Interaction of phytochemicals with dairy ingredients

Abstract

The primary objective of this thesis is to evaluate the interaction of milk protein/polyphenolic complexes and phospholipid membranes. Milk-proteins (casein, whey protein concentrate- WPC or milk protein concentrate-MPC) and polyphenols were either introduced when the liposomes were fabricated or mixed with preformed liposomes. A third combination was also used where polyphenol was incorporated into liposomes and milk proteins were mixed. The challenge of incorporating plant polyphenols into dairy foods is to retain their antioxidant activities. These polyphenols inhibit lipid oxidation in cellular membrane surfaces, although the mechanism of this inhibition is not entirely clear. There is significance of having them together. Polyphenols have significant binding affinity to proteins, which can lead to the formation of soluble and insoluble protein-polyphenol complexes. In the case of the milk proteins, such as casein, whey protein concentrate and milk protein concentrate, of interest here the capacity of inhibition of lipid oxidation in the membrane is significantly enhanced for the polyphenols in the presence of the proteins as assessed by lipid peroxidation inhibition capacity assays (LPIC). Thus the antioxidant action appears to involve these protein/polyphenol complexes, as well as direct antioxidant action by the polyphenol and some milk proteins. But it is not clear whether the interaction between the protein and the antioxidant is located on or near the membrane surface, or whether complexes are dispersed in bulk solution generally. However, the protection occurs for the phospholipids in the membrane in either case. Using small angle X-ray scattering (SAXS) allowed the nature and structure of the three-way interactions to be measured, depending on the interaction conditions, which have been chosen based on functional assays. These screening measurements formed the basis for neutron measurements - small angle neutron scattering (SANS). Polyphenolic antioxidants interact strongly with phospholipid membranes, causing perturbation of the lipid organisation, while their antioxidative ability is enhanced in the presence of milk proteins. The antioxidant perturbs the phospholipid packing, and that its primary location appears to be near the tail/head groups' interface of the phospholipids. With the increase in concentration, polyphenolic molecules travelled towards the inner part of bilayer. In such a position, and lacking electrostatic interactions, the interaction with water-soluble proteins is expected to occur through protein binding to the membrane surface. This knowledge of the nature and location of the protein/antioxidant interactions will shed light on the mechanism of the antioxidant ability and augmentation in these systems, which may be related to the development of functional food systems.