Modelling the colloidal behaviour of food systems in the presence of fragmented proteins/macromolecules: A Self-Consistent Field approach

This thesis presents a theoretical examination of the possibility that the fragments of a protein may provide better colloidal stability than the intact protein itself in the case of αs1-casein. It more generally considers the surface adsorption behaviour of fragmented proteins. In colloidal systems the polymers are mostly present as polydisperse entities. Polydispersity can either be naturally present or be the result of fragmentation, as happens for food proteins during enzymatic modification. Majority of proteins do not possess the most optimum primary structure expected of an ideal colloidal stabiliser. More desirable surface functionality maybe achieved by hydrolysis of edible proteins. For the theoretical examination of this argument we had to extend and develop a new Self Consistent Field (SCF) approach which also had to be validated first. Although this new approach is an extension of the traditional SCF approach, it is capable of modelling highly polydisperse systems in a manner not currently possible with the more usual technique. In this preliminary work we present the results of our method for both homopolymers and proteins. In the homopolymer case, we investigate how the preferential adsorption of homopolymer fragments is influenced by various parameters such as solution concentration, degree of hydrolysis (DH), the intact size of the original homopolymer and the strength of affinity of monomers to the surface. The colloidal stabilising and surface adsorption properties of fragmented proteins were investigated taking the bovine milk protein αs1-casein as an example. The protein was fragmented by selective single bond and also non-selective multiple bond hydrolysis, assumed to be induced by the action of enzyme trypsin. The investigation was carried out at different levels of hydrolysis (DH) and various pH values. We find that the non-selective peptide bond hydrolysis in the case of αs1-casein did not provide a better colloidal stability compared to the intact αs1-casein, at none of the pH values studied here. However, it was shown that a better colloidal stability can be achieved by the selective peptide bond cleavage of particular bonds.