Impact of surfactant type, pH and antioxidants on the oxidation of methyl linoleate in micellar solutions Wenlei Xie * , Junmin Ji, Hongyan Wang School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450052, PR China Received 24 April 2007; accepted 14 August 2007 Abstract The oxidation of methyl linoleate micelles has been studied, aiming at elucidating the effect of various variables, including surfactant type, pH and antioxidants. The progress of the methyl linoleate oxidation was evaluated by measurement of conjugated dienes hydro- peroxides (CDHP) and malondialdehyde (MDA). It was shown that the oxidative stability of methyl linoleate micelles was influenced by surfactant type, with oxidative rate being greater in SDS micelles than in Tween 20 micelles. Besides, the methyl linoleate micelles at pH 6.8 had greater rates of lipid oxidation than their pH 3.0 counterparts. Moreover, the incorporation of VE and VC in the methyl linoleate micelles successfully slowed the formation of hydroperoxides and their subsequent decomposition product MDA. However, the antiox- idant activities of VE and VC were related to their concentrations. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Methyl linoleate; Oxidation; Micelle; Antioxidant 1. Introduction Lipid oxidation is of great concern to the food industry because it causes changes in the quality attributes of foods such as taste, texture, appearance and nutritional profile (McClements & Decker, 2000). The oxidation processes of bulk oils have been extensively studied (Porter, Caldwell, & Mills, 1995; Frankel, 1991). However, in many processed foods, lipids are found in surfactant stabilized dispersions, where oxidation reactions are influenced by factors includ- ing antioxidants, increased surface area and the presence of an interfacial membrane (Coupland & McClements, 1996). Of particular interest is the oxidation of those lipids that have polyunsaturated fatty acid constituents. These lipids are susceptible to oxidation via a chain reaction. A methy- lene group of a polyunsaturated fatty acid that is posi- tioned between cis double bonds can readily lose a hydrogen atom. The residual free radical adds oxygen to yield a peroxyl radical, which can abstract hydrogen atoms from another fatty acid molecular, producing the hydro- peroxides, and initiating another cycle of lipid oxidation. Moreover, the hydroperoxide substitution produces either trans,trans or cis,trans conjugated dienes (CDHP), which are the major intermediate products during the lipid oxida- tion. Further, these conjugated diene hydroperoxides could degrade generating various aldehydes, the major ones being malondialdehydes (MDA) (Brash & Havre, 2002). The incorporation of antioxidants in foods that contain oils and fats is effective in retarding the oxidation of lipids and lipid-containing foods. Considerable interest has long been given to the application of natural antioxidants in foods and biological systems due to their potential nutri- tional and anticarcinogenic effects. In heterogeneous food systems, the effectiveness of antioxidants is heavily depen- dent on several factors such as the polarity of antioxidants, lipid substrate, pH, concentration of antioxidants, and the antioxidant locations in the oil and water phase (Frankel, Huang, Kanner, & German, 1994; Mei, McClements, Wu, & Decker, 1998). Owing to the influence of many factors on the ability of antioxidants to inhibit lipid oxida- tion, great variations in antioxidant activities can be seen in 0963-9969/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2007.08.001 * Corresponding author. Tel./fax: +86 371 67789524. E-mail address: xwenlei@163.com (W. Xie). www.elsevier.com/locate/foodres Food Research International 40 (2007) 1270–1275