The influence of digestion on the behaviour of MDA in oil-in-water emulsions

Abstract

With the increasing consumption of foods rich in PUFA (poly unsaturated fatty acids) and proteins, the interaction between lipids and proteins is of much interest. Malondialdehyde (MDA) a secondary product of lipid peroxidation has been widely studied and associated with food deterioration and deleterious physiological conditions such as heart failure, Alzheimer’s disease, cancer, atherosclerosis, ischemia-reperfusion and other immunological disorders. It is found to be harmful either in its unbound or bound form. MDA binds to proteins and amino acid residues such as lysine, histidine, asparagine and arginine to form adducts which are either stable or unstable depending on external conditions such as temperature, enzymes and pH. This research aimed at studying the influence of digestion on the stability and behaviour of MDA in the presence or absence of proteins in emulsions. To carry this out hydrogenated coconut oil-in-water (O/W) emulsions were stabilized with tween 20 or whey proteins in the presence or absence of food proteins- lysozyme or whey-, pre-incubated at 4⁰C and 40⁰C and digested at 37⁰C. Phosphate Buffer Solution (PBS) and Glycerol triheptanoate oil were used to demonstrate MDA aldol condensation and/or cleavage products are formed easily in the oil phase of emulsions and temperature has no effect on the formation of these products in aqueous phase. PBS samples of 4⁰C with addition of lysozyme or whey, showed formation of stable adducts from stomach to intestine. The use of stabilized emulsions demonstrated that the initial storage temperature influenced MDA reactivity upon digestion. Tween 20-stabilized samples of 40⁰C generally revealed more stable MDA-protein adducts formation than those of 4⁰C. A quantity of ±35% and ±45% of stable adducts remained after intestinal digestion of 40⁰C with added lysozyme and whey proteins respectively. With whey-stabilized emulsions, approximately ±26% of adducts formed remained during the course of digestion with no further formations or breakdown. Generally, the initial incubation temperature had a significant effect on the behaviour of MDA and formation of MDA-protein adducts. There was also a significant difference between stomach and intestinal digestions with respect to MDA behaviour and formation/stability of adducts. The study of MDA kinetics revealed that, the formation of aldol condensation and/or cleavage products is slower in aqueous phase of O/W emulsions with a reaction rate of -0.011 μM/h. Whey proteins bind to MDA at a faster rate and form more stable adducts with MDA (rate of -0.153μM/h) than lysozyme (0.059μM) in the aqueous phase of O/W emulsions. In tween 20-stabilized emulsions, the formation of MDA-protein adducts is faster when whey proteins are added at -0.184μM/h reaction rate than lysozyme (0.129μM/h). Proteins in tween 20 stabilized emulsions formed stable adducts at a faster rate than with whey-stabilized emulsion (reaction rate of -0.084μM/h)