Chainbreaking Antioxidants

Control of free radicals is an effective method to prevent both autoxidation and oxidation accelerated by pro-oxidants, since free radicals are a universal reaction intermediate for all lipid oxidation reactions. Chain-breaking antioxidants inhibit free-radical reactions by scavenging radicals resulting in inactivation of the original free radical and formation of a more stable antioxidant radical (1). The effectiveness of a chain-breaking antioxidant is a function of its chemical and physical properties. For a chain-breaking antioxidant to be effective, it must possess a hydrogen, which is weakly bound to the antioxidant so that it can be freely donated to the free radical. As the bond energy of the hydrogen on the chain-breaking antioxidant decreases, the transfer of the hydrogen to the free radical is more rapid (1,2). The effectiveness of a chain-breaking antioxidant is also related to its ability to decrease radical energy so that the antioxidant radical cannot promote further oxidation of unsaturated fatty acids. Effective antioxidants decrease free radical energy by resonance derealization (2,3). Effective chain-breaking antioxidants also do not react rapidly with oxygen to form peroxides. Degradation of antioxidant peroxides can deplete the system of the antioxidant and can result in the formation of additional free radicals (1).

The most common functional groups that inactivate free radicals are hydroxyl (eg, phenolics), sulfhydryl (eg, cysteine and glutathione) and amino (eg, uric acid, spermine, and proteins) groups (4). Phenolic compounds are the most common forms of synthetic and natural chain-breaking antioxidants in foods. Examples of naturally occurring phenolics that inhibit lipid oxidation include «-tocopherol, epicatechin, ferulic acid, and carnosic acid (Fig. 1). Many of the natural antioxidants are used in foods as plant extracts such as rosemary and mixed tocopherol isomers. Synthetic phenolics approved as food additives include bu-tylated hydroxytoluene, butylated hydroxyanisole, tertiary butylhydroquinone, and propyl gallate (Fig. 2). These phenolics are effective because of their ability to inactivate free radicals and form low-energy phenolic radicals. Natural phenolic extracts are typically added to foods at concentrations <500 ppm whereas the synthetic antioxidants are normally limited to concentrations <200 ppm of the fat content (5,6).

Effective utilization of phenolic antioxidants in foods often depends on the physical characteristics of the food and the solubility characteristics of the phenolics. Porter (7) originally described the "antioxidant paradox" as a phenomenon where the ability of a chain-breaking antioxidant to inhibit oxidation in bulk oils increased as the polarity of the antioxidant increased. Conversely in oil-in-water lipid emulsions, the effectiveness of lipid-soluble antioxidants increased as their polarity decreased. This phenomenon is due to the ability of polar, lipid-soluble chain-breaking antioxidants to concentrate at the oil-air interface of bulk oils (where oxidation was most prevalent) and the ability of nonpolar lipid-soluble chain-breaking antioxidants to be retained in the droplet of emulsified lipids. The structural diversity of naturally occurring phenolic antioxidants means that they vary greatly in polarity. Trolox (a water-

soluble analog of tocopherol) and gallic acid inhibit lipid oxidation more effectively in bulk oils than their nonpolar counterparts, a-tocopherol and propyl gallate. In an oil-in-water emulsion, a-tocopherol is more effective at inhibiting lipid oxidation than Trolox, and methyl carnosate and car-nosol are more effective than their more polar counterpart, carnosoic acid. In lipid emulsions, the phenolic antioxidants can partition into the continuous phase, the emulsifier and droplet interior. As the polarity of phenolic antioxidants decreases, the amount of the phenolic partitioning in the lipid droplet increases as does the ability of the phenolic to inhibit lipid oxidation (8-12).

Ascorbic acid and glutathione (Fig. 1) can also inactivate free radicals and act as chain-breaking antioxidants. Glutathione and ascorbate are endogenous food antioxidants, and ascorbate is often used as an antioxidant food additive. Since ascorbate and glutathione are highly water soluble, they would primarily interact with water-soluble free radicals and reactive oxygen species (4). However, ascorbate and glutathione are strong reducing compounds that can convert transition metals to their reduced state that, in turn, can promote the formation of free radicals from hydrogen and lipid peroxides (13). Therefore, ascorbate and glutathione can exhibit pro-oxidative activity if the activity of transition metals is not controlled.

Chain-breaking antioxidants will go through a series of reactions with free radicals leading to their eventual inactivation (1). However, two or more chain-breaking antioxidants can interact synergistically, resulting in antioxidant regeneration. Synergistic antioxidant interactions can occur when one chain-breaking antioxidant preferentially reacts with free radicals due to its lower bond disassociation energies or physical location (eg, it is the same environment as the free radical) (3). Such a chain breaking is very effective at inhibiting oxidation; however, it is consumed rapidly during oxidation. However, it is possible that the highly reactive chain-breaking antioxidant can interact with another chain-breaking antioxidant resulting in antioxidant regeneration. An example of such a system is a-tocopherol and ascorbic acid (14). a-Tocopherol reacts more readily with lipid radicals due to its presence in the lipid phase. Ascorbic acid can then reduce the oxidized a-tocopherol back to its active form. In turn, the resulting dehydroascorbate is regenerated by enzymes that utilize reduced nicotinamide adenine dinucleotide (NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH) as reducing equivalents (1).

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