whey protein isolate (WPI) and hydrophobin HFBII at air/water interface using Langmuir trough and relate them to foam ability and stability. In spread and adsorbed systems, a gradual increase in modulus with the fraction of HFBII on the surface or in the bulk is observed and we can identify distinct regimes of WPI-dominated and HFBII-dominated behaviour. The dominance of HFBII is further substantiated by visual observation of microscopic wrinkles appearing on the surface of the trough. When comparing spread to adsorbed systems, it was found that a higher HFBII fraction is needed to obtain HFBII dominant behaviour in spread layers than in case of adsorbing layers (fHFBII = 0.6 and 0.2 respectively). Furthermore, our results indicate that the HFBII-contribution to the interfacial behaviour becomes more pronounced upon sequential large scale compression/expansion cycles. In order to explain this non-trivial behaviour, we propose that there is multi-layer formation at the interface, having a top layer enriched in HFBII and bottom layer enriched in WPI. It is also concluded that coarsening stability in foams corresponds more closely to the surface dilational properties measured in adsorbing layers than those in spread layers. Finally, it was observed that in mixed systems of HFBII and WPI, the coarsening process levels off, which corresponds to the increased dominance of HFBII in mixed WPI:HFBII-layers upon large surface deformation that is occurring at the surface of shrinking bubbles.