This article is a review of the fundamental chemistry of the tocopherols and tocotrienols relevant to their antioxidant action. Despite the general agreement that α-tocopherol is the most efficient antioxidant and vitamin E homologuein vivo, there was always a considerable discrepancy in its “absolute” and “relative” antioxidant effectivenessin vitro, especially when compared to γ-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherolsin vivo andin vitro. This paper aims at highlighting some possible reasons for the observed differences between the tocopherols (α-, β-, γ-, and δ-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists. Although literature reports related to the chemistry of the tocotrienols are quite meager, they also were included in the discussion in virtue of their structural and functional resemblance to the tocopherols.

The chemistry and antioxidant properties of tocopherols and tocotrienols

Natural antioxidants from residual sources.

: Edible oil is oxidized during processing and storage via autoxidation and photosensitized oxidation, in which triplet oxygen (3O2) and singlet oxygen (1O2) react with the oil, respectively. Autoxidation of oils requires radical forms of acylglycerols, whereas photosensitized oxidation does not require lipid radicals since 1O2 reacts directly with double bonds. Lipid hydroperoxides formed by 3O2 are conjugated dienes, whereas 1O2 produces both conjugated and nonconjugated dienes. The hydroperoxides are decomposed to produce off-flavor compounds and the oil quality decreases. Autoxidation of oil is accelerated by the presence of free fatty acids, mono- and diacylglycerols, metals such as iron, and thermally oxidized compounds. Chlorophylls and phenolic compounds decrease the autoxidation of oil in the dark, and carotenoids, tocopherols, and phospholipids demonstrate both antioxidant and prooxidant activity depending on the oil system. In photosensitized oxidation chlorophyll acts as a photosensitizer for the formation of 1O2; however, carotenoids and tocopherols decrease the oxidation through 1O2 quenching. Temperature, light, oxygen concentration, oil processing, and fatty acid composition also affect the oxidative stability of edible oil.

Mechanisms and Factors for Edible Oil Oxidation

Recently, there has been growing interest in research into the role of plant-derived antioxidants in food and human health. The beneficial influence of many foodstuffs and beverages including fruits, vegetables, tea, coffee, and cacao on human health has been recently recognized to originate from their antioxidant activity. For this purpose, the most commonly methods used in vitro determination of antioxidant capacity of food constituents are reviewed and presented. Also, the general chemistry underlying the assays in the present paper was clarified. Hence, this overview provides a basis and rationale for developing standardized antioxidant capacity methods for the food, nutraceutical, and dietary supplement industries. In addition, the most important advantages and shortcomings of each method were detected and highlighted. The chemical principles of these methods are outlined and critically discussed. The chemical principles of methods of 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS·+) scavenging, 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radical scavenging, Fe3+–Fe2+ transformation assay, ferric reducing antioxidant power (FRAP) assay, cupric ions (Cu2+) reducing power assay (Cuprac), Folin-Ciocalteu reducing capacity (FCR assay), peroxyl radical scavenging, superoxide anion radical (O
 2
 ·−
 ) scavenging, hydrogen peroxide (H2O2) scavenging, hydroxyl radical (OH·) scavenging, singlet oxygen (1O2) quenching assay and nitric oxide radical (NO·) scavenging assay are outlined and critically discussed. Also, the general antioxidant aspects of main food components were discussed by a number of methods which are currently used for detection of antioxidant properties food components. This review consists of two main sections. The first section is devoted to main components in the foodstuffs and beverages. The second general section is some definitions of the main antioxidant methods commonly used for determination of antioxidant activity of components in the foodstuffs and beverages. In addition, there are given some chemical and kinetic basis and technical details of the used methods.

Antioxidant activity of food constituents: an overview

Phenolic compounds and their role in oxidative processes in fruits

Purified lard triacylglycerols have been oxidised at 25, 50, 75 and 100°C in the presence and absence of 0.2–2.0 g kg−1 (0.45–10.3 mmol litre−1) of the antioxidants α-tocopherol and ferulic acid. Increasing the temperature does not influence the effect of ferulic acid on the induction period or the rate of autoxidation, α-tocopherol is more effective at extending the induction period and reducing the rate of autoxidation as the temperature is increased. The rate of substrate oxidation increases with increasing α-tocopherol concentration, especially at lower temperatures, but decreases with increasing ferulic acid concentration. This behaviour is explained in terms of the participation of antioxidant molecules and radicals in the autoxidation process. High-temperature oxidation results can be extrapolated only to room temperature when no change occurs in the antioxidative mechanism.

Effect of temperature on the antioxidative action of inhibitors in lipid autoxidation

Antioxidative properties ofp-hydroxybenzoic, vanillic, syringic, 3,4-dihydroxybenzoic,p-coumaric, ferulic, sinapic and caffeic acids were studied in the concentration range 0.02–0.20 wt% during autoxidation at 100°C of lard and sunflower oil methyl esters (MEL and MESO, respectively). In both lipid systems, the derivatives of benzoic acid had weaker inhibiting properties than did the corresponding analogues of cinnamic acid. The effectiveness and strength of the antioxidative action were considerably lower in the lipid system MESO, which was rich in linoleic acid and was more easily oxidized. Thep-hydroxybenzoic, vanillic, syringic andp-coumaric acids in this system exercised no inhibiting effect. We established that the molecules of the investigated phenolic acids initiated the chain radical process of autoxidation, and the formed antioxidant radicals propagated the chains as a result of the reaction with the lipid substrate. These reactions proceeded at a higher rate in MESO than in MEL.

Effect of lipid unsaturation on the antioxidative activity of some phenolic acids

Antioxidative activity of phenolic acids on triacylglycerols and fatty acid methyl esters from olive oil

The kinetics of peroxide accumulation during oxidation of triacylglycerols of sunflower oil at 100 °C in presence of different concentrations of hexane, ethylacetate and ethanol extracts of Melissa officinalis L., Mentha piperita L., Mentha spicata L., Ocimum basilicum L., Origanum vulgare L. and Saturejae hortensis L., was followed. Ethanol extracts are the most active in retarding the autoxidation process. The most effective are the extracts of Saturejae hortensis L. (stabilization factor F for 0.5% direct hexane and 0.5% direct ethanol extracts being 14.8 and 24.0, respectively), followed by the extracts of Mentha piperita L. (F = 7.4 and 19.6, respectively) and Melissa officinalis L. (F = 7.4 and 18.6, respectively). Some of the spices are recommended as sources of natural antioxidants for stabilization of polyunsaturated lipid systems.

Antioxidant activity of selected species of the family Lamiaceae grown in Bulgaria

The antioxidative properies of p-hydroxy-benzole acid, vanillic acid, syringic acid, 3,4-dihydroxy-benzoic acid, p-coumaric acid, ferulic acid, sinaptic acid and caffeic acid in lard autoxidation at 100 o C are compared. It is proved that the derivatives of the benzoic acid have weaker inhibiting properties than is the case of the corresponding analogues of the cinnamic acid due to the more active participation of the inhibitor in the elementary reactions of propagation and initiation of the radical chain process

Nedjalka Vl. Yanishlieva

Papers

Effect of temperature on the antioxidative action of inhibitors in lipid autoxidation

Effect of lipid unsaturation on the antioxidative activity of some phenolic acids

Antioxidative activity of phenolic acids on triacylglycerols and fatty acid methyl esters from olive oil

Antioxidant activity of selected species of the family Lamiaceae grown in Bulgaria

Inhibited Oxidation of Lipids II: Comparison of the Antioxidative Properties of Some Hydroxy Derivatives of Benzoic and Cinnamic Acids