NATURAL ANTIOXIDANTS

VI. NATURAL ANTIOXIDANTS

A. Tocopherol and Nonessential Polyphenols as Antioxidants

Tocopherols are the most active chain-breaking antioxidants, and there is an explicit di- etary requirement for tocopherols as vitamin E. Tocopherols occur to varying extents in most foods unless they are removed by specific processes during manufacture. Loss of the free radical–scavenging properties of tocopherol is believed to be the basis for its essentiality and the pathologies associated with its deficiency. That the basis for the essen- tiality of tocopherol lies in its ability to prevent oxidative damage raises an important nutritional question, ‘‘Are these actions also provided by nonessential polyphenolics pres- ent in plants and foods derived from them?’’ Many phytochemicals that are present in the food supply have been implicated as being capable of interfering with and inhibiting free radical chain reactions of lipids. Plant phenolics inhibit lipid ROOH formation cata- lyzed by metals, radiation, and heme compounds (Buettner, 1993; Hanasaki et al., 1994), and also scavenge peroxy, alkoxy, and hydroxy radicals and singlet oxygen (Hanasaki et al., 1994; Laughton et al., 1991; Tournaire et al., 1993). Tocopherol in oxidizing lipid systems are spared by flavonoids (Terao et al., 1994; Jessup et al., 1990). If α-tocopherol ( Fig. 2 ) is an essential antioxidant that acts where no other compound can, the sparing effect of nonessential antioxidants may be one of their most important actions.

B. Inhibition of Hydroperoxide Decomposition, Alkoxyl Radical Reduction, and Aldehyde Scavenging

Antioxidant activity is not limited to prevention of ROOH formation. ROOH are not dam-

Figure 2 α-Tocopherol.

has occurred. Although ROOH are not directly damaging, their decomposition by reduced metals generates the reactive hydroxyl radical HO • or the alkoxyl radical RO • . These strongly electrophilic oxidants react with and oxidize virtually all biological macromole- cules. The alkoxy radical typically fragments the parent lipid molecule and liberates elec- trophilic aldehydes, hydrocarbons, ketones, and alcohols. Both the highly reactive hydroxy and alkoxy radicals and the electrophilic aldehydes liberated with their reduction react readily with polypeptides (proteins) and polynucleotides (DNA). Thus, additional antioxi- dant actions include preventing ROOH decomposition, reducing alkoxyl radicals, or scav- enging the electrophilic aldehydes. The efficacy of different antioxidants varies during this phase of the oxidation process. Even tocopherol isomers differ with respect to their ability to prevent decomposition of ROOH (Huang et al., 1994, 1995). Plant phenolics vary in their ability to interrupt a free radical chain reaction, with differences detectable among different lipid systems, oxidation initiators, and other antioxygenic components.

As discussed, the chemistry of free radical oxidations is multistage and complex. Oxidation is not a single catastrophic event. There is no single initiating oxidant that generates all free radicals; there are a great many sources of single electrons. Similarly, there is no single reactive product of oxidation; there are classes of products, many of which are both selectively and broadly damaging. Free radicals and their products react with virtually all biological molecules, and there is no single defense against all targets of oxidative damage. Thus, a variety of mechanisms have evolved to prevent or respond to oxidative stresses and free radicals and their products at one or more of the many steps of oxidation.

C. Natural Antioxidants Added to Foods

Soluble chain-breaking antioxidants used in foods include ascorbate, as either the naturally occurring free acid or as synthetic ascorbate, and various soluble and insoluble ester forms. The acid form is an excellent electron donor in foods. This is the principle property that makes it an excellent antioxidant at low concentrations, but at high concentrations its ability to reduce metal initiators can actually lead to a prooxidant effect (Frankel, 1989).

In response to a perceived desire by consumers for less chemically processed food ingredients, several naturally occurring, chain-breaking antioxidants are being introduced to accomplish essentially the same effects as those of substituted phenols such as butylated hydroxy hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) (Aruoma et al., 1992). These natural antioxidants are primarily extracts of herbs or plant materials with In response to a perceived desire by consumers for less chemically processed food ingredients, several naturally occurring, chain-breaking antioxidants are being introduced to accomplish essentially the same effects as those of substituted phenols such as butylated hydroxy hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) (Aruoma et al., 1992). These natural antioxidants are primarily extracts of herbs or plant materials with

Because once initiated, lipid oxidation of PUFA tends to proceed autocatalytically, breaking the chain reaction is a critical step in affording stability. This stability is afforded both in vivo and in food lipids by the presence of scavengers of the peroxy radical oxidant. The abundance of scavengers of oxidants and peroxyl radicals is therefore a critical vari- able limiting the progress of lipid oxidation.