Stability and rheology of protein foams: contribution of interfacial properties, involvement of film relaxation dynamics

International audienceThe stability and flow properties of surfactant-stabilised aqueous foams are keys to their use invarious application contexts. The main destabilisation processes in aqueous foams are liquiddrainage, coalescence and disproportionation. They behave as yield-stress fluids, in which flowleads to film-scale topological rearrangements known as T1s.Still, the contribution to these features of the properties imparted by the surfactants to the air-liquid interface is far from fully understood.Proteins are amphiphilic macromolecules. They lower the air-water surface tension, as small-molecular-weight surfactants do, but in contrast with the latter, they adsorb irreversibly to theinterface and give specific interfacial visco-elasticity.In order to correlate the stability and rheology of protein foams to the properties of interfaces,we adopted a multi-scale approach combining the interfacial rheology, the dynamics of foamfilms after T1 topological rearrangements, and macroscopic foam characterisations: the foamstability against drainage was evaluated by following the evolution of the liquid fraction as afunction of both time and height in the foam column[1], and the foam complex modulus and yieldstress were measured under oscillatory shear. We investigated the behaviour of dairy proteins(whey protein isolate or purified β-lactoglobulin), either in the native state or after modificationby dry-heating and/or pH adjustment prior to dehydration, to vary interfacial properties.Our results show that small-extent structural modifications of proteins had a dramatic impact oninterfacial rheology, liquid film dynamics, foam stability and foam rheology.This approach, correlating multiple investigation scales, sheds light on the contribution of theinterfacial rheology to protein foam properties, in particular through the involvement of filmrelaxation dynamics