Food Sci. Biotechnol. Vol. 14, No. 1, pp. 152 ~ 163 (2005) 152 Mechanism of Lipid Peroxidation in Meat and Meat Products -A Review B. Min and D. U. Ahn Department of Animal Science, Iowa State University, Ames, IA 50011 Abstract Lipid peroxidation is a primary cause of quality deterioration in meat and meat products. Free radical chain reaction is the mechanism of lipid peroxidation and reactive oxygen species (ROS) such as hydroxyl radical and hydroperoxyl radical are the major initiators of the chain reaction. Lipid peroxyl radical and alkoxyl radical formed from the initial reactions are also capable of abstracting a hydrogen atom from lipid molecules to initiate the chain reaction and propagating the chain reaction. Much attention has been paid to the role of iron as a primary catalyst of lipid peroxidation. Especially, heme proteins such as myoglobin and hemoglobin and “free” iron have been regarded as major catalysts for initiation, and iron-oxygen complexes (ferryl and perferryl radical) are even considered as initiators of lipid peroxidation in meat and meat products. Yet, which iron type and how iron is involved in lipid peroxidation in meat are still debatable. This review is focused on the potential roles of ROS and iron as primary initiators and a major catalyst, respectively, on the development of lipid peroxidation in meat and meat products. Effects of various other factors such as meat species, muscle type, fat content, oxygen availability, cooking, storage temperature, the presence of salt that affect lipid peroxidation in meat and meat products are also discussed. Keywords: Lipid peroxidation, mechanism, reactive oxygen species, catalyst, meat Introduction Consumer concerns on the quality of meat and meat products have greatly increased during past decades. “Quality” and “healthfulness” were reported to be one of the most important factors for influencing consumers choice for foods (1). Three sensory quality characteristics appearance/color, texture, and flavor are the main quality attributes that affect consumer acceptance of meat, and lipid peroxidation is the primary cause of these quality deteriorations in meat and meat products (2). Lipid peroxidation primarily occurs through a free radical chain reaction, and oxygen is the most important factor on the development of lipid peroxidation in meat (3, 4). Theoretically, oxygen molecule and polyunsaturated fatty acid (PUFA) cannot interact with each other because of thermodynamic constraints. Ground state oxygen does not have strong enough reactivity, but can be converted to reactive oxygen species (ROS) such as hydroxyl radical ( OH), superoxide anion (O 2 - ), hydrogen peroxide (H 2 O 2 ), hydroperoxyl radical (HO 2 ), lipid peroxyl radical (LOO ), alkoxyl radical (LO ), iron-oxygen complexes (ferryl- and perferryl radical) and singlet oxygen ( 1 O 2 ), some of which are highly reactive to initiate lipid peroxidation. In addition, numerous agents such as enzymes and transition metals can directly or indirectly catalyze these oxidative processes through enzymic and nonenzymic mechanisms. Especially, iron plays a critical role in lipid peroxidation process as a major catalyst. Many comprehensive reviews on the mechanism of lipid peroxidation in muscle foods, including the major initiators and catalysts for the oxidative process (5-8), have been published. This review is focused on the potential roles of reactive oxygen species (ROS) and iron as primary initiators and a major catalyst, respectively, on the development of lipid peroxidation in meat and meat products. Mechanism of lipid peroxidation Lipid peroxidation is a free radical chain reaction that is comprised of three primary steps: initiation, propagation, and termination. Initiation of lipid peroxidation takes place by attack of any species that has sufficient reactivity to abstract a labile hydrogen atom from a methylene group in lipid molecules (LH) to form lipid radicals (L ) (Equation 1). LH + Initiator +L + InitiatorH (reduced form) (Equation 1) Wagner et al. (9) reported that the amount of lipid radical generated increased with the total number of bis-allylic carbons, and suggested that the number of bis-allylic carbons in lipid molecules determines their susceptibility to lipid peroxidation. More importantly, the rate of lipid peroxidation exponentially increased with the number of bis-allylic carbons although lipid chain length had no relationship with the rate of radical formation. The differences in the initiation rate of lipid peroxidation are closely related to the dissociation energies of various carbon-hydrogen (C-H) bonds in fatty acid chains. The weakest C-H bond is at bis-allylic position, whose bond energy is 75-80 kcal/mol, and those at allylic position and alkyl C-H bond are ≈ 88 kcal/mol and ≈ 101 kcal/mol, respectively (10, 11). Consequently, the C-H bond at the bis-allylic position is the most reactive site for hydrogen abstraction. The well-known species capable of abstracting hydrogen atom are ROS, especially OH. Koppenol (10) estimated that the reduction potential of PUFA radical/ PUFA couple was +0.60 V at neutral pH, suggesting that PUFA could be readily oxidized by OH (+2.31 V) as well as other ROS. The abstraction of hydrogen atom (H ) from lipid chain *Corresponding author: Tel: 515-294-6595; Fax: 515-294-9143 E-mail: duahn@iastate.edu Received August 24, 2004; accepted October 12, 2004 MINIREVIEW The Korean Society of Food Science and Technology