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Comprehensive

Food Scienceand

Food Safety

Reviewsin

Mechanismsand Factors

for Edible OilOxidation

Eunok Choe and David B. Min

ABSTRACT: Edible oil is oxidized during processing and storage via autoxidation and photosensitized oxidation, inwhichtripletoxygen(3O2)andsingletoxygen(

1O2)reactwiththeoil,respectively.Autoxidationofoilsrequiresradicalformsof acylglycerols, whereasphotosensitized oxidationdoes notrequire lipidradicals since 1O2 reacts directlywithdouble bonds. Lipid hydroperoxides formed by3 O2 are conjugated dienes, whereas

1 O2 produces both conjugatedandnonconjugated dienes. Thehydroperoxides aredecomposed to produceoff-flavor compounds andtheoil qualitydecreases. Autoxidation of oilis acceleratedbythe presence of freefattyacids, mono- anddiacylglycerols,metalssuchas iron, and thermally oxidized compounds. Chlorophylls and phenolic compounds decrease the autoxidation of oilin the dark, and carotenoids, tocopherols, and phospholipids demonstrate both antioxidant and prooxidant activitydepending on the oil system. In photosensitized oxidation chlorophyll acts as a photosensitizer for the formationof1 O2; however, carotenoids and tocopherols decrease the oxidation through

1 O2 quenching. Temperature, light,oxygen concentration, oil processing, and fatty acid composition also affect the oxidative stability of edible oil.

IntroductionOxidative stability of oils is the resistance to oxidation during

processing and storage (Guillen and Cabo 2002). Resistance tooxidation can be expressed as the period of time necessary to at-tain the critical point of oxidation, whether it is a sensorial changeor a suddenacceleration of the oxidative process (Silva andothers2001). Oxidative stability is an important indicator to determineoil quality and shelf life (Hamilton 1994) because low-molecular-weight off-flavor compounds are produced during oxidation. Theoff-flavor compounds make oil less acceptable or unacceptableto consumers or for industrial use as a food ingredient. Oxidationof oil also destroys essential fatty acids and produces toxic com-pounds and oxidized polymers. Oxidation of oil is very importantin terms of palatability, nutritional quality, and toxicity of edibleoils.

Different chemical mechanisms, autoxidation and photosensi-tized oxidation, are responsible for the oxidation of edible oilsduring processing and storage depending upon the types of oxy-gen. Two types of oxygen can react with edible oils. One is called

atmospheric triplet oxygen, 3O2, and the other is singlet oxy-gen,1O2.

3 O2 reacts with lipid radicals and causes autoxidation,which is a free radical chain reaction. The chemical propertiesof3 O2 to react with lipid radicals can be easily explained by themolecular orbital of the oxygen as shown in Figure 1. The 3 O2inthe ground state with 2 unpaired electrons in the 2p antibond-

MS 20060292 Submitted5/23/06, Accepted 7/13/06. Author Choe is with Dept.of Food and Nutrition, The Inha Univ., Incheon, Korea. Author Min is with Dept.of Food Science and Technology, The Ohio State Univ., Columbus, OH. Directinquiries to author Min (E-mail: [email protected]).

ing orbitals has a permanent magnetic moment with 3 closelygrouped energy states under a magnetic field and is called tripletoxygen. 3O2is a radical with 2 unpaired orbitals in the molecule.It reacts with radical food compounds in normal reaction condi-

tions according to spin conservation. Photosensitized oxidationof edible oils occurs in the presence of light, sensitizers, and at-mospheric oxygen, in which 1 O2 is produced.

The electron configuration in the 2p antibonding orbitals of1O2 is shown in Figure 2. Since 1 orbital of the 2p antibond-ing orbitals of1O2 has paired electrons and the other is com-pletely empty, 1O2 has 1 energy level under a magnetic fieldand is electrophilic. The nonradical electrophilic singlet oxygenreadily reacts with compounds with high electron densities, suchas the double bonds of unsaturated fatty acids. The 1O2 has anenergy of 93.6 kJ above the ground state of 3O2 (Korycka-Dahland Richardson 1978; Girotti 1998). High-energy 1O2 in solu-tion is deactivated by transferring its energy to the solvent, andits lifetime depends on the solvent. The lifetime of singlet oxygenis about 2, 17, and 700 s in water, hexane, and carbon tetra-

chloride, respectively (Merkel and Kearns 1972; Long and Kearns1975).Oxidation of edible oils is influenced by an energy input such

as light or heat, composition of fatty acids, types of oxygen, andminor compounds such as metals, pigments, phospholipids, freefatty acids, mono- and diacylglycerols, thermally oxidized com-pounds, and antioxidants. Many efforts have been made to im-prove the oxidative stabilities of oils by systematic studies on theeffects of these factors.

This article reviews the reaction mechanism, kinetics, andoxidation products of autoxidation and photosensitized ox-idation. Factors affecting the oxidation of edible oils, freefatty acids, mono- and diacylglycerols, metals, phospholipids,

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CRFSFS: Comprehensive Reviews in FoodScienceandFoodSafety

*

*

S2S2

Energy

2px 2py 2pz 2pz 2py 2px

Molecular

Atomic Atomic

*

1S 1S

* *

Figure1---Molecular orbitalof tripletoxygen,3O2

Figure2---Electronicconfiguration of2p antibonding

orbitalof singletoxygen,1O2

pigments, thermally oxidized compounds, and antioxidants, are

also discussed.

Mechanisms of Autoxidation in Edible OilAutoxidation of oils, free radical chain reaction, includes initi-

ation, propagation, and termination steps:

Initiation RH R + HPropagation R + 3O2 ROO

ROO + RH ROOH + RTermination ROO + R ROOR

R + R RR(R : lipid alkyl)

Autoxidation of oils requires fatty acids or acylglycerols to bein radical forms. Fatty acids or acylglycerols are in nonradicalsinglet states, and the reaction of fatty acids with radical stateatmospheric3 O2 is thermodynamically unfavorable due to elec-tronic spin conservation (Min and Bradley 1992). The hydrogenatom in the fatty acids or acylglycerols in edible oil is removedand lipid alkyl radicals are produced in the initiation step. Heat,metal catalysts, and ultraviolet and visible light can acceleratefree radical formation of fatty acids or acylglycerols. The energyrequired to remove hydrogen from fatty acids or acylglycerolsis dependent on the hydrogen position in the molecules. Thehydrogen atom adjacent to the double bond, especially hydro-gen attached to the carbon between the 2 double bonds, is re-moved easily. Hydrogen at C11 of linoleic acid is removed at

Table1 ---Hydroperoxidesof fatty acids byautoxidationa

Fatty acid Hydroperoxides at Relative amount (%)

Oleic acid C8 2628C9 2225C10 2224C11 2628

Linoleic acid C9 4853C13 4853

Linolenic acid C9 2835C12 813C13 1013C16 2835

a Frankel (1985).

50 kcal/ mol. The energy required to remove hydrogen in C8 andC14of linoleic acid is 75 kcal/ mol andthe homolytic dissociationenergy between hydrogen and C17 or C18 is about 100 kcal/mol(Min and Boff 2002). Thedouble bond adjacent to the carbonrad-ical in linoleic acid shifts to the more stable next carbon and fromthe cis to the trans form. Autoxidation of linoleic and linolenicacids produces only conjugated products. The hydroperoxide po-sitional isomers formed in the autoxidation of oleic, linoleic, andlinolenic acids are shown in Table 1.

The lipid alkyl radical reacts with 3 O2 and forms lipid peroxyradical, another reactive radical. Reaction between lipid alkylradical and 3O2 occurs very quickly at normal oxygen pres-sure and, consequently, the concentration of lipid alkyl radicalis much lower than that of lipid peroxy radical (Aidos and oth-ers 2002). The lipid peroxy radical abstracts hydrogen from otherlipid molecules and reacts with the hydrogen to form hydroper-oxide and another lipid alkyl radical. These radicals catalyze theoxidation reaction, and autoxidation is called the free radicalchain reaction. Figure 3 shows the hydroperoxide formation in

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Mechanisms and factors for edible oil oxidation

CH3(CH2)4 (CH2)7COOH





-H




-H

HOO OOH

+3O2 +H,3O2, H

1213 10 9 13 12 10 9

13 12 10 9 13 12 10 9

13 12 10 9 13 12 10 9

13 12 10 9 13 12 10 9

13-Hydroperoxide 9-Hydroperoxide

Figure3---Hydroperoxide

formation in the

autoxidationof linoleic

acid

the autoxidation of linoleic acid. The rates for the formation oflipid peroxy radical and hydroperoxide depend only on oxygenavailability and temperature (Velasco and others 2003). Whenradicals react with each other, nonradical species are producedand the reaction stops.

The primary oxidation products, lipid hydroperoxides, are rel-atively stable at room temperature and in the absence of metals.However, in thepresenceof metals or at high temperature they arereadily decomposed to alkoxy radicals and then form aldehydes,ketones, acids, esters, alcohols, and short-chain hydrocarbons.The most likely pathway of hydroperoxide decomposition is ahomolytic cleavage between oxygen and the oxygen bond, inwhich alkoxy and hydroxy radicals are produced. The activationenergy to cleave the oxygenoxygen bond is 46 kcal/mol lowerthan that to cleave the oxygenhydrogen bond (Hiatt and others1968). The alkoxy radical then undergoes homolytic-scission ofthe carboncarbon bond and produces oxo-compounds and sat-urated or unsaturated alkyl radicals (Figure 4). After electron rear-

rangement, the addition of hydroxyl radical, or hydrogen transfer,the ultimate secondary lipid oxidation products are mostly low-molecular-weight aldehydes, ketones, alcohols, and short-chainhydrocarbons, as shown in Table 2.

The time for secondary product formation from the primaryoxidation product, hydroperoxide, varies with different oils. Sec-ondary oxidation products are formed immediately after hy-droperoxide formation in olive and rapeseed oils. However, insunflower and safflower oils, secondary oxidation products areformed when the concentration of hydroperoxides is appreciable(Guillen and Cabo 2002).

Most decomposition products of hydroperoxides are responsi-ble for the off-flavor in the oxidized edible oil. Aliphatic carbonylcompounds have more influence on the oxidized oil flavor dueto their low threshold values. Threshold values for hydrocarbons,alkanals, 2-alkenals, and trans, trans-2,4-alkadienals are 90 to2150, 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

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H

CH CH CH

OOH

OH-

CH CH CH

O

AB

O 2

CH CH OHC+

CH CH OH

CH 2 CHOCH CH 2

OH-

B 1

B 2

R 3H

B

A

CH CH CHO +

A1 A2

OH-

R 3H

OH R 2H

R 1

R 2

R 2 R 1

R 1

R 2 R 1

R 2

R 2

R 3

R 2 R 1

R 2R 3

R 2R 2

Figure4 ---Mechanismsofhydroperoxide

decompositionto formsecondaryoxidation

products

2-heptenal (29.5%) andtrans-2-octenal (18.1%), were the majorvolatile compounds detected by the solid-phase microextractionmethod in soybean and corn oils (peroxide value of 5), respec-tively (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 themost significant compound in determining the oxidized flavorof oil, followed by trans, trans-2,4-decadienal, trans, cis-2,4-heptadienal, 1-octen-3-ol, butanal, and hexanal. Hexanal, pen-tane, and 2,4-decadienal were suggested and used as indicatorsto determine the extent of the oil oxidation (Warner and others1978; Przybylski and Eskin 1995; Choe 1997; Heinonen and oth-ers 1997). Trans-2-hexenal, and trans, cis, trans-2,4,7-decatrienaland 1-octen-3-one, were reported to give grass-like and fish-likeflavor in oxidized soybean oil, respectively (Min and Bradley1992). No single flavor compound is mainly responsible for theoxidized flavor of vegetable oils.

Singlet Oxygen Formation and Mechanismsof Photosensitized Oxidation of Edible Oil

Oil oxidation is accelerated by light, especially in the presenceof sensitizers such as chlorophylls. Sensitizers in singlet state ab-sorb light energy very rapidly, in picoseconds, and become ex-

cited. Excited singlet sensitizers can return to their ground statevia emission of light, internal conversion, or intersystem crossing(Figure 5). Fluorescence and heat are produced by emission oflight and internal conversion, respectively. Intersystem crossingresults in excited triplet state of sensitizers.

Excited triplet sensitizers may accept hydrogen or an electronfrom the substrate and produce radicals (type I) as shown inFigure 6. Excited triplet sensitizers react with 3O2and produce su-peroxide anion by electron transfer. Superoxide anion produceshydrogen peroxide, one of the reactive oxygen species by spon-taneous dismutation, and the reaction of hydrogen peroxide withsuperoxides results in singlet oxygen formation by HaberWeissreaction in the presence of transition metals such as iron or copper


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Table2---Secondary oxidationproducts of fatty acidmethyl ester byautoxidationa

Class Oleic acid Linoleic acid Linolenic acid

Aldehydes Octanalb Pentanal PropanalNonanal Hexanal Butanal2-Decenal 2-Octenal 2-Butenal

Decanal 2-Nonenal 2-Pentenal2,4-Decadienal 2-Hexenal

3,6-NonadienalDecatrienal

Carboxylic acid Methyl heptanoate Methyl heptanoate Methyl heptanoateMethyl octanoate Methyl octanoate Methyl octanoateMethyl 8-oxooctanoate Methyl 8-oxooctanoate Methyl nonanoateMethyl 9-oxononanoate Methyl 9-oxononanoate Methyl 9-oxononanoateMethyl 10-oxodecanoate Methyl 10-oxodecanoate Methyl 10-oxodecanoateMethyl 10-oxo-8-decenoateMethyl 11-oxo-9-undecenoate

Alcohol 1-Heptanol 1-Pentanol1-Octene-3-ol

Hydrocarbons Heptane Pentane EthaneOctane Pentane

aFrankel (1985).

Figure5---Excitationanddeactivation ofsensitizer

(Sen; sensitizers, ISC; intersystemcrossing)

(Kellogg and Fridovich 1975):

O2 + O2 + 2H

+ H2O2 + O2

H2O2 + O2 HO + OH

+ 1O2

The excitation energy of triplet sensitizers can be trans-ferred onto adjacent 3O2 to form

1O2 by triplettriplet anni-hilation, and the sensitizers return to their ground singlet state(type II). Kochevar and Redmond (2000) reported that a sensi-tizer molecule may generate 103 to 105 molecules of1 O2 beforebecoming inactive.

The rate of the type I or II processes depends on the kinds ofsensitizers and substrates (Chen and others 2001), and concentra-tions of substrates and oxygen (Foote 1976). Phenols or amines,which are readily oxidized, or readily reducible quinones favorthe type I process. On the other hand, olefins, dienes, and aro-matic compounds, which are not so readily oxidized or reduced,more often favor type II. Photosensitized oxidation of edible oilfollows the singlet oxygen oxidation pathway. 1O2was suggestedto be involved in the initiation of the oil oxidation (Rawls and VanSanten 1970; Lee and Min 1988).

1O2 either reacts chemically with other molecules or transfersits energy to them. When 1 O2reacts with unsaturated fatty acids,

mostly allyl hydroperoxides are formed by ene reaction (Gollnick1978), as shown in Figure 7.

Electrophilic1 O2can directly react with high-electron-densitydouble bonds without the formation of alkyl radical, and formhydroperoxides at the double bonds. When hydroperoxide isformed, double bond migration and trans fatty acid occur, pro-ducing both conjugated and nonconjugated hydroperoxides, asshown in Table 3. Production of nonconjugated hydroperoxidesis not observed in the autoxidation. Figure 8 shows the oxidationpathway of linoleic acid by 1 O2.

Hydroperoxides formed by 1 O2 oxidation are decomposed bythe same mechanisms for the hydroperoxides formed by 3O2in autoxidation. 1O2 oxidation produces more 2-decenal andoctane, two of the decomposition products of hydroperoxides,in oleate than autoxidation does (Frankel 1985). The contentsof octanal and 10-oxodecanoate in autoxidized oleate werehigher than those of 1O2-oxidized oleate. 2-Heptenal and 2-butenal were noticeable in 1O2-oxidized linoleic and linolenicacids, whereas they were negligible in autoxidized linoleic andlinolenic acids. Heptenal was formed only in 1O2-oxidized soy-bean oil in the presence of chlorophyll and light (Min and others2003).

A beany flavor, which is a unique and undesirable flavor insoybean oil with low peroxide value, has been a problem for the


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3Sen*

+ 3O2

+ 3O2

+ 3O2

+ RH

+ RH

ROOH+ Sen+ROOH

R + Sen H ( or R++ Sen-) 1O2

Type IIType I

O2

Figure6---Reaction of tripletsensitizerswith

substrates

(CH2)6COOHCH3(CH2)6 OOH

(CH2)6COOHCH3(CH2)6

(CH2)6COOHCH3(CH2)6

OOH1O2

(CH2)6COOHCH3(CH2)6

1O2O

H O

O

O H

(CH2)6COOHCH3(CH2)6

Figure7 --- Formationof

allyl hydroperoxideof

oleicacidbyene

reaction

last 70 years and studied extensively both nationally and inter-nationally (Chang and others 1966; Smouse and Chang 1967).2-Pentylfuran and pentenylfuran were reported to be responsiblefor the beany flavor (Chang and others 1966, 1983; Smouse andChang 1967; Ho and others 1978; Smagula and others 1979), and1O2 was involved in their formation from linoleic and linolenicacids present in soybean oil, as shown in Figure 9 and 10(Minandothers 2003). Min and others (2003) strongly indicated that thereversion flavor of soybean oil can be decreased or eliminated byremoving chlorophylls from the oil during processing. The soy-bean oil industry currently removes effectively chlorophylls fromsoybean oil using bleaching materials during the refining process,and the beany flavor is no longer a serious problem in soybeanoil.

Factors Affecting the Oxidation of Edible OilThe oxidation of oil is influenced by the fatty acid compo-

sition of the oil, oil processing, energy of heat or light, the

Table3 ---Hydroperoxidesof fatty acids bysingletoxygenoxidation

Relative

Fatty acid Hydroperoxides at amount (%)a Type of acid

Oleic acid C9 48

C10 52Linoleic acid C9 32 Conjugated

C10 17 NonconjugatedC12 17 NonconjugatedC13 34 Conjugated

Linolenic acid C9 23 ConjugatedC10 13 NonconjugatedC12 12 ConjugatedC13 14 ConjugatedC15 13 NonconjugatedC16 25 Conjugated

a Frankel (1985).


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(CH2)7COOH

OOH

H O

O

OOH1O2

O

O H

Conjugated

Nonconjugated

O

O H

Conjugated

CH3(CH2)4CH3(CH2)4 (CH2)7COOH

(CH2)7COOHCH3(CH2)4


Nonconjugated

1O2

1

O2

1O2

OOH

OOH

CH3(CH2)4

CH3(CH2)3CH

CH3(CH2)4

CH3(CH2)4 CH(CH2)6COOH

(CH2)7COOH

(CH2)7COOH(CH2)7COOH

H O

O(CH2)7COOH

CH3(CH2)4

OOH

Figure8---Hydroperoxide

formation in linoleicacid

oxidation bysinglet

oxygen

concentration and type of oxygen, and free fatty acids, mono- anddiacylglycerols, transition metals, peroxides, thermally oxidizedcompounds, pigments, and antioxidants. These factors interac-tively affect the oxidation of oil and it is not easy to differentiatethe individual effect of the factors.

Fatty acid composition of oilsOils that are more unsaturated are oxidized more quickly than

less unsaturated oils (Parker and others 2003). As the degree ofunsaturation increases, both the rate of formation and the amountof primary oxidation compounds accumulated at the end of theinduction period increase (Martin-Polvillo and others 2004). Soy-bean, safflower, or sunflower oil (iodine values > 130) stored inthe dark showed a significantly (P

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CH3 (CH2)4 CH CH CH2 CH CH CH2 (CH2)6 COOH

1O2

CH3 (CH2)4

CH CH CH2 CH CH CH (CH2)6 COOH

O

OH

CH3 (CH2)4 CH CH CH2 CH

O1O2


OO O


OO O

+ 2 RH

CH3 (CH2)4 CH CH2 CH2 CH

OO

OH

+ 2 R

CH3 (CH2)4 CH CH2 CH2 CH

OO

CH3 (CH2)4 C CH2 CH2 CH

OO

CH3 (CH2)4 C CH CH CH

OHOH- H2O

O

CH3 (CH2)4

Figure9---Formation of 2-pentylfuran fromlinoleicacid bysingletoxygenoxidation

and Saari Csallani 1998). Light is much more important than tem-perature in 1 O2oxidation. Light of shorter wavelengths had moredetrimental effects on the oils than longer wavelengths (Sattarand others 1976). Reportedly, the effect of light on oil oxidationbecomes less as temperature increases (Velasco and Dobarganes2002).

Because 1O2 oxidation occurs in the presence of light, thepackaging of oils is very important. Transparent plastic bot-tles 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 trans-parent plastic bottles improved oxidative and sensory stability ofsoybean oil under light (Pascall and others 1995; Azeredo andothers 2003).

OxygenThe oxidation of oil can often take place when oil, oxygen, and

catalysts are in contact. Both concentration and type of oxygenaffect the oxidation of oils. The oxygen concentration in the oilis dependent on the oxygen partial pressure in the headspace ofthe oil (Andersson 1998). A higher amount of oxygen is dissolvedin the oil when the oxygen partial pressure in the headspace ishigh. Oxidation of the oil increased with the amount of dissolvedoxygen (Min and Wen 1983). The solubility of oxygen is higherin the oil than in water, and also in crude oil than refined oil (Ahoand Wahroos 1967). One gram of soybean oil dissolves 55 goxygen at room temperature (Andersson 1998). The amount ofoxygen dissolved in oil is sufficient to oxidize the oil to a peroxidevalue of approximately 10 meq/kg in the dark (Przybylski andEskin 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 inthedarkwere 0.049, 0.058, 0.126, and 0.162 ppm/ h, respectively.

The effect of oxygen concentration on the oxidation of oil in-creased at high temperature and in the presence of light and met-als such as iron or copper. Higher oxygen dependence of oil ox-idation at high temperature is due to low solubility of oxygen inthe oil at high temperature (Andersson 1998). The oxygen has tobe transported into the oil by diffusion when the oil is not stirred,such as during storage at low temperature. Convection is anotherimportant pathway for oxygen penetration into the oil from thesurface when the oil is stirred, for example, during processing athigh temperature.

Oil oxidation rate is independent of oxygen concentration atsufficiently 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 dependenton oxygen concentration and is independent of lipid concentra-tion (Andersson 1998). The autoxidation rate of oil at more than4% to 5% oxygen in the headspace was independent of oxy-gen concentration and was directly dependent on the lipid con-centration (Labuza 1971). However, the reverse was true at lowoxygen pressure of less than 4% in the headspace (Karel 1992).The oxidation of rapeseed oil at 50 C in the dark, measured asoxygen consumption or peroxide values, became faster as theoxygen concentration increased at less than 0.5% oxygen in theheadspace, whereas the oxidation rate decreased with oxygenconcentration at more than 1% oxygen (Andersson and Lingnert1999).

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

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

The reaction rate between lipid and oxygen is dependent onthe type of oxygen; the reaction rate of 1O2 with lipid is much


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CH3 CH2 CH CH2 CH CHCHCH2CHCH CH2 (CH2)6 COH

O

CH3 CH2 CH CH2 CH CHCHCH2CHCH CH (CH2)6 COH

O

1

O2

O

OH

CH3 CH2 CH CH2 CH CHCHCH2CHCH CH (CH2)6 COH

O

O

CH3 CH2 CH CH2 CH CHCH2CHCH

O


O

1O2

O

OH


OO

CH3 CH2 CH CH2 CH2 CHCH2CCH

OO

CH3 CH2 CH CH2 CH CHCHCCH

OHOH

- H2O

CH3 CH2 CH CH2CH

O

Figure10---Formation of 2-pentenylfuranfrom linolenic

acidby singletoxygenoxidation


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higher than that of3O2because1O2can directly react with lipids.

3O2 reacts with the radical state of lipids. Linoleates react with1O2 at a rate 1,450 times faster than with

3O2 (Rawls and VanSanten 1970).

Minor components present in oil

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

Free fatty acid and mono- and diacylglycerols. Crude oil con-tains free fatty acids, and oil processing, such as refining, de-creases the free fatty acid contents. Crude soybean oil containsabout 0.7% free fatty acids; however, refined oil contains 0.02%free fatty acids (Jung and others 1989). Sesame oil extracted fromroasted sesame seeds contains 0.72% free fatty acids and bleach-ing with acid clay decreases free fatty acid contents of the oil to0.56% (Kim and Choe 2005). Free fatty acids are more suscepti-ble to autoxidation than esterified fatty acids (Kinsella and others1978). Free fatty acids act as prooxidants in edible oil (Miyashitaand Takagi 1986; Mistry and Min 1987a). They have hydrophilic

and hydrophobic groups in the same molecule and prefer to beconcentrated on the surface of edible oils. The hydrophilic car-boxy groups of the free fatty acids will not easily dissolve in thehydrophobic edible oil and are present on the surface of edibleoil. Mistry and Min (1987a) reported that free fatty acids decreasethe surface tension of edible oil and increase the diffusion rate ofoxygenfrom theheadspace into theoil 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 proox-idants to increase oxidation at 55 C in the dark (Mistry and Min1987b; Mistry and Min 1988). The mono-and diacylglycerols,which have hydrophilic hydroxy groups and hydrophobic hy-drocarbons, decrease the surface tension of edible oil and in-crease the diffusion rate of oxygen from the headspace to theoil to accelerate the oxidation of oil. Mono- and diacylglyc-

erols should be removed from the oil during the oil-refining pro-cess to improve the oxidative stability of edible oils (Mistry andMin 1988).

Metals. Crude oil contains transition metals such as iron orcopper. For example, crude soybean oil contains 13.2 ppb and2.80 ppm of copper and iron, respectively. However, refining re-duces their contents. Edible oils manufactured without refining,such as extra virgin olive oil and sesame oil, contain relativelyhigh quantities of transition metals (Table 4). Metals increase therate of oil oxidation due to the reduction of activation energy ofthe initiation step in the autoxidation down to 63104 kJ/mol(Jadhav and others 1996). Metals react directly with lipids toproduce lipid alkyl radicals. They also produce reactive oxygen

Table4 ---Copperandironcontents inedibleoils

Metal contenta

Oils Copper (ppb) Iron (ppm)

Cold-pressed sesame oilb 16 (3.038) 1.16 (0.181.52)Crude soybean oilc 13.2 2.80Virgin olive oilb 9.8 (1.079) 0.73 (nd9.79)Cold-pressed sunflower oilb 5.2 (2.28.5) 0.26 (0.220.31)Refined olive oild 15 0.08Refined soybean oilc 2.5 0.20

aNumbers in parentheses are the ranges reported.bMAFF (1997).cSleeter (1981).dLeonardis and Macciola (2002).

species such as 1O2 and hydroxy radical from 3O2 and hydro-

gen peroxide, respectively (Andersson 1998). Lipid alkyl radicaland reactive oxygen species accelerate oil oxidation. Copper ac-celerates hydrogen peroxide decomposition 50 times faster thanferrous ion (Fe2+), and ferrous ion acts 100 times faster than ferricion (Fe3+):

Fe3+ + RH Fe2+ + R + H+

Fe2+ + 3O2 Fe3+ + O2

O2 + O2 + 2H

+ 1O2 + H2O2

H2O2 + O2

metal catalystHO + OH + 1O2

Fe2+ + H2O2 Fe3+ + OH + HO

Metals also accelerate autoxidation of oil by decomposing hy-droperoxides (Benjelloun and others 1991):

ROOH + Fe2+(or Cu+) RO + Fe3+(or Cu2+) + OH

ROOH + Fe3+(or Cu2+) ROO + Fe2+(or Cu+) + H+

Fe2+ is much more active with a rate of 1.5 103 M1s1

(Halliwell and Gutteridge 2001) than Fe3+ in decomposing thelipid hydroperoxides to catalyze autoxidation (Mei and others1998). Fe3+ also causes decomposition of phenolic compoundssuch as caffeic acid in olive oil and decreases oil oxidative sta-bility (Keceli and Gordon 2002).

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

Phospholipids. Crude oils contain phospholipids such asphosphatidylethanolamine, phosphatidylcholine, phosphatidyli-nositol, phsphatidylserine, and phosphatidic acid, but most

of them are removed by processing such as degumming.Crude soybean oil contains phosphatidylcholine and phos-phatidylethanolamine at 500.8 and 213.6 ppm, respectively.However, refined, bleached, and deodorized (RBD) soybeanoil contains 0.86 and 0.12 ppm of phosphatidylcholine andphosphatidylethanolamine, respectively (Yoon and others 1987).Phospholipids act as antioxidants and prooxidants depending ontheir concentration and presence of metals. Oxidation of docosa-hexaenoic acid at 25 C to 30 C in the dark decreased whenmixed with phosphatiylcholine at a molar ratio of 1:1 (Lybergand others 2005).

The mechanism of the antioxidative effects of phospholipidshas not yet been elucidated in detail, but the polar group plays animportant role and the nitrogen-containing phospholipids such asphosphatidylcholine and phosphatidylethanolamine are efficientantioxidants under most conditions (King and others 1992). Phos-pholipids decrease oil oxidation by sequestering metals, and con-centration forthe maximal antioxidant activity wasbetween 3 and60 ppm. Soybean oil oxidation decreased with addition of 5 to10ppm phospholipids, and higher amount of phospholipids actedas prooxidants. Yoon and Min (1987) reported that phospholipidsacted as antioxidants only in the presence of Fe2+ by chelatingiron. In purified soybean oil, which did not contain any met-als, phospholipids worked as prooxidants. Phospholipids havehydrophilic and hydrophobic groups in the same molecule. Thehydrophilic groups of the phospholipids are on the surface of oiland hydrophobic group are in the edible oil. The phospholipidsdecrease the surface tension of edible oil and increase the diffu-sion rate of oxygen from the headspace to the oil to accelerate oil


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oxidation. Soybean oil oxidation in the dark at 60 C was the low-est with phosphatidic acid and phosphatidylethanolamine, fol-lowed by phosphatidylcholine, phosphatidylglycerol, and phos-phatidylinositol (Yoon and Min 1987).

Chlorophylls. Chlorophylls are common pigments present inedible vegetable oil. Virgin olive oil and rapeseed oil contain

chlorophyll at 10 ppm and 5 to 35 ppm, respectively (Salvadorand others 2001). Virgin olive oil also contains 4 to 15 ppm pheo-phytin (Psomiadou and Tsimidou 2002). The chlorophyll concen-trations in crude and bleached soybean oils are 0.30 and 0.08ppm, respectively (Jung and others 1989). Chlorophylls are gen-erally removed during oil processing, especially the bleachingprocess (Table 5).

Chlorophylls and their degradation products, pheophytins andpheophorbides, act as sensitizers to produce 1 O2in the presenceof light and atmospheric 3 O2, and accelerate the oxidation of oil(Fakourelis and others 1987; Whang and Peng 1988; Gutierrez-Rosales and others 1992). Pheophytins have a higher sensitizingactivity than chlorophylls, but lower than that of pheophorbides(Endo and others 1984; Rahmani and Csallany 1998). Soybean oilpurified by silicic acid column chromatography did not containany chlorophylls and did not produce volatiles in the headspace

under light at 10 C, but purified soybean oil with added chloro-phyll and RBD soybean oil produced headspace volatiles un-der the same experimental conditions (Lee and Min 1988). Also,appreciable amounts of volatiles were produced in soybean oilcontaining chlorophyll only under light, but not in the dark. Theheadspace volatile formation in soybean oil under light at 10 Cincreased as the concentration of chlorophyll increased.Rahmaniand Csallani (1998) showed that the oxidation of virgin olive oilcontaining pheophytin was accelerated by the illumination offluorescent light.

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

Thermally oxidized compounds. Processing of crude oils to re-

fined oils is generally performed at high temperature, and itcan produce oxidized compounds such as cyclic and noncycliccarbon-to-carbon-linked dimers and trimers, hydroxy dimers, anddimers and trimers joined through carbon-to-oxygen linkage. TheRBD soybean oil contains 1.2% thermally oxidized compounds(Yoon and others 1988). Thermally oxidized compounds accel-erated the soybean oil autoxidation at 55 C (Yoon and others

Table5---Chlorophyllcontents ofcanolaoilduringprocessing(ppm)a

Oil Chlorophyll a Pheophytin a Pheophytin b Pyropheophytin a Pyropheophytin b

Extracted 1.88 3.31 1.34 16.57 3.13Degummed 0.27 7.16 1.07 9.40 1.84Refined 0.22 6.27 1.12 9.13 1.79

Bleached --- 0.56 0.32 0.21 0.25

aSuzuki and Nishioka (1993).

OHROO ROOH

O OO O Figure11---Resonancestabilization ofan

antioxidant radical

1988). The acceleration of oil oxidation increases with the con-centration of thermally oxidized compounds. Lipid hydroperox-ides were also shown to act as prooxidants (Hahm 1988). Theoxidized compounds formed by hydroperoxide decompositioncan act as an emulsifier, which contain both hydrophilic and hy-drophobic 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, andsterols. Antioxidants are sometimes intentionally added to oil toimprove oxidative stability. Antioxidants are compounds that ex-tend the induction period of oxidation or slow down the oxidationrate. Antioxidants scavenge free radicals such as lipid alkyl rad-icals or lipid peroxy radicals, control transition metals, quenchsinglet oxygen, and inactivate sensitizers.

Antioxidants can donate hydrogen atoms to free radicals andconvert them to more stable nonradical products (Decker 2002).The major hydrogen-donating antioxidants are monohydroxy orpolyhydroxy phenolic compounds with various aromatic ringsubstitutions. Any compound whose reduction potential is lowerthan that of a free radical can donate hydrogen to that radi-

cal unless the reaction is kinetically unfavorable. Standard 1-electron reduction potentials of alkoxy, peroxy, and alkyl radicalsof polyunsaturated fatty acids are 1600, 1000, and 600 mV, re-spectively (Buettner 1993). The standard reduction potential ofantioxidants is generally 500 mV or below. This clearly showsthat antioxidants react with lipid peroxy radicals before the per-oxy radicals react with other lipid molecules to produce anotherfree radical. Any antioxidant radical produced from the reactionwith lipid peroxy radical has lower energy than the lipid peroxyradical itself due to resonance structure (Figure 11).

Metal chelators such as phosphoric acid, citric acid, ascorbicacid, and EDTA (ethylenediaminetetraacetic acid) decrease oiloxidation in an indirect way. They can convert iron or copperions into insoluble complexes or can sterically hinder the forma-tion of the complexes between metals and lipid hydroperoxides

(Halliwell and others 1995). Citric acid improved the sensoryquality of soybean oil containing 1 ppm iron during storage at55 C (Min and Wen 1983). Citric acid is often added to oil in thefield to reduce its oxidation during storage before processing. Minand Wen (1983) reported that the antioxidant effects of citric acidincreased as its content increased; and more than 150 ppm citricacid was necessary to overcome the catalyticeffect of 1 ppm iron.


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When oil contains 0.1 ppm or less iron such as in RBD oil, thecommon practice of adding about 150 ppm citric acid to the oilfor the improvement of oxidative stability is not necessary (Minand Wen 1983).

Some antioxidants quench 1O2 or excited sensitizers. 1O2 is

quenched physically and chemically. In physical quenching, 1O2

is converted into3

O2 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).

TocopherolsTocopherols are the most important antioxidants present in ed-

ible oil. Animal fats also contain tocopherols, but at lower con-centrations (Table 6). Among vegetable oils, soybean, canola,sunflower, and corn oils contain relatively high amounts of to-copherols. Palm oil does not have large amounts of tocopherols(118 to 146 ppm); however, it contains a higher concentrationof tocotrienols, that is, -, -, and -tocotrienols at 211, 353 to372, and 56 to 67 ppm, respectively (Al-Saqer and others 2004).Safflower oil also contains - and -tocotrienols in addition totocopherols at 3.8 to 7.0 and 7.5 to 8.4 ppm, respectively (Leeand others 2004). Tocopherol contents of edible oils are affectedby the cultivar, processing, and storage of the oil (Deiana andothers 2002). Tocopherol contents of sesame oil range between404 and 540 ppm depending on the cultivars (Mohamed andAwatif 1998). Thetocopherol contents of rapeseed oil are794 and749 ppm for hexane-extracted oil and pressed oil, respectively(Krygier and Platek 1999). The refining process, especially de-odorization, reduces tocopherol contents (Jung and others 1989;Reische and others 2002; Eidhin and others 2003). The crude,bleached, and deodorized soybean oil contains tocopherols at1670, 1467, and 1138 ppm, respectively (Jung and others 1989).In virgin olive oil, -tocopherol concentration decreased withstorage time of the oil in the dark (Deiana and others 2002).There was no tocopherol left in olive oil stored in the dark atroom temperature for 12 mo (Morello and others 2004).

Tocopherols compete with unsaturated fats and oils for lipidperoxy radicals. Lipid peroxy radicals react with tocopherolsmuch faster at 104 to 109 M1s1 than with lipids (10 to 60M1s1). One tocopherol molecule can protect about 103 to108 polyunsaturated fatty acid molecules at low peroxide value(Kamal-Elden and Appelqvist 1996). Tocopherols can transfer a

Table6 ---Tocopherol contentsof edibleoil

Tocopherol (ppm)

Oil Total

Soybeana 116.0 34.0 737.0 275.0 1162Canolaa 272.1 0.1 423.2 695.4

Sunflowera

613.0 17.0 18.9 648.9Corna 134.0 18.0 412.0 39.0 603Roasted sesameb 4 584 9 597Rapeseedc 252 314 566Safflowerd 386520 8.612.4 2.47.7 --- 397540Olivee 168226 --- --- --- 168226Beef tallowf 30.4 --- 3.8 --- 34.2Lardg 18.0 --- --- --- 18.0

aPrzybylski (2001).bKamal-Eldin and Andersson (1997).cVelasco and others (2003).dLee and others (2004)eSalvador and others (2001); Gutierrez and others (2001).f Choe and Lee (1998).gDrinda and Baltes (1999).

hydrogen atom at the 6-hydroxy group on its chroman ring to lipidperoxy radical and scavenge the peroxy radicals. Tocopherol (T),with a reduction potential of 500 mV, donates hydrogen to lipidperoxy radicals (ROO) that have a reduction potential of 1000mV, and produces lipid hydroperoxide (ROOH) and tocopheroxyradicals (T). Tocopheroxy radicals are more stable than lipid per-

oxy radicals due to resonance structures. This ultimately slowsdown the oil oxidation rate in the propagation stage of autoxida-tion. Reaction rates of peroxy radicals in stearic and oleic acidswith -tocopherol were reported to be 2.8 106 and 2.5 106

M1 s1, respectively (Simic 1980). Tocopheroxy radicals can in-teract with other compounds or each other, depending on thelipid oxidation rates. Tocopheroxy radicals react with each otherat low lipid oxidation rates and produce tocopheryl quinone andtocopherol. At higher oxidation rates, tocopheroxy radicals mayreactwith lipid peroxy radicals and produce tocopherollipidper-oxy complexes ([T-OOR]); these can be hydrolyzed to tocopherylquinone and lipid hydroperoxide (Liebler and others 1990):

T + ROO T + ROOH

T + T T + Tocopheryl quinone

T + ROO [T OOR] Tocopheryl quinone + ROOH

The effectiveness of tocopherols as antioxidants depends onthe isomers and concentration of tocopherols. Free radical scav-enging activity of tocopherols is the highest in -tocopherol fol-lowed by -, -, and -tocopherol (Reische and others 2002).-Tocopherol was more effective in reducing oil oxidation than-tocopherol up to 200 ppm and less effective above this con-centration (Yanishlieva and others 2002).

The optimal concentration of tocopherols as antioxidants isdependent on their oxidative stability; the lower the oxidativestability of the tocopherol, the lower the optimal concentrationof tocopherol for the maximal antioxidant activity.-Tocopherol,which is the least stable among tocopherol isomers, showed themaximal antioxidant activity at 100 ppm in the oxidation of soy-

bean oil at 55 C in the dark, but the optimal concentrations of- and -tocopherols as an antioxidant were 250 ppm and 500ppm, respectively (Jung and Min 1990).

Tocopherols, particularly -tocopherol, act as prooxidantswhen present in high concentrations in vegetable oils viapropagation of free radicals, depending on the hydroperoxide


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concentration (Cillard and others 1980; Terao and Matsushita1986; Jung and Min 1990). When the concentration of lipid per-oxy radical is very low, the tocopheroxy radical abstracts hy-drogen from the lipid and produces tocopherol and lipid alkylradical; however, the reaction rate is very low. Formation oflipid alkyl radicals by tocopherols accelerates lipid oxidation,

which is called tocopherol-mediated peroxidation (Bowry andStocker 1993; Yamamoto 2001). The highest prooxidant activitywas shown in -tocopherol followed by - and -tocopherol insoybean oil autoxidation (Jung and Min 1990). Addition of 100ppm-tocopherol increased the oxidation of purified olive oil atan early stage of autoxidation; however, -tocopherol added tomoderately oxidized (POV = 15) purified olive oil or lard signifi-cantly decreased the oil oxidation (Blekas and others 1995). Thethreshold value for -tocopherol as a prooxidant in virgin oliveoil oxidation was 60 to 70 ppm. When less -tocopherol was ini-tially present in the oil, the threshold value for prooxidant activitywas reached more quickly (Deiana and others 2002). Prooxidantactivity of-tocopherol decreased as the temperature increased,even at a high concentration (Marinova and Yanishlieva 1992).Ascorbic acid (Asc) can reduce tocopheroxy radical and preventtocopherol-mediated peroxidation (Yamamoto 2001):

T + RH T + R

R + O2 ROO

T + Asc T + Dehydroascorbic acid

Oxidized tocopherols increase soybean oil oxidation andprooxidant activity was the highest in oxidized -tocopherol fol-lowed by oxidized - and -tocopherol (Jung and Min 1992).Prevention of tocopherol oxidation and removal of oxidized to-copherols during oil processing are strongly recommended toimprove oil oxidative stability.

In addition to free radical scavenging activity, tocopherols de-crease soybean oil oxidation under light by only 1 O2 quenching(Min and Lee 1988). 1O2 quenching rate by -tocopherol was

reported to be 2.7 107 M1s1 (Jung and others 1991). 1O2quenching effect in soybean oil oxidation under light is depen-dent on tocpherol concentration and the type; at 1 103 M,the activity was in the decreasing order of -, -, and - to-copherol; however, there was no significant difference in 1O2quenching activity among tocopherols at 4 103 M (Jung andothers 1991). Tocopherols can form a charge transfer complex([T+ 1O2]1) with

1O2 by electron donation to 1O2. The singlet

state of tocopherol1O2complex undergoes intersystem crossinginto the triplet state ([T+ 1O2]3) and produces less reactive

3 O2and tocopherol. Since this does not involve chemical reactionsbetween tocopherol and 1 O2, it is called physical quenching:

T + 1O2 [T+

1O2]1 [T+

1O2]3 T +3O2

Tocopherols react irreversibly with 1O2 in chemical quench-ing and produce tocopherol hydroperoxydienone, tocopherylquinone, and quinone epoxide (Figure 12). The reaction rate oftocopherols with 1O2is affected by their structures.-Tocopherolshowed the highest reaction rate of 2.1 108 M1s1, followedby -tocopherol with 1.5 108 M1s1, -tocopherol with 1.4 108 M1s1, and-tocopherol with 5.3 107 M1s1 (Mukai andothers 1991).

CarotenoidsCarotenoids are a group of tetraterpenoids consisting of iso-

prenoid units. Double bonds in carotenoids are conjugated formsand usually are all trans forms. -Carotene is one of the most

studied carotenoids. Edible oils, especially unrefined oils, con-tain -carotene. Crude palm oil and red palm olein contain 500to 700 ppm carotenoids (Bonnie and Choo 2000). Virgin oliveoil contains 1.0 to 2.7 ppm -carotene as well as 0.9 to 2.3 ppmlutein (Psomiadou and Tsimidou 2002).-Carotene can slow down oil oxidation by light filtering, 1 O2

quenching, sensitizer inactivation, and free radical scavenging.Fakourelis and others (1987) reported that oxidation of olive oilcontaining only -carotene under light at 25 C was decreasedby filtering out some light energy, mainly between 400 and 500nm. In the co-presence of chlorophylls, -carotene decreasedthe oxidation of soybean oil stored under light by 1 O2quenching(Lee and Min 1988). 1O2 quenching by carotenoids is mainlyby energy transfer from 1O2 to carotenoids, without generatingoxidized products. Excited carotenoids return to the ground stateby giving off heat:

1O2 +1Carotenoid 3O2 +

3Carotenoid

3Carotenoid

Carotenoid + Heat

One mole of-carotene can quench 250 to 1000 molecules of1O2 at a rate of 1.3 1010M1s1 (Foote 1976). 1O2 quenchingof carotenoids is dependent on the number of conjugated dou-ble bonds (Beutner and others 2001). Carotenoids having at least9 conjugated double bonds act as an effective 1O2 quencher;-carotene, lycopene, and lutein are good 1 O2 quenchers; how-ever, phytoene, phytofluene, and -carotene,which are precursorcompounds of lycopene, are not. The 1O2 quenching activity ofcarotenoids increases as the number of conjugated double bondsincreases (Min and Boff 2002; Foss and others 2004). Lycopeneand -carotene demonstrate higher 1O2 quenching ability than-carotene (Miller and others 1996).

Carotenoids inactivate excited sensitizers physically by absorb-ing energy from sensitizers. The excited carotenoids return to theground state by transferring the energy to the oil (Stahl and Sies1992). Lee and others (2003) reported that -carotene with a

high 1-electron reduction potential of 1060 mV had great diffi-culty in donating hydrogen to alkyl (Eo = 600 mV) or peroxyradical (Eo = 770 to1440 mV) of polyunsaturated fatty acid. Nu-clear magnetic resonance study showed that -carotene did notdonate a hydrogen atom to quench these free radicals (Lee andothers 2003). However, -carotene can donate hydrogen to hy-droxy radical, which has a high reduction potential (2310 mV),and produces a carotene radical (Car). A carotene radical is afairly stable species due to delocalization of unpaired electronsthrough its conjugated polyene system. At low oxygen concen-tration, a carotene radical may react with other radicals such aslipid peroxy radicals and form nonradical products (Burton andIngold 1984; Beutner and others 2001):

Car + HO Car + H2O

Car + ROO Car OOR

A lipid peroxy radical may be added to -carotene and pro-duce carotene peroxy radical (ROOCar), especially at higherthan 150 mm Hg of oxygen (Burton and Ingold 1984). Caroteneperoxy radical reacts with 3O2and then with lipid molecules, andproduces lipid alkyl radicals, which propagate the chain reactionof lipid oxidation (Iannone and others 1998):

ROO + Car ROO Car

ROO Car + 3O2 ROO Car OO

ROO Car OO + RH ROO Car OOH + R


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CH3

O

HO

H3C

CH3

C16H33O

O

H3C

CH3

C16H33

1O2

OOH

CH3

O

HO

H3C

CH3

C16

H33

OO

O

O

H3C

CH3

C16H33HOO

O

H3C

CH3

C16H33

O

1O2

+

Hydroperxydienone-Tocopherol

-Tocopherol endoperoxide

-Tocopheryl quinone-Tocopheryl quinone epoxide

CH3

CH3CH3

Figure12---Singletoxygen

oxidationof -tocopherol

Antioxidant activity of-carotenewas notshownin soybean oilstored in the dark (Lee and others 2003). During photosensitizedoxidation of soybean or rapeseed oil in open vessels, -caroteneincreased oil oxidation, but the co-presence of tocopherols de-creased oxidation of the oil (Haila and Heinonen 1994). The an-tioxidant activities of carotenoids by hydrogen donation remaincontroversial.-Carotene may also donate electrons to free radicals and

become a -carotene cation radical (Liebler 1993; Mortensenand others 2001). The transfer of hydrogen or electrons fromcarotenoids to free radicals depends on the reduction potentialsof the free radicals and chemical structures of the carotenoids,especially the presence of oxygen-containing functional groups(Edge and others 1997). The -carotene cation radical has a rel-atively high standard 1-electron reduction potential (1060 mV),and does not readily react with oxygen to form peroxide (Edgeand others 2000). The -carotene cation radical may react withalkyl, alkoxy, or peroxy radicals formed in soybean oil duringoxidation. Lee and others (2003) reported that -carotene was aprooxidant in soybean oil oxidation in the dark due to the elec-tron transfer mechanism in their 2, 2-diphenyl-1-picryl hydrazyland NMR study.

Carotenoids are degraded by hydroperoxides to hydroxyl- orepoxycarotenes, and the degradation slows down at higher con-centration; the rate of degradation of carotenes is lycopene >-carotene -carotene (Anguelova and Warthesen 2000).

Other phenolic compoundsPhenolic compounds other than tocopherols in edible oils also

exert antioxidant activity. Sesame oil, which contains a highamount of unsaturated fatty acids (iodine value = 109), has goodoxidative stability (Tan and others 2002). The autoxidation rate ofsesame oils at 60 C was much lower than that of corn oil, saf-flower oil, and a mixture of soybean and rapeseed oils (Fukudaand Namiki 1988). Roasted sesame oil was more stable for theoxidation than unroasted sesame oil (Yoshida and Takagi 1997;Yoshida and others 2000). The remarkable oxidative stability ofsesame oil is due to the presence of lignan compounds as well astocopherols (Yoshida 1994; Namiki 1995). Lignan compounds insesame oil include sesamin, sesamol, sesamolin, sesaminol, andsesamolinol (Figure 13). Sesamin is the predominant lignan com-pound in unroasted sesame oil (474 ppm) followed by sesamolin(159 ppm). Sesamol concentration in unroasted sesame oil wasless than 7 ppm (Fukuda and others 1986; Dachtler and others


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O

O

OH

sesamol O

O

O

O

O

O

sesamin

O

O

O

O

O

O

O

sesamolin

O

O

O

O

O

O

HO

sesaminol

HO

HO COOH

HO

OH

OH

HO

OH

Caffeic acid Hydroxytyrosol Tyrosol

Figure13---Phenolic

acids present inoils

2003); however, roasted sesame oil contains a higher concen-tration of sesamol (36 ppm) than unroasted oil (Kim and Choe2005). Sesamol is produced by hydrolysis of sesamolin duringoil processing such as heating and bleaching (Fukuda and others1985; Osawa and others 1985; Kim and Choe 2005). Sesamol isconverted to sesamol dimer and then to sesamol dimer quinone(Kurechi and others 1981; Kikugawa and others 1983). Sesamoland sesaminol showed higher antioxidant activity than sesaminin sunflower oil autoxidation by scavenging radicals (Dachtlerand others 2003).

Sesamol acts as an antioxidant in the oxidation under light aswell as in the dark. The antioxidation by sesamol in chlorophyll-sensitized photooxidation of soybean oil was lower than by -

tocopherol, similar to-tocopherol, and higher than DABCO (di-azabicyclo[2,2,2]octane) at the same molar concentration (Kimand others 2003). The decrease of photooxidation of soybean oilby sesamol was due to its 1O2quenching, and the quenching ratewas 1.9 107 M1 s1 at 20 C (Kim and others 2003).

Sesame oil also contains phytosterols such as campesterol, stig-masterol, -sitosterol, and 4,5-avenasterol, with -sitosterol asthe predominant sterol (Dachtler and others 2003). Sitosterol be-haves partly as a prooxidant by increasing the solubility of oxygenin the oil (Yanishlieva and Schiller 1983) and partly as a weakantioxidant in sunflower oil and lard by competing with lipidmolecules for oxidation at the oil surface (Maestroduran and Bor-japadilla 1993; Brimberg and Kamal-Eldin 2003).


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Table7 ---Phenoliccompounds invirginolive oila

Phenolic alcohols 3,4-Dihydroxyphenyl ethanol (3,4-DHPEA, hydroxytyrosol),p-hydroxyphenylethanol (p-HPEA, tyrosol),3,4-dihydroxyphenylethanol-glucoside

Phenolic acid and its derivatives Vanillic 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

Secoiridoids Dialdehydic form of elenolic acid linked to 3,4-DHPEA, dialdehydic form of elenolic acid linked top-HPEA,

oleuropein aglycon, ligstroside aglycon, oleuropein, p-HPEA derivativeLignans 1-Acetoxypinoresinol, pinoresinol, 1-hydroxypinoresinolFlavones Apigenin, luteolin

aServili and Montedoro (2002).

Olive oil, which is very stable to autoxidation (Guillenand Cabo 2002), contains phenolic compounds and toco-pherols. 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 andhydroxytyrosol (Papadopoulos and Boskou 1991; Tsimidouand 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). Thephenolic compounds in olive oil acted as antioxidants mainlyat the initial stage of autoxidation (Deiana and others 2002) byscavenging free radicals and chelating metals (Chimi and others1991). Hydroxytyrosol was the most effective antioxidant inolive oil oxidation (Papadopoulos and Boskou 1991; Tsimidouand others 1992; Baldioli and others 1996). Among phenoliccompounds, o-diphenols such as caffeic acid are oxidized toquinones by ferric ions and become ineffective in inhibitingiron-dependent free radical chain reactions in oil (Keceli andGordon 2002). However, hydroxytyrosol, tyrosol, vanillic acids,and p-coumaric acid were not oxidized by the ferric ions. Fkiand others (2005) reported high radical scavenging effects of3,4-dihydroxyphenyl acetic acid present in olive mill wastewateron the oxidation of olive and husk refined oils. Chlorogenic acidand caffeic acid are the principal phenols found in sunflower oil

(Leonardis and others 2003). Osborne and Akoh (2003) reportedprooxidant effects of polar phenolic acids such as quercetin andgallic acid on a canola oil and caprylic acid structured lipid inthe presence of iron at pH 3.0, due to an increased ability toreduce iron.

Antioxidant interactionsEdible oil often contains multicomponent antioxidants and

demonstrates interactions among them. Metal chelator and a freeradical scavenger demonstrate better antioxidant activities in oilautoxidation when present together than when used separately.The improved antioxidation of oil by a combination of metalchelator and free radical-scavenging antioxidant is caused mainlyby the action of the chelator at the initiation step of oxidation andthat of the free radical scavenger at the propagation step.

Because tocopherols are the most common antioxidants in ed-ible oil, they have been used in research on interactions amongantioxidants. -Tocopherol showed synergistic effects with -carotene to decrease the autoxidation (Palozza and Krinsky 1992)and photosensitized oxidation of soybean oil (Choe and Min1992). The synergistic effects of tocopherol and -carotene weresuggested to be due to the protection of -carotene from degra-dation by tocopherols (Shibasaki-Kitakawa and others 2004). -Tocopherol shows synergism with ascorbic acid and phospho-lipids. Ascorbic acid gives hydrogen to the tocopheroxy radicalproduced from the reaction between the lipid peroxy radical andtocopherol, and then regenerates tocopherol. Nitrogen moiety ofphosphatidylethanolamine and phosphatidylcholine, or the re-

ducing sugar of phosphatidylinositol, donates hydrogen or elec-tron to tocopherols and delays the oxidation of tocopherols to to-copheryl quinone (Yoon and Min 1987). Sesamol and sesaminolshowed synergistic antioxidant activities with-tocopherol in theautoxidation of sunflower oil (Dachtler and others 2003).

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

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

ConclusionsEdible oil undergoes autoxidation and photosensitized oxi-

dation during processing and storage. The oxidation of edibleoil produces off-flavor compounds and decreases oil quality.Free fatty acids, mono- and diacylglycerols, metals, chlorophylls,carotenoids, tocopherols, phospholipids, temperature, light, oxy-

gen, oil processing methods, and fatty acid composition affectthe oxidative stability of edible oil. To minimize the oxidation ofedible oil during processing and storage, it is recommended todecrease temperature, exclude light and oxygen, remove metalsand oxidized compounds, and use appropriate concentrations ofantioxidants such as tocopherols and phenolic compounds.

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