Investigations on protein-lipid interactions under oxidative conditions

Oxidative reactions in food systems during processing and storage constitute a significant problem that determines the nutritional and sensory qualities of the food product as well as the textural and functional properties. Proteins and lipids, as essential components of foods, are highly prone to oxidative degradation that results in undesired modifications in food systems. Although lipid oxidation as a topic has been given a widespread attention, protein oxidation and its consequences in foods have been studied relatively recently. In particular, co-oxidation of food proteins and lipids, in terms of their interactions within the complex mechanism of oxidative reactions, has been gathering interest only lately. The behavior of proteins from various food sources and technological pre-treatments as well as the outcomes of this behavior under oxidative conditions in the presence of lipids is a much required subject on which to focus both by academia and industry. This study investigated the oxidative modifications taking place in food proteins and lipids as well as their consequences in model systems. Firstly, molecular level changes between oxidized lipid product malondialdehyde (MDA) and β-lactoglobulin (β-Lg) peptides were characterized via utilization of LC-MS/MS techniques. The results showed that the main reactions occurred as formation of two Schiff base adducts between MDA and peptides located at either N-terminus amino groups or side-chains of amino acids. These adducts were identified as enaminaland dihydropyridine-type derivatives that were observed with +54 and +134 Da mass increments of native peptides, respectively. Following studies focused on emulsion model systems that were stabilized by plant proteins in varying compositions. Emulsions were stored at different temperatures during which oxidation of lipids and proteins was monitored. Emulsions prepared with quinoa and amaranth proteins were compared to Tween®20-stabilized emulsions. Quinoa protein-emulsions showed the least oxidative and physical stability due to rapid protein degradation while oxidation of amaranth proteins proceeded in a slower fashion. Tween®20-stabilized emulsions exhibited higher oxidative and physical stability compared to protein-stabilized emulsions. Final model study focused on the oxidation of continuous and interfacial proteins in emulsions prepared with faba bean proteins that had undergone microwave (MWT) and conventional thermal treatment (CTT) in order to inhibit native lipoxygenase enzyme activity. Enzymatic oxidation pathway had a profound effect on the extent of oxidation. Emulsions prepared with CTT proteins exhibited higher stability than MWT protein-stabilized emulsions. Furthermore, degradation of interfacial proteins was more emphasized than those located in the aqueous phase in both MWT and CTT emulsions. In contrast, emulsions prepared with proteins from untreated faba beans contained highly oxidized aqueous phase proteins due to extensive enzymatic oxidation. This thesis demonstrates that interdependent relations between proteins and lipids such as adduct formation, free-radical transfer, and reactions between oxidized species have a significant effect on the overall course of oxidation of the food system which affect the observed modifications. Therefore, customized solutions against oxidation should regard the intricate relations of protein-lipid co-oxidation in a food system that contains proteins and lipids as major constituents.