Propagation lipid oxidation

The often conflicting reports in the literature indicate that more research is needed to clarify the role of interacting enzyme systems that control the generation and survival of active forms of oxygen and their involvement in the initiation and propagation of lipid oxidation in milk. 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

The inhibition of lipid (LH) oxidation may be considered as one of the most important chemical reaction mechanisms that could explain the antioxidant function of flavonoids. In general terms, chain-breaking antioxidants (AH) inhibit or retard lipid oxidation (reactions 1-7) by interfering with initiation [generically represented by reaction 1] or with chain propagating reactions (reactions 2 and 3) by readily donating hydrogen atoms to lipid peroxyl radicals (LOO ) or lipid radicals (L ) (reactions 4 and 5) [Frankel, 1998] ... 

Part of the problem stems from considering lipid oxidation as precisely following classic free radical chain reactions. To be sure, lipids do oxidize by a radical chain mechanism, and they show initiation, propagation, and termination stages... 

Figure 1. Classic free radical chain reaction mechanism of lipid oxidation with propagation by a series of hydrogen abstractions.

Citation of the classic chain reaction for lipid oxidation persists even though, as product analysis and studies of mechanisms have become more sophisticated, there is now considerable evidence that only Reactions 1, 2, and 5 (and perhaps also 6) of Figure 1 are always present. Research has shown that, although hydrogen abstraction ultimately occurs, it is not always the major fate of the initial peroxyl or alkoxyl radicals. Indeed, lipid alcohols from H abstraction are relatively minor products of lipid oxidation. There are many competing alternative reactions for LOO and LO that propagate the radical chain but lead to different kinetics and different products than expected from the classic reaction sequence (5, 6, 21). A more detailed consideration of each stage shows how this basic radical chain sequence portrays only a small part of the lipid oxidation process and products, and a new overall reaction scheme for lipid oxidation is needed. 

It is important to recognize that scission does not necessarily stop after reaction of initial alkoxyl radicals. Scissions of secondary products generated during lipid oxidation also contribute to propagation and to the ultimate product mix (346). Malonaldehyde is perhaps the best known example of this, as will be discussed further in Section 4.2. 

At the risk of being redundant, let me summarize conditions that shift chain propagation mechanisms in lipid oxidation ... 

Peroxyl Radicals Secondary peroxyl radicals, as are found in most lipid acyl chains, recombine rapidly (2k = 10 -10 M s ) (192, 362) to form a variety of products, including alcohols and ketones (Reaction 67) (361, 362, 366), ketones and alkanes (Reaction 68) (60, 292), or acyl peroxides and peroxyl radicals (Reaction 69) (264, 367, 369). The alcohols thus produced are indistinguishable from H abstraction products of an original LO, but the ketones and dialkyl peroxides are unique to recombination reactions. As any R3OO and RO released from Reaction 68 or Reaction 69a react further, peroxyl radical recombinations also have the potential for propagating lipid oxidation (Section 3.1.4). 

Therefore, a new integrated paradigm for lipid oxidation is proposed in which the major alternative pathways are added to the classic free radical chain (Figure 15). The traditional reaction sequence involving hydrogen abstractions is presented vertically down the center of the scheme because most radicals formed in alternative reactions ultimately abstract hydrogens to propagate the chain. This is the core of the oxidation process. Pathways that compete with H abstraction are... 

Lipid oxidation in both food systems and biological tissues exhibit the same temporal three-stage pattern of initiation, propagation, and termination... 

Effects of ionizing radiation on lipid molecules have been understood by studying model systems which are simpler than the real biological membranes, such as PUFA micelles and liposomes. The formation of lipid oxidative modifications of PUFAs appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products. Kinetic data, such... 

Lipid oxidation is an autocatalysed free-radical chain reaction which is normally divided into three phases initiation, propagation and termination (Figure 3.33). 

Figure 1. lipid oxidation with an initiation phase, propagation phase and icrmina-tion phase. 

The kinetic stability of a radical is largely controlled by steric factors. When the radical center is crowded, the radical becomes less reactive and persists longer under normal conditions (it has a longer life-time). Aromatic compounds that can form allylic radicals show similar benzylic stabilization. If the radical center is sterically crowded by bulky tertiary butyl substituents, the allylic radical intermediates formed by hydrogen transfer have kinetic stability that imparts important antioxidant properties (see Chapter 9). Thus, when phenolic compounds contain three bulky tertiary butyl substituents, they form persistent radicals after hydrogen donation and inhibit lipid oxidation by intermpting the propagation of free radicals (see Chapter 9). 

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