Iron Catalyzed Lipid Oxidation in Emulsions and the Influence of Antioxidants
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Lipids from marine sources contain high amounts of omega-3 fatty acids which are known to have several beneficial effects on human health. Their use as food ingredients is however limited due to their high susceptibility to lipid oxidation resulting in development of rancidity. Liposomes prepared from marine phospholipids have previously been used as a model system to study lipid oxidation by measurement of oxygen consumption. It was of interest to study lipid oxidation by this method in oil-in-water emulsions which can be considered to be more similar to foods. Incorporation of antioxidants in foods is an approach to increase the stability of foods containing omega-3 fatty acids. Emulsions were used as a model system to study the influence of EDTA, citric acid, caffeic acid, propyl gallate, α-tocopherol, ascorbic acid, β-carotene and astaxanthin on iron catalyzed lipid oxidation. Crude herring oil was washed with water to remove impurities. Analyses showed that impurities including carbonyl compounds and carotenoids were removed by the washing, while the oxidation level of the oil slightly increased. The herring oil was used for preparation of emulsions with herring phospholipids as emulsifier by the use of a dispersing tool where increased dispersing time resulted in larger oil droplets with a wider size distribution.Iron catalyzed oxidation of lipids in the emulsions occurred at a lower rate than what has previously been measured in liposomes, but the initial reaction between lipid hydroperoxides and Fe2+ occurred at the same rate in the two systems. The use of soy lecithin as emulsifier inhibited oxidation of lipids in the emulsions. Interactions between iron and antioxidants had a major impact on oxidation in the emulsions. EDTA and citric acid completely inhibited the oxidation when they were added in twice the ratio to iron. Citric acid was not able to inhibit the initial reaction between lipid hydroperoxides and Fe2+ which was thought to be due to its inability to bind Fe2+. Caffeic acid and α-tocopherol enhanced the oxidation rates by reducing Fe3+ to the more catalytically active Fe2+. The prooxidative activity of caffeic acid was significantly greater than that of α-tocopherol. Caffeic acid, α-tocopherol and propyl gallate inhibited the initial reaction between lipid hydroperoxides and Fe2+ to a similar degree which was thought to be related to their free radical scavenging and metal chelating activities. Propyl gallate was also able to reduce the oxidation rates. Ascorbic acid was itself oxidized by Fe2+ and Fe3+ which resulted in increased initial consumption of oxygen, but not increased oxidation of the lipids. Ascorbic acid was able to decrease the prooxidative activity of α-tocopherol by regeneration of α-tocopherol from the α-tocopheroxyl radical. β-Carotene and astaxanthin showed only minor influences on lipid oxidation in the emulsions.