NATURAL ANTIOXIDANTS: SOURCES, EFFECTS AND APPLICATIONS 
 
Fereidoon Shahidi,PhD., Department of Biochemistry, Memorial University of Newfoundland,
St. John’s, NF, Canada A1B 3X               

Antioxidants are of interest to both food scientists and health professionals. Whilst antioxidants are often added to foods to stabilize them and prevent off-flavour development, considerable interest has been expressed for their potential role as therapeutic agents. Antioxidants are substances that when present in food or in the body at low concentrations compared to that of an oxidizable substrate markedly delay or prevent the oxidation of that substrate. In food, antioxidants occur either as endogenous constituents or are added for enhancing product quality by controlling oxidation with its deleterious consequences. The mechanism by which antioxidants protect food from oxidation is by scavenging of free radicals via donation of an electron or a hydrogen atom, or by deactivation of metal ions and singlet oxygen.

The most widely used synthetic antioxidants in food are butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tert-butylhdroquinone (TBHQ). These antioxidants may be added to food, individually or in combination, at a total level of up to 200 ppm on a fat basis. In Singapore, levels vary between 80 and 200 ppm, depending on the product. The use of synthetic antioxidants in food dates back some 60 years ago when BHA and then alkyl esters of gallic acid were first approved. It was also evident that the pro-oxidant effects of transition metal ions such as iron and copper had to be considered. Thus, certain acids such as citric acid (CA), ethylenediamintetraacetic acid (EDTA) and polyphosphates, or their derivatives, were found to deactivate metal ions by chelation. These antioxidants and chelators have since been used in food, individually or in combination. However, concern has now been expressed about the safety of certain synthetic antioxidants as potential carcinogens. As a result, BHA has been removed from the GRAS (Generally Recognized As Safe) list and use of TBHQ remains prohibited in some countries. Therefore, there has been an interest by the industry and a desire by consumers to replace synthetic compounds with natural alternatives.

Natural Antioxidants

Naturally occurring inhibitors of oxidation in food generally originate from plant-based materials. The active components, namely phenolics and polyphenolics, including tocopherols, are secondary plant metabolites and are first derived from phenylalanine and in certain cases and in some plants from tyrosine. The resultant phenylpropanoids may then undergo further transformation to yield benzoic acid derivatives as well as flavonoids, isoflavons, and other complex polyphenols. Thus, natural food phenolics are present as a complex mixture of compounds that provide a cocktail of many active components present in the free, esterified, glycosylated and bound forms (Shahidi and Naczk, 1995). The potency of preparations is therefore dictated by their chemical structures and governed by the hydrophilic-lipophilic balance (HLB) of the participating molecules in a concentration- and system-dependent manner. Thus, the mode of action of natural antioxidants may involve multiple mechanisms, depending on the source material and possible presence of synergists and antagonists.

In order to use any antioxidant preparation in food, it must be safe, easy to incorporate, effective at low concentrations, with no undesirable odour, flavour or colour, heat stable, nonvolatile and with good carry through properties and cost-effective. In addition, presence and possible effects of antagonists must be carefully considered as an antioxidant may become a prooxidant in the presence of certain other molecules. As an example, chlorophylls may overwhelm the antioxidant effect of phenolics due to photosensitized oxidation and transition metal ions such as those of iron and copper may render conditions that favour oxidation. Synergism among different phenolic antioxidants and between phenolics and non-phenolics should be considered in all application areas.

Table 1. Natural Antioxidants in Selected Plant Sources

COMMERCIAL SOURCES OF NATURAL ANTIOXIDANTS

The most common natural antioxidant preparations in the market are mixed tocopherols, which are by-products of vegetable oil refining. In addition, spices or their oleoresins and extracts, such as those of rosemary and sage, green tea extracts, other plant-based mixtures, such as those of mustard and certain unsaponifiables of edible oils, and, of course, carotenoids are also important (Table 1) ( Ho et al., 1994; Shahidi, 1997).

Tocopherols and Tocotrienols

Edible oils and their source material usually contain a high amount of tocopherols and tocotrienols, collectively known as “tocols”. These are present as constituents of unsaponifiable matter and may occur together with phospholipids, carotenoids, chlorophylls and triterpenyl alcohols. Deodourization of edible oils via molecular distillation or other processes yields a substantial amount of tocols , depending on the source material. Most of the world supply of tocopherols originates from soybean oil processing while tocotrienols are procured from palm and rice bran oils or processing by-products in their production.

Tocopherols and tocotrienols are monophenolic and lipophilic compounds and the former occur as the d-isomer of alpha-, beta-, gamma- and delta-, depending on the number and position of the methyl groups. In terms of vitamin E activity, d-alpha-tocopherol is the reference compound with highest potency. However, in in-vitro systems, the order of activity is delta>gamma>beta>alpha. Products obtained from edible oil processing possess a mixture of tocol isomers and these may be formulated as a 100% dispersible or dry form and can be suspended in water before inclusion into foods. Mixed tocopherols have GRAS status and may be used at 150-450 ppm, based on lipid level, in foods. However, tocopherols may exert a prooxidant effect at high concentrations and their activity is temperature-dependent. Tocopherols, while present at a much lower level in oils, are important contributors to the tocol content of palm, rice, barley, and wheat germ oils. On the other hand, tocotrienols have much less vitamin activity, but are generally better antioxidants than tocopherols in-vitro. For example, alpha-tocotrienol is 6.5-60 times more effective than alpha-tocopherol as an antioxidant. Due to their excellent function in cardiovascular health, tocotrienols serve as effective supplements and over-the-counter (OTC) health products.

Synergism between tocols and ascorbic acid or its derivatives has been well documented and ascorbic acid is known to regenerate alpha-tocopherol in both in-vivo and in-vitro systems. In addition, a strong synergism exists between tocopherols and phospholipids. Ternary mixtures of tocopherols, lecithin or other phospholipids and ascorbic acid or its derivatives exhibit excellent antioxidant activity in bulk oil and emulsified foods. Spices, Herbs and their Extracts

Spices and herbs have traditionally been used in foods to improve or modify their flavour. However, the first report on protective effect of sage, mace and black pepper appeared some 60 years ago for inhibiting oxidation in frozen meat.

Among spices and extracts in commercial production are rosemary and sage. Both of these belong to the family of Labiatae and possess varied, but similar antioxidative components. These include carnosol, carnosic acid, rosmaridiphenol, rosmarinic acid , rosmanol and rosmariquinol. Extracts of rosemary and sage devoid of any flavour are now commercially available and their application in a variety of lipid-containing foods is increasing. The oleoresins of rosemary and sage are usually in the liquid form and may be used at 200-1000 ppm level on a fat basis. These oleoresins, that have GRAS status, have been used in different products, such as potato chips, flakes, sauces, dressings, processed meats, poultry and seafood as well as in cakes and crackers. The antioxidant activity of such extracts/ oleoresins is comparable or exceeds those of BHA and BHT and is always higher than tocopherols.

In addition, certain plant materials are used as components of binder mixes in processed meats or as breading and batter components. For example deheated mustard flour may be used in meats at up to 2% (application of higher amounts may cause yellowing of products). The active antioxidant components of mustard flour include phenolic acids and phenylpropanoids.

Tea Extracts

Non-fermented green tea is used for the production of commercial extracts. The crude extracts contain at least 25% catechins, dominated by epigallocatechin gallate (48-55%), followed by equal amounts of epigallocatechin and epicatechin gallate (each at 9-12%), as well as epicatechin (5-7%), gallocatechin (3.5%), catechin (0.3-0.6%) and gallic acid (0.3-0.5%). Application of green tea extracts in a variety of foods has been demonstrated successfully and activity of such extracts generally exceeds those of mixed tocopherols and herbal preparations.

Carotenoids

Carotenoids, especially beta-carotene, may be prepared, on a commercial scale, from carrot, yam and other sources. Carotenoids are excellent scavengers of singlet oxygen and hence they are used mainly in foods that are exposed to light, such as vegetable oils packaged in clear bottles. Red palm oil is dominated by alpha-carotene (0.25%), but it also contains beta-carotene (0.04%) and minute amounts of other carotenoids, including lycopene – the major source of which is tomato. The beneficial health effects of carotenoids, including their oxygenated derivatives, known as xanthophylls, are well documented (eg., Ong and Packer, 1992; Rao and Agarwal, 1999).

EFFICACY OF NATURAL ANTIOXIDANTS IN DIFFERENT SYSTEMS

The chemical composition and structures of active extract components are important factors governing the efficacy of natural antioxidants in different foods. Thus, phenolic compounds with ortho- and para- dihydroxylation or a hydroxy and a methoxy group are more effective than simple phenolics. In addition, phenylpropanoids with extended conjugation are more effective than benzoic acid derivatives. Furthermore, hydrophilicity and lipophilicity of the active components is dictated by the appropriateness of antioxidants in systems. In general, more hydrophilic antioxidants are better in stabilizing bulk oil than oil-in-water emulsions while the activity of lipophilic antioxidants follows the opposite trend. There are also many other factors that must be taken into account when considering and selecting antioxidants and extracts for food application. Specifically, attention should be paid to the photosensitizing effect of chlorophylls in natural extracts. In addition, the level of incorporation of antioxidants in foods should be optimized and the use of chelating agents considered, when and where appropriate. Many antioxidants behave prooxidatively at high concentrations or when present together with ions of transition metals; such effects are also important when considering the in-vivo activity of antioxidants ( Shahidi and Ho, 2000). Some chelators, such as polyphosphates, in addition to metal sequestration, may also exert other beneficial effects such as to improve the cooking yield and juiciness of meat and poultry products or keeping quality of fresh seafoods. The role of natural antioxidants in foods is expected to rise over the years to come.

REFERENCES:

Ho, C-T., Osawa, T., Huang, M-T. and Rosen, R.T. 1994. Food Phytochemicals and Cancer Prevention. II. Teas, Spices and Herbs. ACS Symposium Series 547, American Chemical Society, Washington, DC.

Ong, A.S.H. and Packer, L. 1992. Lipid Soluble Antioxidants: Biochemistry and Clinical Applications. Birkhavser Verlog, Basel, Switzerland.

Rao, A.V. and Agarwal, S. 1999. Role of lycopene as antioxidant carotenoid in the prevention of chronic diseases: a review. Nutr. Res. 19: 305-323.

Shahidi, F. 1997. Natural Antioxidants: Chemistry, Health Effects and Applications. American Oil Chemists’ Society, Champaign, IL.

Shahidi, F. and Naczk, M. 1995. Food Phenolics: Sources, Chemistry, Effects and Applications. Technomic Publishing Company, Lancaster, PA.

Shahidi, F. and Ho, C-T. 2000. Phytochemicals and Phytopharmaceuticals. American Oil Chemists’ Society, Champaign, IL.

[1] Prof. Shahidi was a visiting Professor in the Food Science and Technology Programme at the National university of Singapore, c/o Dept. of Chemistry, 3 Science Drive 3, Singapore 117543