Competitive adsorption of macromolecules during emulsification

Abstract: The adsorption behavior of macromolecular emulsifiers is important in the understanding of their functionality. Understanding of the adsorption behavior can ultimately enable us to control emulsion properties. Adsorption during emulsification differs from many adsorption processes as the interface at which adsorption occurs is created in parallel with the adsorption. In this thesis two different main types of competitive macromolecular adsorption during emulsification are illustrated, kinetically controlled and affinity controlled. Kinetically controlled adsorption is illustrated with OSA-starch which is an amphiphilic and polydisperse macromolecule with ultra-high molar mass. The other case is affinity controlled adsorption illustrated both by OSA-starch and egg yolk proteins. Emulsions are often formed through high-pressure homogenization. When macromolecules are used as emulsifiers this process can cause degradation of the molecules and can have a profound effect on the molecular conformation. Conformational changes induced by high-pressure homogenization were twofold. The treatment resulted in an increase in the apparent density of the molecules but also in a change in molecular shape. The change in molecular shape is described as a change in the relation between the molecular mass and hydrodynamic radius as a result of homogenization. OSA-starch proved to be a reasonably efficient emulsifier which could give rise to very high surface loads. The high surface loads were related to the relationship between bulk macromolecule concentration and surface area created during emulsification. A small interfacial area and high bulk concentration lead to very high surface loads. The high surface loads were likely to be caused by the short times allowed for adsorption during emulsification. This is likely to cause jamming and close-packing at the interface. In the turbulent flow conditions where emulsification takes place convective mass transport becomes important. Hence, another contribution to the high surface load was the sequence of adsorption of molecules which resulted in selective adsorption of high molar mass molecules present within a sample. Affinity controlled adsorption is related to the number of and distribution of groups with high affinity to the interface in the macromolecule. For OSA-starch it is suggested that the density of charges or hydrophobic groups at the surface of the molecules determined the affinity, depending on the properties of the interface. For proteins it is suggested that affinity to the oil/water interface was determined by the length of hydrophobic domains in the amino acid sequence.