Chelation lipid oxidation

Over the years, scientists have used many types of antioxidants in a variety of foods to retard or inhibit lipid oxidation and, thus, increase shelf-life and preserve quality. These antioxidants include free radical scavengers, chelators, and oxygen absorbers. While there are numerous antioxidants available to food scientists, the objective of this report was to discuss several of these antioxidants as they relate to meat flavor quality research and to show how they were used to retard lipid oxidation and prevent meat flavor deterioration in ground beef patties. 

Secondary antioxidants, i.e., sequestrants or chelators, are important compounds in the prevention of lipid oxidation. The effect of chelators tested varied with the different compounds. Of those chelators tested, ethylenediaminetetraacetic acid (EDTA, tetrasodium salt) and sodium phytate were the most effective inhibitors of lipid oxidation (so indicated by low hexanal and TEARS values) and MFD (as seen by high CBB and low PIT and CBD intensity values), see Table 5. Sodium phytate was previously shown to chelate iron and thus, was proposed as a food antioxidant(7J). Sodium citrate at a concentration of 500... 

The other important property affecting lipid oxidation is the chelating effect of chlorogenic acids. It is important to keep in mind that the influence of biometals (Fe, Cu etc.) on lipid free radical oxidation is essential. It is well known that iron can react with hydrogen peroxide by the Fenton reaction (Equation 3). The hydroxyl radical formed in the Fenton reaction is capable of reacting with lipid and PUFA as the initiation stage. Iron can also participate in alkyl peroxide or lipid peroxide decomposition. Therefore, the nature of iron chelation in a biological system is an important aspect in disease prevention. 

Application of data obtained from simple clean reaction systems in biological or chemical studies of heme catalysis also has its problems. Chemical model systems use chelators, model hemes, and substrate structures that are quite different from those existing in foods. Reaction sequences change with heme, substrate, solvent, and reaction conditions. Intermediates are often difficult to detect (141), and derivations of mechanisms by measuring products and product distributions downstream can lead to erroneous or incomplete conclusions. It is no surprise, then, that there remains considerable controversy over heme catalysis mechanisms. Furthermore, mechanisms determined in these defined model systems with reaction times of seconds to minutes may or may not be relevant to lipid oxidation being measured in the complex matrices of foods stored for days or weeks under conditions where phospholipids, fatty acid composition, heme state, and postmortem chemistry complicate the oxidation once it is started (142). Hence, the mechanisms outlined below should be viewed as guides rather than absolutes. More research should be focused on determining, by kinetic and product analyses, which reactions actually occur and are of practical importance in specific food systems. 

Several proteins that exist in food (e.g., lactoferrin, ferritin, transferritin, heme protein) possess strong binding sites for iron. Reducing agents (ascorbate, cysteine, superoxide anion) to low pH causes release of iron from proteins and accelerates lipid oxidation (34). Some amino acids and peptides found in muscle foods (e.g., carnosine) are capable of chelating metal ions and inhibit their prooxidant activity (35, 36). 

In addition to influencing the rate of lipid oxidation via activation of thiol groups and metallo-enzymes, heating milk may also affect oxidation via redistribution of Cu (which migrates to the FGM on heating) and possibly by the formation of Maillard browning products, some of which have metal chelating and antioxidant properties. 

MA, a three carbon dialdehyde, can experience a number of configurational modifications as discussed by Kwon and Watts (6). Enolization of the diketo form may take place. The enolic tautomer may further undergo molecular rearrangement into its open cis-, open trans-, or chelated forms. At pH 3 or lower, MA is chelated and exists as 3 hydroxy-acrolein above pH 6.5 MA is completely dissociated and exists as an enolate anion. Between pH 3 and 6.5, MA is an equilibrium mixture of enolate anion and chelated forms. MA (also malondialdehyde), is one of the main secondary products of lipid oxidation. It forms a pink color by condensing with 2 moles of TBA (7). 

Various proteins inhibit lipid oxidation in different lipid systems by their capacity to bind or chelate metal ions. The relative antioxidant potency of phenolic compounds in liposomes was mediated to different extents by the presence of protein. At high relative metal concentrations, however, these complexes can also promote lipid oxidation (see Chapter 10.B.2). 

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