Towards tailoring the viscoelasticity of liquid-liquid interfaces in emulsions: understanding phospholipid-protein interactions at the oil-water interface

In the food industry, oil-water emulsions often contain a mixture of phospholipids (PL) and proteins such as β-lactoglobulin (β-LG), resulting in either the displacement of β-LG by PL on the interface or co-existence driven by β-LG + PL interactions. The PL's molecular structure (headgroup, fatty acyl chain), as well as the system's pH and temperature, impact the extent of intermolecular PL-PL interactions. Differences in β-LG + PL interactions as a function of these parameters are also expected.

This study aimed to analyse the effects of the molecular structure of PL on the interaction with β-LG at the oil-water interface, taking temperature cycles and the system's pH into account. PL with varying headgroups (choline PC, ethanolamine PE) and fatty acyl chain (FA; C18:0, C18:1) were used. The interfacial rheological properties at pH 3.5 and 6.5 were investigated within and outside the linear viscoelastic regime via dilatational and interfacial shear rheological measurements. Possible β-LG + PL interactions were tested in bulk via FTIR measurements.

In the case of β-LG + saturated PL, an increase in the storage modulus was measured, while the interface behaved predominated viscous in the case of β-LG + unsaturated PL. It is, thus, assumed that the unsaturated PL fully displaced the protein from the interface while the saturated PL co-exist with β-LG, allowing β-LG + PL interactions to occur. In both dilatation and shear rheology, the PE 18:0 + β-LG (small headgroup, saturated FA) initially showed the strongest interface, possibly due to the formation of a crystalline PE:18:0 sublayer on the interface during the cooling step. The storage modulus increased further with decreasing pH due to attractive interactions between β-LG and PL's charged headgroup.