Dynamic phenomena in caseinate–monoglyceride mixed films at the air–water interface

Abstract

The aim of this work is to perform a comparative study of the long-term relaxation phenomena and surface dilatational characteristics in protein and monoglyceride (monopalmitin, monoolein and monolaurin) mixed monolayers spread at the air–water interface as a function of processing variables (surface composition, surface pressure and temperature) and aqueous phase pH. The study has been centred on a real protein (caseinate), to see if the relaxation phenomena observed in protein–monoglyceride mixed films with a model protein (β-casein) are generic. Different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial dilatational rheology) have been used to analyse the static (structure, reflectivity, miscibility, and interactions) and dynamic characteristics (long-term relaxation phenomena and surface dilatational properties) of protein–monoglyceride mixed films spread on the air–water interface. The static and dynamic characteristics of the mixed films depend on the protein–monoglyceride ratio and the surface pressure. At higher surface pressures, collapsed protein residues may be displaced from the interface by monoglyceride molecules with important repercussions on the interfacial characteristics of the mixed films. From the frequency dependence of the surface dilatational modulus we have deduced the relationships between interfacial dilatational rheology and changes in molecular structure, interactions, miscibility, and relaxation phenomena.