Interactions at the interface between hydrophobic and hydrophilic emulsifiers: Polyglycerol polyricinoleate (PGPR) and milk proteins, studied by drop shape tensiometry

The equilibrium interfacial tension and dilational elasticity at the soy oil–water interface were studied in the presence of a lipophilic emulsifier, polyglycerol polyricinoleate (PGPR), in the continuous oil phase, and dairy proteins, β-lactoglobulin (β-lg) or sodium caseinate, in the aqueous phase using drop shape tensiometry. The interfacial tension decreased with increasing PGPR concentration, and was <2 mN m−1 at PGPR concentrations beyond 1%. The presence of proteins in the water phase, β-lg and sodium caseinate, further reduced the interfacial tension. Even at the low concentrations (0.008%) tested, PGPR dominated the interfacial elasticity, which was only slightly affected by the addition of elevated levels of protein. While in the presence of β-lg (0.1%) in isolation, the system showed a high interfacial elasticity, the addition of PGPR lowered the elasticity, suggesting that PGPR interfered with protein–protein interactions at the interface, or caused displacement of β-lg. Interfacial elasticity at the oil–water interface showed little dependence on dilation or frequency of the sinusoidal oscillation, when the interface was dominated by PGPR.