Low molecular weight (LMW) iron is present in most food in traces or significant amounts. Upon contact with unsaturated lipids LMW iron acts as a prooxidant. This creates oxidative stability problems for products containing marine polyunsaturated lipids. In this work, LMW iron-mediated oxidation in fish oil emulsions stabilized with phospholipids and Tween and in liposomes made from phospholipids was studied. Marine and non-marine sources of phospholipids were used. The aim was to evaluate how physicochemical factors and lipid properties affect the prooxidant activity of LMW iron. The oxidation was followed by measuring the rate of the dissolved oxygen consumption by fatty acids. Red-ox cycling of iron (Fe3+/Fe2+) aided by decomposition of the pre-formed lipid hydroperoxides was the major prooxidation mechanism in both emulsions and liposomes. The elimination of the pre-formed lipid hydroperoxides prevented LMW iron-mediated oxidation. The oxygen uptake rates were highest for various liposomes/emulsions at pH 4–5 making this an optimum pH for iron activity. The pro-oxidant effect of iron was reduced by using less unsaturated phospholipids, specific amounts of emulsifiers, a presence of chloride anions, or xanthan gum. This work suggests that the iron ions are tightly associated with the phosphate groups within the phospholipids heads. The measurement of oxygen consumption is a good tool for systematic oxidation studies in emulsions and liposomes, and may be useful for assessment of optimal conditions for reduction of LMW iron-mediated oxidation in emulsion-like systems.
Practical applications: For a successful addition of marine polyunsaturated lipids into processed food, it is important to understand the pro-oxidation mechanisms of ubiquitous LMW iron, as well as how physicochemical conditions affect the pro-oxidative activity or LMW iron. This work addresses these issues in emulsions and liposomes and the knowledge may help to characterize effective hurdles for pro-oxidant activity of LMW iron. There is a need for quick and inexpensive oxidation assays in order to be able to screen and model the effects of different factors on the oxidative stability of lipids. The measurement of dissolved oxygen in emulsions and liposomes has been used in this study for evaluating the effect of the different psychochemical and chemical factors on iron-mediated oxidation. This method has a potential as a tool for assessment of optimal conditions for prevention of LMW iron-mediated lipid oxidation in emulsion type systems and understanding the possible mechanisms.