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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

ABSTRACT: 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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

Oxidative stability of oils is the resistance to oxidation during processing and storage (Guillen and Cabo 2002). Resistance to oxidation can be expressed as the period of time necessary to attain the critical point of oxidation, whether it is a sensorial change or a sudden acceleration of the oxidative process (Silva and others 2001). Oxidative stability is an important indicator to determine oil quality and shelf life (Hamilton 1994) because low-molecular-weight off-flavor compounds are produced during oxidation. The off-flavor compounds make oil less acceptable or unacceptable to consumers or for industrial use as a food ingredient. Oxidation of oil also destroys essential fatty acids and produces toxic compounds and oxidized polymers. Oxidation of oil is very important in terms of palatability, nutritional quality, and toxicity of edible oils.

Different chemical mechanisms, autoxidation and photosensitized oxidation, are responsible for the oxidation of edible oils during processing and storage depending upon the types of oxygen. Two types of oxygen can react with edible oils. One is called atmospheric triplet oxygen, 3O2, and the other is singlet oxygen,1O2. 3O2 reacts with lipid radicals and causes autoxidation, which is a free radical chain reaction. The chemical properties of 3O2 to react with lipid radicals can be easily explained by the molecular orbital of the oxygen as shown in Figure 1. The 3O2 in the ground state with 2 unpaired electrons in the 2pπ antibonding orbitals has a permanent magnetic moment with 3 closely grouped energy states under a magnetic field and is called triplet oxygen. 3O2 is a radical with 2 unpaired orbitals in the molecule. It reacts with radical food compounds in normal reaction conditions according to spin conservation. Photosensitized oxidation of edible oils occurs in the presence of light, sensitizers, and atmospheric oxygen, in which 1O2 is produced.

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Figure 1—. Molecular orbital of triplet oxygen, 3O2

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The electron configuration in the 2pπ antibonding orbitals of 1O2 is shown in Figure 2. Since 1 orbital of the 2pπ antibonding orbitals of1O2 has paired electrons and the other is completely empty, 1O2 has 1 energy level under a magnetic field and is electrophilic. The nonradical electrophilic singlet oxygen readily reacts with compounds with high electron densities, such as the double bonds of unsaturated fatty acids. The 1O2 has an energy of 93.6 kJ above the ground state of 3O2 (Korycka-Dahl and Richardson 1978; Girotti 1998). High-energy 1O2 in solution is deactivated by transferring its energy to the solvent, and its lifetime depends on the solvent. The lifetime of singlet oxygen is about 2, 17, and 700 μs in water, hexane, and carbon tetrachloride, respectively (Merkel and Kearns 1972; Long and Kearns 1975).

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Figure 2—. Electronic configuration of 2pπ antibonding orbital of singlet oxygen, 1O2

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Oxidation of edible oils is influenced by an energy input such as light or heat, composition of fatty acids, types of oxygen, and minor compounds such as metals, pigments, phospholipids, free fatty acids, mono- and diacylglycerols, thermally oxidized compounds, and antioxidants. Many efforts have been made to improve the oxidative stabilities of oils by systematic studies on the effects of these factors.

This article reviews the reaction mechanism, kinetics, and oxidation products of autoxidation and photosensitized oxidation. Factors affecting the oxidation of edible oils, free fatty acids, mono- and diacylglycerols, metals, phospholipids, pigments, thermally oxidized compounds, and antioxidants, are also discussed.

Mechanisms of Autoxidation in Edible Oil

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

Autoxidation of oils, free radical chain reaction, includes initiation, propagation, and termination steps:

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Autoxidation of oils requires fatty acids or acylglycerols to be in radical forms. Fatty acids or acylglycerols are in nonradical singlet states, and the reaction of fatty acids with radical state atmospheric 3O2 is thermodynamically unfavorable due to electronic spin conservation (Min and Bradley 1992). The hydrogen atom in the fatty acids or acylglycerols in edible oil is removed and lipid alkyl radicals are produced in the initiation step. Heat, metal catalysts, and ultraviolet and visible light can accelerate free radical formation of fatty acids or acylglycerols. The energy required to remove hydrogen from fatty acids or acylglycerols is dependent on the hydrogen position in the molecules. The hydrogen atom adjacent to the double bond, especially hydrogen attached to the carbon between the 2 double bonds, is removed easily. Hydrogen at C11 of linoleic acid is removed at 50 kcal/mol. The energy required to remove hydrogen in C8 and C14 of linoleic acid is 75 kcal/mol and the homolytic dissociation energy between hydrogen and C17 or C18 is about 100 kcal/mol (Min and Boff 2002). The double bond adjacent to the carbon radical in linoleic acid shifts to the more stable next carbon and from the cis to the trans form. Autoxidation of linoleic and linolenic acids produces only conjugated products. The hydroperoxide positional isomers formed in the autoxidation of oleic, linoleic, and linolenic acids are shown in Table 1.

Table 1—.  Hydroperoxides of fatty acids by autoxidationa
Fatty acidHydroperoxides atRelative amount (%)
Oleic acidC8 26∼28
C9 22∼25
C1022∼24
C1126∼28
Linoleic acidC9 48∼53
C1348∼53
Linolenic acidC9 28∼35
C12 8∼13
C1310∼13
C1628–35

The lipid alkyl radical reacts with 3O2 and forms lipid peroxy radical, another reactive radical. Reaction between lipid alkyl radical and 3O2 occurs very quickly at normal oxygen pressure and, consequently, the concentration of lipid alkyl radical is much lower than that of lipid peroxy radical (Aidos and others 2002). The lipid peroxy radical abstracts hydrogen from other lipid molecules and reacts with the hydrogen to form hydroperoxide and another lipid alkyl radical. These radicals catalyze the oxidation reaction, and autoxidation is called the free radical chain reaction. Figure 3 shows the hydroperoxide formation in the autoxidation of linoleic acid. The rates for the formation of lipid peroxy radical and hydroperoxide depend only on oxygen availability and temperature (Velasco and others 2003). When radicals react with each other, nonradical species are produced and the reaction stops.

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Figure 3—. Hydroperoxide formation in the autoxidation of linoleic acid

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The primary oxidation products, lipid hydroperoxides, are relatively stable at room temperature and in the absence of metals. However, in the presence of metals or at high temperature they are readily decomposed to alkoxy radicals and then form aldehydes, ketones, acids, esters, alcohols, and short-chain hydrocarbons. The most likely pathway of hydroperoxide decomposition is a homolytic cleavage between oxygen and the oxygen bond, in which alkoxy and hydroxy radicals are produced. The activation energy to cleave the oxygen–oxygen bond is 46 kcal/mol lower than that to cleave the oxygen–hydrogen bond (Hiatt and others 1968). The alkoxy radical then undergoes homolytic β-scission of the carbon–carbon bond and produces oxo-compounds and saturated or unsaturated alkyl radicals (Figure 4). After electron rearrangement, the addition of hydroxyl radical, or hydrogen transfer, the ultimate secondary lipid oxidation products are mostly low-molecular-weight aldehydes, ketones, alcohols, and short-chain hydrocarbons, as shown in Table 2.

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Figure 4—. Mechanisms of hydroperoxide decomposition to form secondary oxidation products

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Table 2—.  Secondary oxidation products of fatty acid methyl ester by autoxidationa
ClassOleic acidLinoleic acidLinolenic acid
AldehydesOctanalbPentanalPropanal
NonanalHexanalButanal
2-Decenal2-Octenal2-Butenal
Decanal2-Nonenal2-Pentenal
2,4-Decadienal2-Hexenal
3,6-Nonadienal
Decatrienal
Carboxylic acidMethyl heptanoateMethyl heptanoateMethyl heptanoate
Methyl octanoateMethyl octanoateMethyl octanoate
Methyl 8-oxooctanoateMethyl 8-oxooctanoateMethyl nonanoate
Methyl 9-oxononanoateMethyl 9-oxononanoateMethyl 9-oxononanoate
Methyl 10-oxodecanoateMethyl 10-oxodecanoateMethyl 10-oxodecanoate
Methyl 10-oxo-8-decenoate 
Methyl 11-oxo-9-undecenoate 
Alcohol1-Heptanol1-Pentanol 
1-Octene-3-ol 
HydrocarbonsHeptanePentaneEthane
Octane Pentane

The time for secondary product formation from the primary oxidation product, hydroperoxide, varies with different oils. Secondary oxidation products are formed immediately after hydroperoxide formation in olive and rapeseed oils. However, in sunflower and safflower oils, secondary oxidation products are formed when the concentration of hydroperoxides is appreciable (Guillen and Cabo 2002).

Most decomposition products of hydroperoxides are responsible for the off-flavor in the oxidized edible oil. Aliphatic carbonyl compounds have more influence on the oxidized oil flavor due to their low threshold values. Threshold values for hydrocarbons, alkanals, 2-alkenals, and trans, trans-2,4-alkadienals are 90 to 2150, 0.04 to1, 0.04 to 2.5, and 0.04 to 0.3 ppm, respectively (Frankel 1985). Hexanal (23.5%) and 2-decenal (34.3%), and 2-heptenal (29.5%) and trans-2-octenal (18.1%), were the major volatile compounds detected by the solid-phase microextraction method in soybean and corn oils (peroxide value of 5), respectively (Steenson and others 2002). Pentane, hexanal, propenal, and 2,4-decadienal were present in high amounts in canola oil stored uncovered at 60 °C (Vaisey-Genser and others 1999). Frankel (1985) reported that trans, cis-2,4-decadienal was the most significant compound in determining the oxidized flavor of oil, followed by trans, trans-2,4-decadienal, trans, cis-2,4-heptadienal, 1-octen-3-ol, butanal, and hexanal. Hexanal, pentane, and 2,4-decadienal were suggested and used as indicators to determine the extent of the oil oxidation (Warner and others 1978; Przybylski and Eskin 1995; Choe 1997; Heinonen and others 1997). Trans-2-hexenal, and trans, cis, trans-2,4,7-decatrienal and 1-octen-3-one, were reported to give grass-like and fish-like flavor in oxidized soybean oil, respectively (Min and Bradley 1992). No single flavor compound is mainly responsible for the oxidized flavor of vegetable oils.

Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

Oil oxidation is accelerated by light, especially in the presence of sensitizers such as chlorophylls. Sensitizers in singlet state absorb light energy very rapidly, in picoseconds, and become excited. Excited singlet sensitizers can return to their ground state via emission of light, internal conversion, or intersystem crossing (Figure 5). Fluorescence and heat are produced by emission of light and internal conversion, respectively. Intersystem crossing results in excited triplet state of sensitizers.

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Figure 5—. Excitation and deactivation of sensitizer (Sen; sensitizers, ISC; intersystem crossing)

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Excited triplet sensitizers may accept hydrogen or an electron from the substrate and produce radicals (type I) as shown in Figure 6. Excited triplet sensitizers react with 3O2 and produce superoxide anion by electron transfer. Superoxide anion produces hydrogen peroxide, one of the reactive oxygen species by spontaneous dismutation, and the reaction of hydrogen peroxide with superoxides results in singlet oxygen formation by Haber–Weiss reaction in the presence of transition metals such as iron or copper (Kellogg and Fridovich 1975):

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Figure 6—. Reaction of triplet sensitizers with substrates

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The excitation energy of triplet sensitizers can be transferred onto adjacent 3O2 to form 1O2 by triplet–triplet annihilation, and the sensitizers return to their ground singlet state (type II). Kochevar and Redmond (2000) reported that a sensitizer molecule may generate 103 to 105 molecules of 1O2 before becoming inactive.

The rate of the type I or II processes depends on the kinds of sensitizers and substrates (Chen and others 2001), and concentrations of substrates and oxygen (Foote 1976). Phenols or amines, which are readily oxidized, or readily reducible quinones favor the type I process. On the other hand, olefins, dienes, and aromatic compounds, which are not so readily oxidized or reduced, more often favor type II. Photosensitized oxidation of edible oil follows the singlet oxygen oxidation pathway. 1O2 was suggested to be involved in the initiation of the oil oxidation (Rawls and Van Santen 1970; Lee and Min 1988).

1O2 either reacts chemically with other molecules or transfers its energy to them. When 1O2 reacts with unsaturated fatty acids, mostly allyl hydroperoxides are formed by ene reaction (Gollnick 1978), as shown in Figure 7.

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Figure 7—. Formation of allyl hydroperoxide of oleic acid by ene reaction

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Electrophilic 1O2 can directly react with high-electron-density double bonds without the formation of alkyl radical, and form hydroperoxides at the double bonds. When hydroperoxide is formed, double bond migration and trans fatty acid occur, producing both conjugated and nonconjugated hydroperoxides, as shown in Table 3. Production of nonconjugated hydroperoxides is not observed in the autoxidation. Figure 8 shows the oxidation pathway of linoleic acid by 1O2.

Table 3—.  Hydroperoxides of fatty acids by singlet oxygen oxidation
Fatty acidHydroperoxides atRelative amount (%)aType of acid
Oleic acidC9 48 
C1052 
Linoleic acidC9 32Conjugated
C1017Nonconjugated
C1217Nonconjugated
C1334Conjugated
Linolenic acidC9 23Conjugated
C1013Nonconjugated
C1212Conjugated
C1314Conjugated
C1513Nonconjugated
C1625Conjugated
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Figure 8—. Hydroperoxide formation in linoleic acid oxidation by singlet oxygen

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Hydroperoxides formed by 1O2 oxidation are decomposed by the same mechanisms for the hydroperoxides formed by 3O2 in autoxidation. 1O2 oxidation produces more 2-decenal and octane, two of the decomposition products of hydroperoxides, in oleate than autoxidation does (Frankel 1985). The contents of octanal and 10-oxodecanoate in autoxidized oleate were higher than those of 1O2-oxidized oleate. 2-Heptenal and 2-butenal were noticeable in 1O2-oxidized linoleic and linolenic acids, whereas they were negligible in autoxidized linoleic and linolenic acids. Heptenal was formed only in 1O2-oxidized soybean oil in the presence of chlorophyll and light (Min and others 2003).

A beany flavor, which is a unique and undesirable flavor in soybean oil with low peroxide value, has been a problem for the last 70 years and studied extensively both nationally and internationally (Chang and others 1966; Smouse and Chang 1967). 2-Pentylfuran and pentenylfuran were reported to be responsible for the beany flavor (Chang and others 1966, 1983; Smouse and Chang 1967; Ho and others 1978; Smagula and others 1979), and 1O2 was involved in their formation from linoleic and linolenic acids present in soybean oil, as shown in Figure 9 and 10 (Min and others 2003). Min and others (2003) strongly indicated that the reversion flavor of soybean oil can be decreased or eliminated by removing chlorophylls from the oil during processing. The soybean oil industry currently removes effectively chlorophylls from soybean oil using bleaching materials during the refining process, and the beany flavor is no longer a serious problem in soybean oil.

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Figure 9—. Formation of 2-pentylfuran from linoleic acid by singlet oxygen oxidation

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Figure 10—. Formation of 2-pentenylfuran from linolenic acid by singlet oxygen oxidation

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Factors Affecting the Oxidation of Edible Oil

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

The oxidation of oil is influenced by the fatty acid composition of the oil, oil processing, energy of heat or light, the concentration and type of oxygen, and free fatty acids, mono- and diacylglycerols, transition metals, peroxides, thermally oxidized compounds, pigments, and antioxidants. These factors interactively affect the oxidation of oil and it is not easy to differentiate the individual effect of the factors.

Fatty acid composition of oils

Oils that are more unsaturated are oxidized more quickly than less unsaturated oils (Parker and others 2003). As the degree of unsaturation increases, both the rate of formation and the amount of primary oxidation compounds accumulated at the end of the induction period increase (Martin-Polvillo and others 2004). Soybean, safflower, or sunflower oil (iodine values > 130) stored in the dark showed a significantly (P < 0.05) shorter induction period than coconut or palm kernel oil whose iodine value is less than 20 (Tan and others 2002). High oleic and high stearic oils by gene silencing of the oilseeds or hydrogenated soybean oil showed higher autoxidative stability (Evans and others 1973; Liu and others 2002). The autoxidation rate greatly depends on the rate of fatty acid or acylglycerol alkyl radical formation, and the radical formation rate depends mainly on the types of fatty acid or acylglycerol. The relative autoxidation rate of oleic, linoleic, and linolenic acids was reported as 1:40 to 50:100 on the basis of oxygen uptake (Min and Bradley 1992).

The difference in1O2 oxidation rate among fatty acids is lower than that for autoxidation. The reaction rates between 1O2 and stearic, oleic, linoleic, and linolenic acids are 1.2 × 104, 5.3 × 104, 7.3 × 104, and 10.0 × 104 M−1s−1, respectively (Vever-Bizet and others 1989). The relative reaction rates of 1O2 with oleic, linoleic, and linolenic acids are 1.0:1.4:1.9, respectively. Soybean oil reacts with 1O2 at a rate of 1.4 × 105 M−1s−1 in methylene chloride at 20 °C (Lee and Min 1991). The type of polyunsaturated fatty acids, nonconjugated or conjugated dienes or trienes, has little effect on the reaction between the lipid and 1O2 (Rahmani and Saari Csallani 1998).

Oil processing

The oil-processing method affects the oxidative stability of an oil. Crude soybean oil was the most stable to the oxidation followed by deodorized, degummed, refined, and bleached oil during 6 d storage at 55 °C in the dark (Jung and others 1989). Higher oxidative stability of crude oil than refined oil was suggested to be partly due to higher concentrations of tocopherols in crude oil (1670 ppm) than refined oil (1546 ppm). The induction time in hexane-extracted rapeseed oil oxidation at 90 °C was 10.5 ± 1.9 h compared with an induction time of 8.1 ± 0.7 h for pressed rapeseed oil (Krygier and Platek 1999). Oxidative stability was significantly lower in supercritical carbon dioxide-extracted walnut oil than in pressed oil (Crowe and White 2003). Roasting of safflower and sesame seeds before oil extraction improved the oxidative stability of their oils (Yen and Shyu 1989; Lee and others 2004), partly due to the Maillard products produced during roasting. Some Maillard reaction products were reported to be antioxidants. Oxidative stability increased as the roasting and expelling temperatures of the seeds increased.

Temperature and light

Autoxidation of oils and the decomposition of hydroperoxides increase as the temperature increases (Shahidi and Spurvey 1996; St. Angelo 1996). The formation of autoxidation products during the induction period is slow at low temperature (Velasco and Dobarganes 2002). The concentration of the hydroperoxides increases until the advanced stages of oxidation. The content of polymerized compounds increases significantly at the end of the induction period of autoxidation (Marquez-Ruiz and others 1996). The hydroperoxide decomposition rate of crude herring oil stored at 50 °C in the dark was higher than the formation rate of hydroperoxide. The reverse phenomena were observed in the same crude herring oil at 0 or 20 °C in the dark (Aidos and others 2002).

Temperature has little effect on 1O2 oxidation due to the low activation energy of 0 to 6 kcal/mole (Yang and Min 1994; Rahmani and Saari Csallani 1998). Light is much more important than temperature in 1O2 oxidation. Light of shorter wavelengths had more detrimental effects on the oils than longer wavelengths (Sattar and others 1976). Reportedly, the effect of light on oil oxidation becomes less as temperature increases (Velasco and Dobarganes 2002).

Because 1O2 oxidation occurs in the presence of light, the packaging of oils is very important. Transparent plastic bottles increase oil oxidation. The incorporation of Tinuvin 234 (2-(2-hydroxy-3,5-di(1,1-dimethylbenzyl)phenyl) benzotriazole) or Tinuvin 326 (2-(3'-tert-butyl-2'-hydroxy-5'-methylphenyl)-5-chlorobenzo-triazole), which is a UV absorber, into the transparent plastic bottles improved oxidative and sensory stability of soybean oil under light (Pascall and others 1995; Azeredo and others 2003).

Oxygen

The oxidation of oil can often take place when oil, oxygen, and catalysts are in contact. Both concentration and type of oxygen affect the oxidation of oils. The oxygen concentration in the oil is dependent on the oxygen partial pressure in the headspace of the oil (Andersson 1998). A higher amount of oxygen is dissolved in the oil when the oxygen partial pressure in the headspace is high. Oxidation of the oil increased with the amount of dissolved oxygen (Min and Wen 1983). The solubility of oxygen is higher in the oil than in water, and also in crude oil than refined oil (Aho and Wahroos 1967). One gram of soybean oil dissolves 55 μg oxygen at room temperature (Andersson 1998). The amount of oxygen dissolved in oil is sufficient to oxidize the oil to a peroxide value of approximately 10 meq/kg in the dark (Przybylski and Eskin 1988). Min and Wen (1983) reported that the rate constants for oxygen disappearance in soybean oil containing 2.5, 4.5, 6.5, and 8.5 ppm dissolved oxygen during storage at 55 °C in the dark were 0.049, 0.058, 0.126, and 0.162 ppm/h, respectively.

The effect of oxygen concentration on the oxidation of oil increased at high temperature and in the presence of light and metals such as iron or copper. Higher oxygen dependence of oil oxidation at high temperature is due to low solubility of oxygen in the oil at high temperature (Andersson 1998). The oxygen has to be transported into the oil by diffusion when the oil is not stirred, such as during storage at low temperature. Convection is another important pathway for oxygen penetration into the oil from the surface when the oil is stirred, for example, during processing at high temperature.

Oil oxidation rate is independent of oxygen concentration at sufficiently high oxygen concentrations, for example, above 10% in the oxidation of methyl linoleate (Kacyn and others 1983). When the oxygen content is low, the oxidation rate is dependent on oxygen concentration and is independent of lipid concentration (Andersson 1998). The autoxidation rate of oil at more than 4% to 5% oxygen in the headspace was independent of oxygen concentration and was directly dependent on the lipid concentration (Labuza 1971). However, the reverse was true at low oxygen pressure of less than 4% in the headspace (Karel 1992). The oxidation of rapeseed oil at 50 °C in the dark, measured as oxygen consumption or peroxide values, became faster as the oxygen concentration increased at less than 0.5% oxygen in the headspace, whereas the oxidation rate decreased with oxygen concentration at more than 1% oxygen (Andersson and Lingnert 1999).

Oxygen and edible oil can react more efficiently when an oil sample size is small or an oil sample has a high ratio of surface to volume (Tan and others 2002; Kanavouras and others 2005). When the surface-to-volume ratio increases, the relative rate of oxidation is less oxygen-dependent with a low oxygen content. The container surface can act as a reduction catalyst, and its effect was shown to be proportional to the area of the container in contact with the oils (Brimberg and Kamal-Eldin 2003).

The interaction between temperature and oxygen concentration affects the volatile formation in rapeseed oil in the dark; production of 2-pentenal and 1-pentene-3-one correlated positively with oxygen concentration at 50 °C, but negatively at 35 °C (Andersson and Lingnert 1999).

The reaction rate between lipid and oxygen is dependent on the type of oxygen; the reaction rate of 1O2 with lipid is much higher than that of 3O2 because 1O2 can directly react with lipids. 3O2 reacts with the radical state of lipids. Linoleates react with 1O2 at a rate 1,450 times faster than with 3O2 (Rawls and Van Santen 1970).

Minor components present in oil

Edible oil consists of mostly triacylglycerols, but it also contains minor components such as free fatty acids, mono- and diacylglycerols, metals, phospholipids, peroxides, chlorophylls, carotenoids, phenolic compounds, and tocopherols. Some of them accelerate the oil oxidation and others act as antioxidants.

Free fatty acid and mono- and diacylglycerols Crude oil contains free fatty acids, and oil processing, such as refining, decreases the free fatty acid contents. Crude soybean oil contains about 0.7% free fatty acids; however, refined oil contains 0.02% free fatty acids (Jung and others 1989). Sesame oil extracted from roasted sesame seeds contains 0.72% free fatty acids and bleaching with acid clay decreases free fatty acid contents of the oil to 0.56% (Kim and Choe 2005). Free fatty acids are more susceptible to autoxidation than esterified fatty acids (Kinsella and others 1978). Free fatty acids act as prooxidants in edible oil (Miyashita and Takagi 1986; Mistry and Min 1987a). They have hydrophilic and hydrophobic groups in the same molecule and prefer to be concentrated on the surface of edible oils. The hydrophilic carboxy groups of the free fatty acids will not easily dissolve in the hydrophobic edible oil and are present on the surface of edible oil. Mistry and Min (1987a) reported that free fatty acids decrease the surface tension of edible oil and increase the diffusion rate of oxygen from the headspace into the oil to accelerate oil oxidation.

Mono- and diacylglycerol, usually present at 0.07% to 0.11% and 1.05% to1.20% in soybean oil, respectively, acted as prooxidants to increase oxidation at 55 °C in the dark (Mistry and Min 1987b; Mistry and Min 1988). The mono-and diacylglycerols, which have hydrophilic hydroxy groups and hydrophobic hydrocarbons, decrease the surface tension of edible oil and increase the diffusion rate of oxygen from the headspace to the oil to accelerate the oxidation of oil. Mono- and diacylglycerols should be removed from the oil during the oil-refining process to improve the oxidative stability of edible oils (Mistry and Min 1988).

Metals Crude oil contains transition metals such as iron or copper. For example, crude soybean oil contains 13.2 ppb and 2.80 ppm of copper and iron, respectively. However, refining reduces their contents. Edible oils manufactured without refining, such as extra virgin olive oil and sesame oil, contain relatively high quantities of transition metals (Table 4). Metals increase the rate of oil oxidation due to the reduction of activation energy of the initiation step in the autoxidation down to 63∼104 kJ/mol (Jadhav and others 1996). Metals react directly with lipids to produce lipid alkyl radicals. They also produce reactive oxygen species such as 1O2 and hydroxy radical from 3O2 and hydrogen peroxide, respectively (Andersson 1998). Lipid alkyl radical and reactive oxygen species accelerate oil oxidation. Copper accelerates hydrogen peroxide decomposition 50 times faster than ferrous ion (Fe2+), and ferrous ion acts 100 times faster than ferric ion (Fe3+):

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Table 4—.  Copper and iron contents in edible oils
OilsMetal contenta
Copper (ppb)Iron (ppm)
  1. aNumbers in parentheses are the ranges reported.

Cold-pressed sesame oilb16 (3.0–38)1.16 (0.18–1.52)
Crude soybean oilc13.22.80
Virgin olive oilb9.8 (1.0–79)0.73 (nd–9.79)
Cold-pressed sunflower oilb5.2 (2.2–8.5)0.26 (0.22–0.31)
Refined olive oild150.08
Refined soybean oilc2.50.20

Metals also accelerate autoxidation of oil by decomposing hydroperoxides (Benjelloun and others 1991):

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Fe2+ is much more active with a rate of 1.5 × 103 M−1s−1 (Halliwell and Gutteridge 2001) than Fe3+ in decomposing the lipid hydroperoxides to catalyze autoxidation (Mei and others 1998). Fe3+ also causes decomposition of phenolic compounds such as caffeic acid in olive oil and decreases oil oxidative stability (Keceli and Gordon 2002).

Shiota and others (2006) reported that the prooxidant activity of iron was suppressed by lactoferrin in fish oil or soybean oil oxidation at 50 °C to 120 °C, possibly due to the iron-binding ability of lactoferrin. Depletion of iron from the surroundings by lactoferrin can suppress the iron-catalyzed oxidation of the oil.

Phospholipids Crude oils contain phospholipids such as phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, phsphatidylserine, and phosphatidic acid, but most of them are removed by processing such as degumming. Crude soybean oil contains phosphatidylcholine and phosphatidylethanolamine at 500.8 and 213.6 ppm, respectively. However, refined, bleached, and deodorized (RBD) soybean oil contains 0.86 and 0.12 ppm of phosphatidylcholine and phosphatidylethanolamine, respectively (Yoon and others 1987). Phospholipids act as antioxidants and prooxidants depending on their concentration and presence of metals. Oxidation of docosahexaenoic acid at 25 °C to 30 °C in the dark decreased when mixed with phosphatiylcholine at a molar ratio of 1:1 (Lyberg and others 2005).

The mechanism of the antioxidative effects of phospholipids has not yet been elucidated in detail, but the polar group plays an important role and the nitrogen-containing phospholipids such as phosphatidylcholine and phosphatidylethanolamine are efficient antioxidants under most conditions (King and others 1992). Phospholipids decrease oil oxidation by sequestering metals, and concentration for the maximal antioxidant activity was between 3 and 60 ppm. Soybean oil oxidation decreased with addition of 5 to10 ppm phospholipids, and higher amount of phospholipids acted as prooxidants. Yoon and Min (1987) reported that phospholipids acted as antioxidants only in the presence of Fe2+ by chelating iron. In purified soybean oil, which did not contain any metals, phospholipids worked as prooxidants. Phospholipids have hydrophilic and hydrophobic groups in the same molecule. The hydrophilic groups of the phospholipids are on the surface of oil and hydrophobic group are in the edible oil. The phospholipids decrease the surface tension of edible oil and increase the diffusion rate of oxygen from the headspace to the oil to accelerate oil oxidation. Soybean oil oxidation in the dark at 60 °C was the lowest with phosphatidic acid and phosphatidylethanolamine, followed by phosphatidylcholine, phosphatidylglycerol, and phosphatidylinositol (Yoon and Min 1987).

Chlorophylls Chlorophylls are common pigments present in edible vegetable oil. Virgin olive oil and rapeseed oil contain chlorophyll at 10 ppm and 5 to 35 ppm, respectively (Salvador and others 2001). Virgin olive oil also contains 4 to 15 ppm pheophytin (Psomiadou and Tsimidou 2002). The chlorophyll concentrations in crude and bleached soybean oils are 0.30 and 0.08 ppm, respectively (Jung and others 1989). Chlorophylls are generally removed during oil processing, especially the bleaching process (Table 5).

Table 5—.  Chlorophyll contents of canola oil during processing (ppm)a
OilChlorophyll aPheophytin aPheophytin bPyropheophytin aPyropheophytin b
Extracted1.883.311.3416.57 3.13
Degummed0.277.161.079.401.84
Refined0.226.271.129.131.79
Bleached0.560.320.210.25

Chlorophylls and their degradation products, pheophytins and pheophorbides, act as sensitizers to produce 1O2 in the presence of light and atmospheric 3O2, and accelerate the oxidation of oil (Fakourelis and others 1987; Whang and Peng 1988; Gutierrez-Rosales and others 1992). Pheophytins have a higher sensitizing activity than chlorophylls, but lower than that of pheophorbides (Endo and others 1984; Rahmani and Csallany 1998). Soybean oil purified by silicic acid column chromatography did not contain any chlorophylls and did not produce volatiles in the headspace under light at 10 °C, but purified soybean oil with added chlorophyll and RBD soybean oil produced headspace volatiles under the same experimental conditions (Lee and Min 1988). Also, appreciable amounts of volatiles were produced in soybean oil containing chlorophyll only under light, but not in the dark. The headspace volatile formation in soybean oil under light at 10 °C increased as the concentration of chlorophyll increased. Rahmani and Csallani (1998) showed that the oxidation of virgin olive oil containing pheophytin was accelerated by the illumination of fluorescent light.

Although chlorophylls are strong prooxidants under light via acting as a sensitizer to produce 1O2, they act as antioxidants in the dark possibly by donating hydrogen to free radicals (Endo and others 1985; Francisca and Isabel 1992).

Thermally oxidized compounds Processing of crude oils to refined oils is generally performed at high temperature, and it can produce oxidized compounds such as cyclic and noncyclic carbon-to-carbon-linked dimers and trimers, hydroxy dimers, and dimers and trimers joined through carbon-to-oxygen linkage. The RBD soybean oil contains 1.2% thermally oxidized compounds (Yoon and others 1988). Thermally oxidized compounds accelerated the soybean oil autoxidation at 55 °C (Yoon and others 1988). The acceleration of oil oxidation increases with the concentration of thermally oxidized compounds. Lipid hydroperoxides were also shown to act as prooxidants (Hahm 1988). The oxidized compounds formed by hydroperoxide decomposition can act as an emulsifier, which contain both hydrophilic and hydrophobic groups, lower surface tension in the oil, and increase the introduction of oxygen into the oil to accelerate oil oxidation (Min and Jung 1989).

Antioxidants Edible oils naturally contain antioxidants such as tocopherols, tocotrienols, carotenoids, phenolic compounds, and sterols. Antioxidants are sometimes intentionally added to oil to improve oxidative stability. Antioxidants are compounds that extend the induction period of oxidation or slow down the oxidation rate. Antioxidants scavenge free radicals such as lipid alkyl radicals or lipid peroxy radicals, control transition metals, quench singlet oxygen, and inactivate sensitizers.

Antioxidants can donate hydrogen atoms to free radicals and convert them to more stable nonradical products (Decker 2002). The major hydrogen-donating antioxidants are monohydroxy or polyhydroxy phenolic compounds with various aromatic ring substitutions. Any compound whose reduction potential is lower than that of a free radical can donate hydrogen to that radical unless the reaction is kinetically unfavorable. Standard 1-electron reduction potentials of alkoxy, peroxy, and alkyl radicals of polyunsaturated fatty acids are 1600, 1000, and 600 mV, respectively (Buettner 1993). The standard reduction potential of antioxidants is generally 500 mV or below. This clearly shows that antioxidants react with lipid peroxy radicals before the peroxy radicals react with other lipid molecules to produce another free radical. Any antioxidant radical produced from the reaction with lipid peroxy radical has lower energy than the lipid peroxy radical itself due to resonance structure (Figure 11).

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Figure 11—. Resonance stabilization of an antioxidant radical

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Metal chelators such as phosphoric acid, citric acid, ascorbic acid, and EDTA (ethylenediaminetetraacetic acid) decrease oil oxidation in an indirect way. They can convert iron or copper ions into insoluble complexes or can sterically hinder the formation of the complexes between metals and lipid hydroperoxides (Halliwell and others 1995). Citric acid improved the sensory quality of soybean oil containing 1 ppm iron during storage at 55 °C (Min and Wen 1983). Citric acid is often added to oil in the field to reduce its oxidation during storage before processing. Min and Wen (1983) reported that the antioxidant effects of citric acid increased as its content increased; and more than 150 ppm citric acid was necessary to overcome the catalytic effect of 1 ppm iron. When oil contains 0.1 ppm or less iron such as in RBD oil, the common practice of adding about 150 ppm citric acid to the oil for the improvement of oxidative stability is not necessary (Min and Wen 1983).

Some antioxidants quench 1O2 or excited sensitizers. 1O2 is quenched physically and chemically. In physical quenching, 1O2 is converted into 3O2 by either energy transfer or charge transfer, and there is no oxidation of antioxidants. In chemical quenching, antioxidants react with 1O2 and produce oxidized antioxidants (Min and Boff 2002).

Tocopherols

Tocopherols are the most important antioxidants present in edible oil. Animal fats also contain tocopherols, but at lower concentrations (Table 6). Among vegetable oils, soybean, canola, sunflower, and corn oils contain relatively high amounts of tocopherols. Palm oil does not have large amounts of tocopherols (118 to 146 ppm); however, it contains a higher concentration of tocotrienols, that is, α-, γ-, and δ-tocotrienols at 211, 353 to 372, and 56 to 67 ppm, respectively (Al-Saqer and others 2004). Safflower oil also contains γ- and δ-tocotrienols in addition to tocopherols at 3.8 to 7.0 and 7.5 to 8.4 ppm, respectively (Lee and others 2004). Tocopherol contents of edible oils are affected by the cultivar, processing, and storage of the oil (Deiana and others 2002). Tocopherol contents of sesame oil range between 404 and 540 ppm depending on the cultivars (Mohamed and Awatif 1998). The tocopherol contents of rapeseed oil are 794 and 749 ppm for hexane-extracted oil and pressed oil, respectively (Krygier and Platek 1999). The refining process, especially deodorization, reduces tocopherol contents (Jung and others 1989; Reische and others 2002; Eidhin and others 2003). The crude, bleached, and deodorized soybean oil contains tocopherols at 1670, 1467, and 1138 ppm, respectively (Jung and others 1989). In virgin olive oil, α-tocopherol concentration decreased with storage time of the oil in the dark (Deiana and others 2002). There was no tocopherol left in olive oil stored in the dark at room temperature for 12 mo (Morello and others 2004).

Table 6—.  Tocopherol contents of edible oil
OilTocopherol (ppm)
αβγδTotal
Soybeana116.034.0737.0275.01162
Canolaa272.1 0.1423.2695.4
Sunflowera613.017.018.9648.9
Corna134.0 18.0412.0 39.0603  
Roasted sesameb4584   9 597  
Rapeseedc252   314   566  
Safflowerd386∼5208.6∼12.42.4∼7.7397∼540
Olivee168∼226168∼226
Beef tallowf 30.43.8 34.2
Lardg 18.0 18.0

Tocopherols compete with unsaturated fats and oils for lipid peroxy radicals. Lipid peroxy radicals react with tocopherols much faster at 104 to 109 M−1s−1 than with lipids (10 to 60 M−1s−1). One tocopherol molecule can protect about 103 to 108 polyunsaturated fatty acid molecules at low peroxide value (Kamal-Elden and Appelqvist 1996). Tocopherols can transfer a hydrogen atom at the 6-hydroxy group on its chroman ring to lipid peroxy radical and scavenge the peroxy radicals. Tocopherol (T), with a reduction potential of 500 mV, donates hydrogen to lipid peroxy radicals (ROO·) that have a reduction potential of 1000 mV, and produces lipid hydroperoxide (ROOH) and tocopheroxy radicals (T·). Tocopheroxy radicals are more stable than lipid peroxy radicals due to resonance structures. This ultimately slows down the oil oxidation rate in the propagation stage of autoxidation. Reaction rates of peroxy radicals in stearic and oleic acids with α-tocopherol were reported to be 2.8 × 106 and 2.5 ×106 M−1 s−1, respectively (Simic 1980). Tocopheroxy radicals can interact with other compounds or each other, depending on the lipid oxidation rates. Tocopheroxy radicals react with each other at low lipid oxidation rates and produce tocopheryl quinone and tocopherol. At higher oxidation rates, tocopheroxy radicals may react with lipid peroxy radicals and produce tocopherol–lipid peroxy complexes ([T-OOR]); these can be hydrolyzed to tocopheryl quinone and lipid hydroperoxide (Liebler and others 1990):

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The effectiveness of tocopherols as antioxidants depends on the isomers and concentration of tocopherols. Free radical scavenging activity of tocopherols is the highest in δ-tocopherol followed by γ-, β-, and α-tocopherol (Reische and others 2002). α-Tocopherol was more effective in reducing oil oxidation than γ-tocopherol up to 200 ppm and less effective above this concentration (Yanishlieva and others 2002).

The optimal concentration of tocopherols as antioxidants is dependent on their oxidative stability; the lower the oxidative stability of the tocopherol, the lower the optimal concentration of tocopherol for the maximal antioxidant activity. α-Tocopherol, which is the least stable among tocopherol isomers, showed the maximal antioxidant activity at 100 ppm in the oxidation of soybean oil at 55 °C in the dark, but the optimal concentrations of γ- and δ-tocopherols as an antioxidant were 250 ppm and 500 ppm, respectively (Jung and Min 1990).

Tocopherols, particularly α-tocopherol, act as prooxidants when present in high concentrations in vegetable oils via propagation of free radicals, depending on the hydroperoxide concentration (Cillard and others 1980; Terao and Matsushita 1986; Jung and Min 1990). When the concentration of lipid peroxy radical is very low, the tocopheroxy radical abstracts hydrogen from the lipid and produces tocopherol and lipid alkyl radical; however, the reaction rate is very low. Formation of lipid alkyl radicals by tocopherols accelerates lipid oxidation, which is called tocopherol-mediated peroxidation (Bowry and Stocker 1993; Yamamoto 2001). The highest prooxidant activity was shown in α-tocopherol followed by γ- and δ-tocopherol in soybean oil autoxidation (Jung and Min 1990). Addition of 100 ppm α-tocopherol increased the oxidation of purified olive oil at an early stage of autoxidation; however, α-tocopherol added to moderately oxidized (POV = 15) purified olive oil or lard significantly decreased the oil oxidation (Blekas and others 1995). The threshold value for α-tocopherol as a prooxidant in virgin olive oil oxidation was 60 to 70 ppm. When less α-tocopherol was initially present in the oil, the threshold value for prooxidant activity was reached more quickly (Deiana and others 2002). Prooxidant activity of α-tocopherol decreased as the temperature increased, even at a high concentration (Marinova and Yanishlieva 1992). Ascorbic acid (Asc) can reduce tocopheroxy radical and prevent tocopherol-mediated peroxidation (Yamamoto 2001):

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Oxidized tocopherols increase soybean oil oxidation and prooxidant activity was the highest in oxidized α-tocopherol followed by oxidized γ- and δ-tocopherol (Jung and Min 1992). Prevention of tocopherol oxidation and removal of oxidized tocopherols during oil processing are strongly recommended to improve oil oxidative stability.

In addition to free radical scavenging activity, tocopherols decrease soybean oil oxidation under light by only 1O2 quenching (Min and Lee 1988). 1O2 quenching rate by α-tocopherol was reported to be 2.7 × 107 M−1s−1 (Jung and others 1991). 1O2 quenching effect in soybean oil oxidation under light is dependent on tocpherol concentration and the type; at 1 × 10−3 M, the activity was in the decreasing order of α-, γ-, and δ- tocopherol; however, there was no significant difference in 1O2 quenching activity among tocopherols at 4 × 10−3 M (Jung and others 1991). Tocopherols can form a charge transfer complex ([T+1O2]1) with 1O2 by electron donation to 1O2. The singlet state of tocopherol–1O2 complex undergoes intersystem crossing into the triplet state ([T+1O2]3) and produces less reactive 3O2 and tocopherol. Since this does not involve chemical reactions between tocopherol and 1O2, it is called physical quenching:

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Tocopherols react irreversibly with 1O2 in chemical quenching and produce tocopherol hydroperoxydienone, tocopheryl quinone, and quinone epoxide (Figure 12). The reaction rate of tocopherols with 1O2 is affected by their structures. α-Tocopherol showed the highest reaction rate of 2.1 × 108 M−1s−1, followed by β-tocopherol with 1.5 × 108 M−1s−1, γ-tocopherol with 1.4 × 108 M−1s−1, and δ-tocopherol with 5.3 × 107 M−1s−1 (Mukai and others 1991).

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Figure 12—. Singlet oxygen oxidation of α-tocopherol

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Carotenoids

Carotenoids are a group of tetraterpenoids consisting of isoprenoid units. Double bonds in carotenoids are conjugated forms and usually are all trans forms. β-Carotene is one of the most studied carotenoids. Edible oils, especially unrefined oils, contain β-carotene. Crude palm oil and red palm olein contain 500 to 700 ppm carotenoids (Bonnie and Choo 2000). Virgin olive oil contains 1.0 to 2.7 ppm β-carotene as well as 0.9 to 2.3 ppm lutein (Psomiadou and Tsimidou 2002).

β-Carotene can slow down oil oxidation by light filtering, 1O2 quenching, sensitizer inactivation, and free radical scavenging. Fakourelis and others (1987) reported that oxidation of olive oil containing only β-carotene under light at 25 °C was decreased by filtering out some light energy, mainly between 400 and 500 nm. In the co-presence of chlorophylls, β-carotene decreased the oxidation of soybean oil stored under light by 1O2 quenching (Lee and Min 1988). 1O2 quenching by carotenoids is mainly by energy transfer from 1O2 to carotenoids, without generating oxidized products. Excited carotenoids return to the ground state by giving off heat:

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One mole of β-carotene can quench 250 to 1000 molecules of 1O2 at a rate of 1.3 × 1010M−1s−1 (Foote 1976). 1O2 quenching of carotenoids is dependent on the number of conjugated double bonds (Beutner and others 2001). Carotenoids having at least 9 conjugated double bonds act as an effective 1O2 quencher; β-carotene, lycopene, and lutein are good 1O2 quenchers; however, phytoene, phytofluene, and ζ-carotene, which are precursor compounds of lycopene, are not. The 1O2 quenching activity of carotenoids increases as the number of conjugated double bonds increases (Min and Boff 2002; Foss and others 2004). Lycopene and α-carotene demonstrate higher 1O2 quenching ability than β-carotene (Miller and others 1996).

Carotenoids inactivate excited sensitizers physically by absorbing energy from sensitizers. The excited carotenoids return to the ground state by transferring the energy to the oil (Stahl and Sies 1992). Lee and others (2003) reported that β-carotene with a high 1-electron reduction potential of 1060 mV had great difficulty in donating hydrogen to alkyl (Eo′= 600 mV) or peroxy radical (Eo′= 770 to1440 mV) of polyunsaturated fatty acid. Nuclear magnetic resonance study showed that β-carotene did not donate a hydrogen atom to quench these free radicals (Lee and others 2003). However, β-carotene can donate hydrogen to hydroxy radical, which has a high reduction potential (2310 mV), and produces a carotene radical (Car·). A carotene radical is a fairly stable species due to delocalization of unpaired electrons through its conjugated polyene system. At low oxygen concentration, a carotene radical may react with other radicals such as lipid peroxy radicals and form nonradical products (Burton and Ingold 1984; Beutner and others 2001):

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A lipid peroxy radical may be added to β-carotene and produce carotene peroxy radical (ROO–Car·), especially at higher than 150 mm Hg of oxygen (Burton and Ingold 1984). Carotene peroxy radical reacts with 3O2 and then with lipid molecules, and produces lipid alkyl radicals, which propagate the chain reaction of lipid oxidation (Iannone and others 1998):

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Antioxidant activity of β-carotene was not shown in soybean oil stored in the dark (Lee and others 2003). During photosensitized oxidation of soybean or rapeseed oil in open vessels, β-carotene increased oil oxidation, but the co-presence of tocopherols decreased oxidation of the oil (Haila and Heinonen 1994). The antioxidant activities of carotenoids by hydrogen donation remain controversial.

β-Carotene may also donate electrons to free radicals and become a β-carotene cation radical (Liebler 1993; Mortensen and others 2001). The transfer of hydrogen or electrons from carotenoids to free radicals depends on the reduction potentials of the free radicals and chemical structures of the carotenoids, especially the presence of oxygen-containing functional groups (Edge and others 1997). The β-carotene cation radical has a relatively high standard 1-electron reduction potential (1060 mV), and does not readily react with oxygen to form peroxide (Edge and others 2000). The β-carotene cation radical may react with alkyl, alkoxy, or peroxy radicals formed in soybean oil during oxidation. Lee and others (2003) reported that β-carotene was a prooxidant in soybean oil oxidation in the dark due to the electron transfer mechanism in their 2, 2-diphenyl-1-picryl hydrazyl and NMR study.

Carotenoids are degraded by hydroperoxides to hydroxyl- or epoxycarotenes, and the degradation slows down at higher concentration; the rate of degradation of carotenes is lycopene > β-carotene ≈α-carotene (Anguelova and Warthesen 2000).

Other phenolic compounds

Phenolic compounds other than tocopherols in edible oils also exert antioxidant activity. Sesame oil, which contains a high amount of unsaturated fatty acids (iodine value = 109), has good oxidative stability (Tan and others 2002). The autoxidation rate of sesame oils at 60 °C was much lower than that of corn oil, safflower oil, and a mixture of soybean and rapeseed oils (Fukuda and Namiki 1988). Roasted sesame oil was more stable for the oxidation than unroasted sesame oil (Yoshida and Takagi 1997; Yoshida and others 2000). The remarkable oxidative stability of sesame oil is due to the presence of lignan compounds as well as tocopherols (Yoshida 1994; Namiki 1995). Lignan compounds in sesame oil include sesamin, sesamol, sesamolin, sesaminol, and sesamolinol (Figure 13). Sesamin is the predominant lignan compound in unroasted sesame oil (474 ppm) followed by sesamolin (159 ppm). Sesamol concentration in unroasted sesame oil was less than 7 ppm (Fukuda and others 1986; Dachtler and others 2003); however, roasted sesame oil contains a higher concentration of sesamol (36 ppm) than unroasted oil (Kim and Choe 2005). Sesamol is produced by hydrolysis of sesamolin during oil processing such as heating and bleaching (Fukuda and others 1985; Osawa and others 1985; Kim and Choe 2005). Sesamol is converted to sesamol dimer and then to sesamol dimer quinone (Kurechi and others 1981; Kikugawa and others 1983). Sesamol and sesaminol showed higher antioxidant activity than sesamin in sunflower oil autoxidation by scavenging radicals (Dachtler and others 2003).

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Figure 13—. Phenolic acids present in oils

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Sesamol acts as an antioxidant in the oxidation under light as well as in the dark. The antioxidation by sesamol in chlorophyll-sensitized photooxidation of soybean oil was lower than by α- tocopherol, similar to δ-tocopherol, and higher than DABCO (diazabicyclo[2,2,2]octane) at the same molar concentration (Kim and others 2003). The decrease of photooxidation of soybean oil by sesamol was due to its 1O2 quenching, and the quenching rate was 1.9 × 107 M−1 s−1 at 20 °C (Kim and others 2003).

Sesame oil also contains phytosterols such as campesterol, stigmasterol, β-sitosterol, and 4,5-avenasterol, with β-sitosterol as the predominant sterol (Dachtler and others 2003). Sitosterol behaves partly as a prooxidant by increasing the solubility of oxygen in the oil (Yanishlieva and Schiller 1983) and partly as a weak antioxidant in sunflower oil and lard by competing with lipid molecules for oxidation at the oil surface (Maestroduran and Borjapadilla 1993; Brimberg and Kamal-Eldin 2003).

Olive oil, which is very stable to autoxidation (Guillen and Cabo 2002), contains phenolic compounds and tocopherols. Phenolic compounds in olive oil include tyrosol (4-hydroxyphenylethanol), hydroxytyrosol (3,4-dihydroxy-phenylethanol), hydroxybenzoic acids, oleuropein, caffeic acid, vanillic acid, p-coumaric acid, and derivatives of tyrosol and hydroxytyrosol (Papadopoulos and Boskou 1991; Tsimidou and others 1992), as shown in Table 7. Contents of tyrosol, hydroxytyrosol, and phenolic acids in olive oil were 34.9, 37.8, and 36.3 ppm, respectively (Keceli and Gordon 2002). The phenolic compounds in olive oil acted as antioxidants mainly at the initial stage of autoxidation (Deiana and others 2002) by scavenging free radicals and chelating metals (Chimi and others 1991). Hydroxytyrosol was the most effective antioxidant in olive oil oxidation (Papadopoulos and Boskou 1991; Tsimidou and others 1992; Baldioli and others 1996). Among phenolic compounds, o-diphenols such as caffeic acid are oxidized to quinones by ferric ions and become ineffective in inhibiting iron-dependent free radical chain reactions in oil (Keceli and Gordon 2002). However, hydroxytyrosol, tyrosol, vanillic acids, and p-coumaric acid were not oxidized by the ferric ions. Fki and others (2005) reported high radical scavenging effects of 3,4-dihydroxyphenyl acetic acid present in olive mill wastewater on the oxidation of olive and husk refined oils. Chlorogenic acid and caffeic acid are the principal phenols found in sunflower oil (Leonardis and others 2003). Osborne and Akoh (2003) reported prooxidant effects of polar phenolic acids such as quercetin and gallic acid on a canola oil and caprylic acid structured lipid in the presence of iron at pH 3.0, due to an increased ability to reduce iron.

Table 7—.  Phenolic compounds in virgin olive oila
Phenolic alcohols3,4-Dihydroxyphenyl ethanol (3,4-DHPEA, hydroxytyrosol), p-hydroxyphenylethanol (p-HPEA, tyrosol), 3,4-dihydroxyphenylethanol-glucoside 
Phenolic acid and its derivativesVanillic acid, syringic acid, p-coumaric acid, o-coumaric acid, gallic acid, caffeic acid, protocatechuic acid, p-hydroxybenzoic acid, ferulic acid, cinnamic acid, 4-(acetoxyethyl)-1,2-dihydroxybenzene, benzoic acid 
SecoiridoidsDialdehydic form of elenolic acid linked to 3,4-DHPEA, dialdehydic form of elenolic acid linked to p-HPEA, oleuropein aglycon, ligstroside aglycon, oleuropein, p-HPEA derivative 
Lignans1-Acetoxypinoresinol, pinoresinol, 1-hydroxypinoresinol 
FlavonesApigenin, luteolin 

Antioxidant interactions

Edible oil often contains multicomponent antioxidants and demonstrates interactions among them. Metal chelator and a free radical scavenger demonstrate better antioxidant activities in oil autoxidation when present together than when used separately. The improved antioxidation of oil by a combination of metal chelator and free radical-scavenging antioxidant is caused mainly by the action of the chelator at the initiation step of oxidation and that of the free radical scavenger at the propagation step.

Because tocopherols are the most common antioxidants in edible oil, they have been used in research on interactions among antioxidants. α-Tocopherol showed synergistic effects with β-carotene to decrease the autoxidation (Palozza and Krinsky 1992) and photosensitized oxidation of soybean oil (Choe and Min 1992). The synergistic effects of tocopherol and β-carotene were suggested to be due to the protection of β-carotene from degradation by tocopherols (Shibasaki-Kitakawa and others 2004). α-Tocopherol shows synergism with ascorbic acid and phospholipids. Ascorbic acid gives hydrogen to the tocopheroxy radical produced from the reaction between the lipid peroxy radical and tocopherol, and then regenerates tocopherol. Nitrogen moiety of phosphatidylethanolamine and phosphatidylcholine, or the reducing sugar of phosphatidylinositol, donates hydrogen or electron to tocopherols and delays the oxidation of tocopherols to tocopheryl quinone (Yoon and Min 1987). Sesamol and sesaminol showed synergistic antioxidant activities with γ-tocopherol in the autoxidation of sunflower oil (Dachtler and others 2003).

A combination of 11 ppm β-carotene, 30 ppm citric acid, 3 to 4 ppm Tinuvin P, and 150 to 200 ppm TBHQ decreased the oxidation rate of soybean oil during storage at 25 °C under light for 6 mo (Azeredo and others 2004).

Synergism between antioxidants is affected by hydroperoxide concentration. Olive oil autoxidation was lower when α-tocopherol and phenolic compounds were used together than when used separately (Velasco and Dobarganes 2002); however, α-tocopherol exerted an adverse effect on the antioxidation of olive oil by phenolic compounds such as tyrosol or oleuropein during the period of low hydroperoxide formation, for example, less than 20 meq (Blekas and others 1995).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES

Edible oil undergoes autoxidation and photosensitized oxidation during processing and storage. The oxidation of edible oil produces off-flavor compounds and decreases oil quality. Free fatty acids, mono- and diacylglycerols, metals, chlorophylls, carotenoids, tocopherols, phospholipids, temperature, light, oxygen, oil processing methods, and fatty acid composition affect the oxidative stability of edible oil. To minimize the oxidation of edible oil during processing and storage, it is recommended to decrease temperature, exclude light and oxygen, remove metals and oxidized compounds, and use appropriate concentrations of antioxidants such as tocopherols and phenolic compounds.

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanisms of Autoxidation in Edible Oil
  5. Singlet Oxygen Formation and Mechanisms of Photosensitized Oxidation of Edible Oil
  6. Factors Affecting the Oxidation of Edible Oil
  7. Conclusions
  8. REFERENCES