Mechanical and conformational aspects of protein layers on water

Keywords: protein film, protein conformation, air/water interface, network formation, foam formation, foam stability, interfacial rheology, fracture behaviour.

The aim of this thesis was to obtain systematic information on the importance of mechanical and conformational aspects for the formation of a visco-elastic protein network at the air/water interface. Such a protein network is formed upon adsorption at the interface and is assumed to play a role in the formation and stabilisation of emulsions and foams. To understand the formation of a visco-elastic layer with specific mechanical properties, one has to study the molecular processes occurring at the interface, namely protein adsorption, conformational changes that occur upon adsorption and the interactions between the adsorbed proteins. A series of proteins was studied with a tertiary structure varying from random coil (flexible) to rigid (globular):b-casein,b-lactoglobulin, ovalbumin and (soy) glycinin. Glycinin has only been studied preliminary in the past but, being an interesting substitute for animal proteins, it was investigated quite extensively in this thesis. The conformation of glycinin was found to be pH-dependent and this change in conformation strongly affected the adsorption behaviour and rheological properties of interfacial glycinin layers. The monomeric glycinin form present at pH 3 behaved as a good foaming agent whereas at pH 6.7 (hexamer form) no foam could be formed. Infrared Reflection Absorption Spectroscopy (IRRAS) showed that only minor changes occurred in the secondary structure of a protein upon adsorption at the interface. Ovalbumin andb-lactoglobulin showed a 10% loss ofb-sheet structures whereas glycinin (pH 3) formed intermolecular anti-parallelb-sheets. The latter is an indication for interfacial aggregation. Mechanical properties were determined by deformation in shear and dilation. Upon large deformations most protein films were found to exhibit fracture behaviour. The differences observed for ovalbumin,b-lactoglobulin and glycinin indicated a transition from a more yielding behaviour to a more brittle fracture behaviour. A correlation was found between several mechanical properties of adsorbed protein films and the stability against disproportionation of foams made with the corresponding proteins. Furthermore, correlations between macroscopic film properties and molecular properties of the proteins in terms of molecular dimensions and secondary structure were studied. It was discovered that the molecular area at the onset of surface pressure per unit protein molecular weight was strongly correlated to the steady-state shear stress of a saturated protein film. This means that protein 'hardness' largely determines the film properties but a quantitative model is yet to be developed. Practical relevance of the mechanical properties of adsorbed protein layers for the stability of emulsions and foams is discussed.