Choe e Min, 2009

Mechanisms ofAntioxidants in

the Oxidationof FoodsEunok Choe and David B. Min

ABSTRACT: Antioxidants delay or inhibit lipid oxidation at low concentration. Tocopherols, ascorbic acid,carotenoids, flavonoids, amino acids, phospholipids, and sterols are natural antioxidants in foods. Antioxidants in-hibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen andphotosensitizers, and inactivating lipoxygenase. Antioxidants show interactions, such as synergism (tocopherolsand ascorbic acids), antagonism (�-tocopherol and caffeic acid), and simple addition. Synergism occurs when oneantioxidant is regenerated by others, when one antioxidant protects another antioxidant by its sacrificial oxidation,and when 2 or more antioxidants show different antioxidant mechanisms.

IntroductionOxidation decreases consumer acceptability of foods by pro-

ducing low-molecular-weight off-flavor compounds, as well as bydestroying essential nutrients, and it produces toxic compoundsand dimers or polymers of lipids and proteins (Aruoma 1998).Oxidation of foods can be minimized by removing prooxidantssuch as free fatty acids, metals, and oxidized compounds, and byprotecting foods from light. Air evacuation by reduced pressureor adding oxygen scavengers can also reduce oxidation. Since itis very difficult to completely remove all the prooxidants and air,antioxidants are now increasingly added to foods to slow downthe process of oxidation.

Antioxidants significantly delay or inhibit oxidation of oxidiz-able substrates at low concentration, compared to the highercontents of lipids and proteins in foods (Halliwell and Gutteridge2001). Antioxidants in foods do not necessarily protect biologicaltissues from free radical oxidative damage because they have tobe converted into usable forms in tissues and interact with othersubstances, in addition to effective concentration differences, andthey must display difficulty in absorption from the diet (Azzi andothers 2004). The antioxidants are naturally present in foods,or can be added or formed during processing. Antioxidants forfoods should be reasonable in cost, nontoxic, stable, effective atlow concentration, have carry-through, and should not changeflavor, color, and texture of the food matrix (Schuler 1990). Theeffects of antioxidants on the oxidation of foods are dependenton their concentration (Frankel and others 1996), polarity, andthe medium (Cuvelier and others 2000; Samotyja and Malecka2007), and also the presence of other antioxidants (Decker 2002).The objective of this article was to discuss the reaction mecha-nisms of antioxidants by focusing on their thermodynamic and

MS 20090169 Submitted 2/26/2009, Accepted 4/15/2009. Author Choe iswith Dept. of Food and Nutrition, Inha Univ., Incheon, Korea. Author Minis with Dept. of Food Science and Technology, The Ohio State Univ., 2015Fyffe Rd., Columbus, Ohio, U.S.A. Direct inquiries to author Min (E-mail:[email protected]).

kinetic characteristics depending on their surroundings duringthe oxidation of foods.

Major Antioxidants in FoodsExtensive research has been done on the isolation, purifica-

tion, and identification of the various antioxidants. Phenoliccompounds and ascorbic acid are the most important naturalantioxidants. Carotenoids, protein-related compounds, Maillardreaction products, phospholipids, and sterols also show naturalantioxidant activities in foods.

Phenolic compoundsPhenolic compounds such as tocopherols, polyphenols, phe-

nolic acids, and lignans are widely distributed in plants (Dickoand others 2006; Wang and Ballington 2007).

Tocopherols. Tocopherols are monophenolic compounds andderivatives of chromanol as shown in Figure 1. They are very sol-uble in oil and thus are the most important antioxidants in ediblefats and oils. Tocopherols are more frequently found in vegetableoils than animal fats, especially soybean, canola, sunflower, corn,and palm oils. Most vegetable oils contain tocopherols at con-centrations higher than 500 ppm; beef tallow and lard containless than 40 ppm (Choe and others 2005). Palm oil contains toco-pherols at 100 to 150 ppm, and also 620 to 650 ppm tocotrienols(Al-Saqer and others 2004). The refining process, especially de-odorization, reduces tocopherol contents in oils (Jung and others1989; Reische and others 2002; Eidhin and others 2003). To-copherols in crude soybean oil (1670 ppm) were decreased to1138 ppm during deodorization (Jung and others 1989).

Polyphenols. Olive oil is oxidation-resistant due to the pres-ence of tyrosol (4-hydroxyphenylethanol; 34.9 ppm), hydroxy-tyrosol (3,4-dihydroxyphenylethanol; 37.8 ppm), and catechol(Keceli and Gordon 2002; Servili and Montedoro 2002). Hydrox-ytyrosol is the most effective antioxidant in olive oil (Papadopou-los and Boskou 1991; Tsimidou and others 1992; Baldioli andothers 1996). Most of these antioxidants are removed during al-kaline refining and deodorization (Garcia and others 2006).

C© 2009 Institute of Food Technologists R© Vol. 8, 2009—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 345

CRFSFS: Comprehensive Reviews in Food Science and Food Safety

O

HO

CH2[CH2CH2CH(CH3)CH2]3HO

HO

O

HO

O

HO

α-tocopherol β-tocopherol

γ-tocopherol δ-tocopherol

O

HO

CH2[CH2CH=C(CH3)CH2]3H

α-tocotrienol

O

HO

β-tocotrienol

O

HO

γ-tocotrienol

O

HO

δ-tocotrienol

CH2[CH2CH=C(CH3)CH2]3H

CH2[CH2CH=C(CH3)CH2]3 HCH 2[CH2CH=C(CH3)CH2]3H

CH2[CH2CH2CH(CH3)CH2]3H

CH2[CH2CH2CH(CH3)CH2]3H CH2[CH2CH2CH(CH3)CH2]3H

Figure 1 ---Structures oftocopherols andtocotrienols.

Flavonoids are major plant polyphenols and are derivativesof diphenylpropanes and a heterocyclic 6-membered ring withoxygen. They include flavanols (catechins, naringin), flavanones(hesperidin, naringenin), flavones (apigenin, luteolin), flavonols(kaempferol, quercitrin, myricetin, quercetin), anthocyanins, andleucoanthocyanidins. The glycosylation of flavonoids resultsin lower antioxidant activity than the corresponding aglycons(Shahidi and Wanasundara 1992). The solubility of flavonoids infats and oils is very low and their role in the oxidation of oil isnot significant; however, they can contribute to decreasing theoxidation of oil in food emulsions (Zhou and others 2005).

Phenolic acids. Phenolic acids are closely related to flavonoids.They include hydroxycinnamic acids (coumaric, ferulic, caffeic,chlorogenic, and sinapic acids), hydroxycoumarin (scopoletin),and hydroxybenzoic acids (ellagic, gallic, gentisic, salicylic, andvanillic acids). Chlorogenic and caffeic acids are present in sun-flower oil, and sinapic and ferulic acids are present in rapeseed(Leonardis and others 2003) and defatted rice bran oils (Devi andothers 2007), respectively. Olive oil contains vanillic, syringic,caffeic, and cinnamic acids (Servili and Montedoro 2002). Phe-nolic acids as antioxidants in oils are also limited due to solubilityproblems.

Lignans. Lignans are phenylpropanoids derived from pheny-lalanine as shown in Figure 2. They include sesamol, sesamin,sesamolin, sesaminol, sesamolinol, pinoresinol, and secoisolari-ciresinol. The major lignans in unroasted sesame oil are sesamin(474 ppm), sesamolin (159 ppm), and sesamol (<7 ppm) (Fukuda

and others 1986; Dachtler and others 2003). Concentration ofsesamol is increased to higher than 36 ppm by roasting thesesame seeds due to hydrolysis of sesamolin to sesamol (Kimand Choe 2005). Sesamin and sesamolin extracted from roastedsesame oil and sesaminol in bleached sesame oil are moreheat-resistant than α-tocopherol (Fukuda and others 1986; Leeand others 2007). Secoisolariciresinol and secoisolariciresinoldiglucoside (14.1 to 30.9 mg/g, dry basis) are found in flaxseed(Eliasson and others 2003).

Ascorbic acidAscorbic acid, sodium ascorbate, and calcium ascorbate are

water soluble and have a limitation as antioxidants for fats andoils. Ascorbyl palmitate is used in fat-containing foods to decreasetheir oxidation.

CarotenoidsCarotenoids are polyenoic terpenoids having conjugated trans

double bonds. They include carotenes (β-carotene and ly-copene), which are polyene hydrocarbons, and xanthophylls(lutein, zeaxanthin, capsanthin, canthaxanthin, astaxanthin, andviolaxanthin) having oxygen in the form of hydroxy, oxo, or epoxygroups (Figure 3). Carotenoids are fat soluble and play an impor-tant role in the oxidation of fats and oils.

Carotene is the major carotenoid in oils, and β-carotene is themost studied. Palm oil is one of the richest sources of carotenoids.Crude palm oil and red palm olein contain 500 to 700 ppm

346 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009

Reaction mechanism of antioxidants . . .

O

O

HO

O

OO

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

HO

sesamol

sesamin

sesamolin

sesaminol

O

OO

O

O

OCH3

OH

sesamolinol

O

O

O

HO

OH

O

pinoresinol

OH

OH

H3CO

HO

OCH3

OH

secoisolariciresinol

Figure 2 --- Structures of lignans.

carotenoids (Bonnie and Choo 2000), but refined plam oil is nota good source of carotenoids. Virgin olive oil contains 1.0 to2.7 ppm β-carotene, as well as 0.9 to 2.3 ppm lutein (Psomiadouand Tsimidou 2002). Corn, soybean, and peanut oils containlower amounts of β-carotene at 1.2, 0.28, and 0.13 ppm, respec-tively (Parry and others 2006).

Protein-related compoundsHypoxanthine, xanthine, glycine, methionine, histidine, tryp-

tophan, proline, lysine, ferritin, transferritin, and carnosine showtheir antioxidant activities in the oxidation of lipid-containingfoods (Reische and others 2002). Enzymes such as glucose oxi-dase, superoxide dismutase, catalase, and glutathione peroxidaseare known to decrease the oxidation of foods (Yuan and Kitts1997). Application of enzymes and proteins as antioxidants islimited to unprocessed oil because oil processing denatures theenzymes and proteins.

Maillard reaction productsMaillard reaction products from amines and reducing sugars or

carbonyl compounds from lipid oxidation slow down lipid oxi-dation (Kumari and Waller 1987; Saito and Ishihara 1997). Thereare a number of Maillard reaction products, but the responsi-ble compounds for the antioxidant activity have not been clearlydetermined to date.

PhospholipidsCrude oil contains phospholipids such as phosphatidyletha-

nolamine, phosphatidylcholine, phosphatidylinositol, and phos-phatidylserine, but most of them are removed by oil process-ing such as degumming (Jung and others 1989). Oils that areconsumed without refining contain higher amounts of phos-pholipids. Crude soybean oil contains phosphatidylcholine andphosphatidylethanolamine at 501 and 214 ppm, respectively;however, RBD soybean oil contains only 0.86 and 0.12 ppm

Vol. 8, 2009—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 347


OH

HO

OH

O

HO

O

OH

O

HO

OH

β-carotene

lycopene

lutein

capsanthin

astaxanthin

violaxanthin

O

O

Figure 3 --- Structuresof carotenoids.

phosphatidylcholine and phosphatidylethanolamine, respec-tively (Yoon and others 1987). Unroasted sesame oil contains690 ppm phospholipids (Yen 1990). Extra virgin olive oil con-tains 34 to 156 ppm phospholipids and filtration of the oil lowersthe contents to 21 to 124 ppm (Koidis and Boskou 2006).

Phosphatidylcholine decreased the oxidation of docosahex-aenoic acid (DHA) and soybean oil in the dark (Koo andKim 2005; Lyberg and others 2005). Egg yolk phospholipidsat 0.031% to 0.097% decreased the autoxidation of DHA-rich oil and squalene, and the antioxidant activity of eggyolk phosphatidylethanolamine was higher than that of

phosphatidylcholine (Sugino and others 1997). Although phos-pholipids are generally known as antioxidants, they can increaselipid oxidation depending on the environment such as presenceof iron (Yoon and Min 1987). Lee (2007) reported that phos-phatidylcholine and phosphatidylethanolamine increased the ox-idation of tocopherol-stripped canola oil with added chlorophyllb under light.

SterolsSterols are steroid alcohols with an aliphatic hydrocarbon side

chain of 8 to 10 carbons at the C17-position and a hydroxy



HO

HO

HO

β-sitosterol

stigmasterol

sitostanol

Figure 4 --- Structures of sterols.

group at the C3-position (Figure 4). β-Sitosterol, stigmasterol, andsitostanol are present in edible oils, with the highest amount ofβ-sitosterol. Corn and rapeseed oils have 8000 ppm sterols, andpalm and coconut oils have 600 to 1000 ppm sterols (Verhe andothers 2006). Virgin and refined olive oils contain β-sitosterol at667 and 898 ppm, respectively (Canabate-Diaz and others 2007).Antioxidant activity of β-sitosterol was lower than those of ferulicacid and tocopherol in the autoxidation of soybean oil (Devi andothers 2007). Solubility of plant sterols in corn oil is 2% to 3% at25 ◦C (Vaikousi and others 2007).

Oxidation Mechanisms of Fats and OilsDifferent chemical mechanisms are responsible for the oxi-

dation of fats and oils during processing, storage, and cooking.Two types of oxygen, atmospheric triplet oxygen and singlet oxy-gen, can react with fats and oils. Triplet oxygen, having a radicalcharacter, reacts with radicals and causes autoxidation. The non-radical electrophilic singlet oxygen does not require radicals toreact with; it directly reacts with the double bonds of unsaturatedfats and oils with high electron densities, which is called type IIphotosensitized oxidation (Choe and Min 2005).

AutoxidationFats and oils should be in radical forms to react with triplet

oxygen in autoxidation. Lipids are normally in nonradical sin-glet state and heat, metals, or light accelerates their radical for-mation. Allylic hydrogen, especially hydrogen attached to thecarbon between 2 double bonds, is easily removed due to lowbond dissociation energy (Min and Boff 2002; Choe and Min2005). The carbon and hydrogen dissociation energies are thelowest at the bis-allylic methylene position (Wagner and others1994). Bis-allylic hydrogen at C11 of linoleic acid is removedat 75 to 80 kcal/mol. The energy required to remove allylic hy-drogen in C8 or C14 of linoleic acid is 88 kcal/mol, and 101kcal/mol is necessary to remove alkyl hydrogen from C17 or C18(Wagner and others 1994; Min and Boff 2002; Choe and Min2005). Upon formation of lipid radicals by hydrogen removal,the double bond adjacent to the carbon radical in linoleic andlinolenic acids shifts to the more stable next carbon, resultingin conjugated diene structures. The shifted double bond mostlytakes the more thermodynamically stable trans form.

The lipid radical reacts with triplet oxygen very quickly at nor-mal oxygen pressure (2 to 8 × 109/M/s; Zhu and Sevilla 1990)and forms lipid peroxy radical. The lipid peroxy radical abstractshydrogen from other lipid molecules to form lipid hydroperoxideand another lipid radical. The radicals automatically catalyze thereaction and the autoxidation is called free radical chain reac-tion. When radicals react with each other, nonradical species areproduced to stop the reaction.

Photosensitized oxidationLight accelerates lipid oxidation, especially in the presence

of photosensitizers such as chlorophylls. Chlorophylls in singletstate become excited upon absorption of light energy in picosecond (Choe and Min 2006). Excited singlet state chlorophyllsbecome excited triplet state via intersystem crossing (k = 1 to20 × 108/s; Min and Boff 2002). Excited triplet state chlorophyllsreact with triplet oxygen and produce singlet oxygen by energytransfer, returning to their ground singlet state. Singlet oxygenis able to diffuse over larger distances, about 270 nm (Skovsenand others 2005), to react with electron-rich compounds. Sincesinglet oxygen is electrophilic due to a completely vacant 2 pπorbital, it directly reacts with high-electron-density double bondsvia 6-membered ring without lipid radical formation (Gollnick1978; Choe and Min 2005). The resulting hydroperoxides bysinglet oxygen are both conjugated and nonconjugated (Frankel1985; Figure 5). Production of nonconjugated hydroperoxidesdoes not occur in autoxidation. The oxidation of linoleic acid bysinglet oxygen produces C9- and C13-hydroperoxides, as well asC10- and C12-hydroperoxides (Frankel 1985).

The reaction rate of lipid with singlet oxygen is much higherthan that with triplet oxygen; the reaction rates of linoleic acidwith singlet oxygen and triplet oxygen are 1.3 × 105 and 8.9 ×101/M/s, respectively (Rawls and Van Santen 1970).

Thermal oxidationHeating of oil produces various chemical changes including

oxidation. The chemical mechanism of thermal oxidation is basi-cally the same as the autoxidation mechanism. The rate of thermaloxidation is faster than the autoxidation, and the unstable primaryoxidation products, hydroperoxides, are decomposed rapidly intosecondary oxidation products such as aldehydes and ketones(Choe and Min 2007). Specific and detailed scientific informa-tion and comparisons of the oxidation rates between thermaloxidation and autoxidation are not yet available.

Thermal oxidation of oil produces many volatiles and non-volatiles. Volatiles such as aldehydes, ketones, short-chain hy-drocarbons, lactones, alcohols, and esters are produced from



Chlorophyll Chlorophyll Chlorophyll1O2

3O2 +light

(CH2)7COOH

OOH

H O

O

OOH

1O2O

O H

Conjugated

Nonconjugated

O

O HConjugated

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

(CH2)7COOHCH3(CH2)4

CH3(CH2)4 (CH2)7COOH

Nonconjugated

1O2

1O2

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)7COOHCH3(CH2)4

910

11

1213

13

13

13

3131

13

13

13

12

12

12

12 12

12

12

12

11

11

11

11 11

11

11

11

10

10

10

0101

10

10

109

9

9

99

9

9

9

*

Figure 5 --- Formation of lipid hydroperoxides by photosensitized oxidation.

decomposition of hydroperoxides by the same mechanisms asthe autoxidation. Many nonvolatile polar compounds and triacyl-glycerol dimers and polymers are produced in thermally oxidizedoil by radical reactions. Dimerization and polymerization are ma-jor reactions in the thermal oxidation in oil. Dimers and polymersare large molecules with a molecular weight range of 692 to 1600Daltons and formed by a combination of –C–C–, –C–O–C–, and–C–O–O–C– bonds (Kim and others 1999). Polymerization oc-curs more easily in oil with high linoleic acid than in high oleicacid oil contents (Bastida and Sanchez-Muniz 2001). C–C bondsare formed between 2 acyl groups to produce acyclic dimers inheated oil under low oxygen (Nawar 1996). The Diels-Alder re-action produces cyclic dimers of tetrasusbtituted cyclohexene,and radical reactions within or between triacylglycerols also pro-duce cyclic polymers (Choe and Min 2007). Polymers are richin oxygen and highly conjugated dienes and produce a brown,resin-like residue (Moreira and others 1999).

Enzymatic oxidationLipid oxidation is catalyzed by lipoxygenase in a nonradical

mechanism (Niki 2004). Lipoxygenase is an iron-bound enzymewith Fe in its active center. Lipoxygenase oxidizes unsaturatedfatty acids having a 1-cis, 4-cis-pentadiene system resulting inoil deterioration (Engeseth and others 1987), and oils containinglinoleic, linolenic, and arachidonic acids are favored substrates(Hsieh and Kinsella 1986). Eicosapentaenoic acid (EPA) and DHAcan also be oxidized by lipoxygenase (Wang and others 1991).

LOX(Fe3+)

LOX(Fe2+)LOX(Fe3+) R

LOX(Fe2+) ROO

RH

H

O2

H

ROOH

ROO

Figure 6 --- Oxidation of linoleic acid by lipoxygenase(LOX).

Lipoxygenase with iron in the ferric state (LOX-Fe3+) formsa stereospecific complex with the unsaturated fatty acid havinga 1,4-pentadienyl system (RH), and it abstracts hydrogens frominterrupted methylenes in the fatty acids (Figure 6). It binds topentadienyl radical which is rearranged into a conjugated diene



S

O

HO

OH

Figure 7 --- Structure of thiocremonone.

system, followed by the reaction with oxygen to produce lipidperoxy radicals (ROO �). The iron in the enzyme is reduced tothe ferrous state (LOX-Fe2+). Lipid peroxy radicals are reduced toROO− by lipoxygenase with iron in a ferric state again, and theattachment of a proton, which is produced by the oxidation ofhydrogen abstracted from fats and oils by lipoxygenase, results inrelease of hydroperoxides (Belitz and Grosch 1999).

Mechanisms of Antioxidants in the Oxidation of FoodsAntioxidants slow down the oxidation rates of foods by a

combination of scavenging free radicals, chelating prooxida-tive metals, quenching singlet oxygen and photosensitizers, andinactivating lipoxygenase.

Free radical scavengingAntioxidants scavenge free radicals of foods by donating hy-

drogen to them, and they produce relatively stable antioxi-dant radicals with low standard reduction potential, less than500 mV (Choe and Min 2005). Rates of hydrogen abstractionfrom lipids and antioxidants are in the order of 10◦/M/s and 105

to 106/M/s, respectively (Burton and others 1985; Mukai andothers 1993; Amorati and others 2007). The higher stability ofantioxidant radicals than that of food radicals is due to resonancedelocalization throughout the phenolic ring structure (Choe andMin 2006). Examples of antioxidants to scavenge free radicalsare phenolic compounds (tocopherols, butylated hydroxytoluene(BHT), butylated hydroxyanisole (BHA), tert-butylhydroquinone(TBHQ), propyl gallate (PG), lignans, flavonoids, and phenolicacids), ubiquinone (coenzyme Q), carotenoids, ascorbic acids,and amino acids. Thiacremonone (Figure 7) extracted fromheated garlic at 130 ◦C has higher radical scavenging activitythan ascorbic acid, α-tocopherol, or BHA (Hwang and others2007).

The effectiveness of antioxidants to scavenge free radicals offoods depends on the bond dissociation energy between oxygenand a phenolic hydrogen, pH related to the acid dissociationconstant, and reduction potential and delocalization of the an-tioxidant radicals (Litwinienko and Ingold 2003; Choe and Min2006; Cao and others 2007). Hydrogen transfer from antioxidantsto the peroxy or alkyl radicals of foods is more thermodynami-cally favorable when the bond dissociation energy for O–H inthe antioxidants is low (Cao and others 2007). Bond dissociationenergy for O–H of phenolic antioxidants corresponds to 70 to80 kcal/mol (Berkowits and others 1994; Lucarini and others1996; Wright and others 2001), and decreases in the order of δ> γ > β > α-tocopherol (Wright and others 2001). Bond dis-sociation energy for O–H of phenolic antioxidants is affected bysurrounding solvents; it is higher in polar solvents such as ace-tonitrile and tert-butyl alcohol than nonpolar benzene (Lucariniand others 2002; Zhang and Wang 2005). Thus, polar solventsdecrease the radical scavenging activity of the antioxidants dueto the intermolecular hydrogen bonding between oxygen or ni-trogen in a polar solvent and OH group in phenolic antioxidants(Amorati and others 2007).

The bond dissociation energy for O–H of the phenolic an-tioxidants also predicts the stabilization of antioxidant radicals.The lower the bond dissociation energy for the O–H group ofthe antioxidants, the more stable the antioxidant radical. Theantioxidants with low bond dissociation energy are thus more ef-ficient hydrogen donors and better antioxidants. The O–H bondstrength of phenolic antioxidants is affected by substitution of hy-drogen in a benzene ring. The antioxidant activity of the phenolicantioxidants is dependent on the balance between the electron-donating effect of the substituents and the steric crowding aroundthe phenolic OH groups which is related to the position of thesubstituents (Amorati and others 2007). Any substituent desta-bilizing the ground-state phenolic antioxidants, and/or stabiliz-ing the phenoxy radical form of the antioxidants, reduces theO–H bond strength. Substituents such as an alkyl or a 2nd hy-droxy group improve stabilization of the antioxidant radicals andincrease radical scavenging activity (Shahidi and Wanasundara1992). A single substitution of methyl, tert-methyl, or methoxygroup at the ortho-position decreased the O–H bond strength by1.75, 1.75, and 0.2 kcal/mol, and the O–H bond strength de-crease by the same substituent at the meta-position was about0.5 kcal/mol (Brigati and others 2002).

An intramolecular hydrogen bond between phenolic hydrogenand the oxygen-containing substituent, such as a methoxy groupat the ortho-position, stabilizes ground-state phenol to cancel theO–H bond strength decrease by the methoxy group, and there isa negligible change in the bond dissociation energy (0.2 kcal/moldecrease; Brigati and others 2002). Double substitution interac-tively (additively or synergistically) contributes to the O–H bondstrength. Electron-withdrawing substituents such as COOR andCOOH at the para-position stabilize the phenol form of antiox-idants, and destabilize the phenoxy radical form of the antioxi-dants, to increase the O–H bond strength and make the antiox-idants less efficient (Rice-Evans and others 1996). However, thesubstituent such as methyl, tert-butyl, methoxy, or phenyl groupdecreases the O–H bond strength (Brigati and others 2002). Whenthe substituent at the para-position is an unsaturated hydrocar-bon in which the unpaired electron is highly delocalized, thephenoxy radical is strongly stabilized and the bond dissociationenergy for the O–H is decreased (Brigati and others 2002). Thehydrogen-donating ability decreases in the order of hydroxyty-rosol, oleuropein, caffeic acid, chlorogenic acid, and ferulic acidin olive oil (Roche and others 2005).

The antioxidant activity of phenolic acids such as caffeic, pro-tocatechuic, and chlorogenic acids is dependent on the pH;they are not efficient radical scavengers under acidic pH, butvery good scavengers above pH 7 to 8 (Mukai and others 1997;Amorati and others 2006). At the basic pH, phenolic acids areionized to a phenolated form. The phenolated antioxidant has ahigher electron-donating capacity than the parent species and ac-tivates the phenolic group to give higher free radical scavengingactivity (Amorati and others 2006). The higher radical scavengingactivity of the phenolated form of phenolic acids was suggestedto be due to a rapid electron transfer to lipid peroxy radicals fromthe anion of the phenolic acids (Amorati and others 2006).

The reduction potential of antioxidant radicals can predict theease of a compound to donate hydrogen to food radicals; thelower the reduction potential of the antioxidant radicals, thegreater the hydrogen donating ability of the antioxidants (Choeand Min 2005). Any compound whose radical has a reductionpotential lower than food radicals or oxygen-related radicals candonate hydrogen to them, and can act as an antioxidant (Choeand Min 2005). The reduction potentials of hydroxy, alkyl, alkoxy,alkyl peroxy, and superoxide anion radicals are approximately2300, 600, 1600, 1000, and 940 mV (Choe and Min 2005),respectively. Tocopherol, ascorbic acid, and quercetin radicals



O

HO

ROO

O

O

C16H33

ROOH

O

O

CH2

+

+ R'OO + T

tocopherol dimer

tocopherol semiquinone

R'OOH

C16H33

C16H33

O

O

C16H33

+

+

(T)

(T )

Figure 8 --- Reaction of α-tocopherol withlipid peroxy radical (R, R′ = alkyl group).

have reduction potentials of 500, 330, and 330 mV (Steenkenand Neta 1982; Jovanovic and others 1996), respectively, whichare lower than peroxy, alkoxy, and alkyl radicals. This enables fortocopherol and ascorbic acid to donate hydrogen to the peroxy,alkoxy, and alkyl radicals to slow down the formation of foodradicals. Phenolic compounds can donate hydrogen to alkyl per-oxy radicals and the resulting phenolic radicals do not catalyzethe oxidation of other molecules due to the low reduction po-tential (Shahidi and Wanasundara 1992). The phenolic radicalsreact with each other to form hydroquinone with regeneration ofphenolic antioxidants or to form phenolic dimers. The phenolicradical can react with lipid peroxy radicals to form phenolic-peroxy species adducts that undergo the degradation reactions(Reische and others 2002).

α-Tocopherol reacts with alkyl peroxy radicals more rapidlythan alkyl radicals since the difference in reduction potential be-tween tocopherol radicals and alkyl peroxy radicals (500 mV) ishigher than that between tocopherol radicals and alkyl radicals(100 mV). Tocopherol donates hydrogen at the 6-hydroxy groupon a chromanol ring to alkyl peroxy radical, and alkyl hydroper-oxide and tocopherol radical are formed. Tocopherol radical isrelatively stable due to a resonance structure (Figure 8). Toco-pherol radical can react with lipid peroxy radical to producetocopherol semiquinone having no vitamin E activity, or reactwith each other for the formation of tocopherol dimer (Reische

and others 2002). Reaction rates of peroxy radical of unsaturatedfatty acids with α-tocopherol are 1.85 × 106/M/s (Kamal-Eldinand others 2008). Tocopherols slowly and irreversibly react withsuperoxide anion radicals in organic solvents and produce toco-pherol radical, but the reaction is insignificant in aqueous solu-tion (Arudi and others 1983; Halliwell and Gutteridge 2001).

Tocopherol radical sometimes reacts with lipid peroxy radi-cals at their very high concentration and produces tocopherolperoxide. Tocopherol peroxide produces 2 isomers of epoxy-8α-hydroperoxytocopherone by elimination of an alkoxy radicalfollowed by oxygen addition and hydrogen abstraction. Epoxy-8α-hydroperoxytocopherone becomes epoxyquinones upon hy-drolysis (Liebler and others 1990). This reaction produces alkoxyradicals, instead of peroxy radicals, and loses only tocopherol.Since there is no net decrease in free radicals in the system,tocopherol does not act as an antioxidant; however, reducingagents such as ascorbic acid can regenerate tocopherols fromtocopherylquinone.

Tocopherol radical at high concentration sometimes abstractshydrogen from lipids having very low concentration of per-oxy radical and produces tocopherol and lipid radical; how-ever, the rate is very low (Kamal-Eldin and others 2008). Theresulting lipid radical can increase the lipid oxidation by re-acting with triplet oxygen, and tocopherol acts as prooxidantinstead of antioxidant (Bowry and Stocker 1993; Yamamoto



O

O

A C

B1'

2

34

5

6

7

89

10

2'

3'

4'

5'

6'

1

Figure 9 ---Molecularstructureofflavonoidbackbone.

2001). Tocopherol-mediated peroxidation is prevented by ascor-bic acid since ascorbic acid quickly reduces tocopherol radicalsto tocopherols (Yamamoto 2001).

Tyrosol and hydroxytyrosol in olive oil (Chimi and others 1991)and sesamol and sesaminol in sesame oil (Dachtler and others2003; Suja and others 2005) scavenge free radicals by a simi-lar mechanism as tocopherols due to the presence of phenolichydrogen. Phenolic hydrogens of tyrosol and hydroxytyrosol aretransferred to food radicals with the production of semiquinoneradicals. The semiquinone radical of tyrosol or hydroxytyrosolmay scavenge another radical to give a quinone, disproportion-ate with another semiquinone radical to give the parent com-pound and quinone, or react with oxygen to produce quinoneand hydroperoxy radical (Niki and Noguchi 2000).

Flavonoids should have special structural features for scaveng-ing free radicals as shown in Figure 9: the ortho-dihydroxy orcatechol group in the B-ring, the conjugation of the B-ring to the4-oxo group (Rice-Evans and others 1995; Van Acker and oth-ers 1996; Pietta 2000; Silva and others 2002). Quercetin, rutin,and luteolin satisfy the requirements and are known as some ofthe most efficient radical scavengers among the nonvitamin plantphenols (Rice-Evans and others 1995). Catechin, an efficient rad-ical scavenger, does not have a 2,3-double bond and 4-carbonylgroup, but it has many hydroxy groups to donate hydrogen (Rice-Evans and others 1996). Catechol-structured flavonoids scavengelipid peroxy radicals by donating hydrogen and become more sta-ble phenoxy radicals. Phenoxy radicals undergo disproportion-ation and produce phenolic quinone and a dihydroxy phenoliccompound, as shown in Figure 10 (Shahidi and Wanasundara1992).

Carotenoids can give electrons and then donate hydrogen asshown in Figure 11. Two electrons rather than 1 are transferredper carotenoid with 2 reduction potentials, E1 and E2. Easeof electron donation of carotenoids depends on the nature ofsubstituents on the carotenoids (Jeevarajan and Kispert 1996).

OH

OH

O

OHO

O

XX X

O

OH

X

OH

OH

XOH

OH

RROO

Figure 10 ---Reactions ofcatechol-structuredflavonoid withlipid peroxyradicals.

CarH HraCHraC 2+

Car

+ e

H e-

2e

+ +

+2E1E

E3

Figure 11 --- Hydrogen release from carotenoids (CarH) viaelectron donation (E; reduction potential).

Reduction potential for sequential transferring 2 electrons aredifferent in canthaxanthin and astaxanthin, generally E1 < E2,while lycopene, β-carotene, and zeaxanthin have similar E1 andE2 values (Jeevarajan and Kispert 1996; Liu and others 2000).Electron donation of carotenoids containing terminal electronacceptor group is difficult and the 2nd electron donation oc-curs at quite a different potential to the 1st oxidation step. As theelectron-accepting strength of the end groups decreases, �E (E1 –E2) decreases or cation radical can be reduced to carotenoidradical with a reduction potential E3 which is generally muchlower than E1 (Jeevarajan and Kispert 1996). The standard re-duction potential of carotenoid radical cation (700 to 1000 mV;Jeevarajan and Kispert 1996; Liu and others 2000; Niedzwiedzkiand others 2005; Han and others 2006) is not low enough thatcarotenoid cation donates hydrogen to alkyl (E◦′ = 600 mV)or peroxy radicals of polyunsaturated fatty acids (E◦′ = 770 to1440 mV). It is easier for carotenoids to give hydrogen to hy-droxy radicals having a high reduction potential (2310 mV) thanto alkyl peroxy radicals. The energy required to remove hydrogenfrom carbons in carotene cation is about 65 kcal/mol (Zhou andothers 2000). Lycopene radical cation has the lowest reductionpotential (748 mV) followed by the radical cations of β-carotene(780 mV), zeaxanthin (812 mV), and canthaxanthin (930 mV)(Jeevarajan and Kispert 1996). Astaxanthin is a weaker antioxi-dant than zeaxanthin (Mortensen and Skibsted 1997a; Edge andothers 1998).

β-Carotene may donate hydrogen to lipid peroxy radical withsome limitations and produce carotene radical (Edge and others1998). Carotene radical is a fairly stable species due to delocal-ization of unpaired electrons in its conjugated polyene, and hasenough lifetime for a reaction with lipid peroxy radicals at lowoxygen concentration and forms nonradical carotene peroxides(Burton and Ingold 1984; Beutner and others 2001). Carotene rad-ical can also undergo oxygen addition, and subsequent reactionwith another carotene molecule, and produce carotene epoxidesand carbonyl compounds of carotene (Beutner and others 2001)as shown in Figure 12.

In addition to the radical scavenging activity of carotenoids bydonating hydrogen to lipid peroxy radicals, carotenoids can en-hance lipid oxidation (Lee and others 2003). Lipid peroxy radicals(ROO �) from the oxidation of oils may be added to β-carotene



Car+ O2 Car OO

+ CarHCar CarO O

O

+ CarH

carotene epoxide

Car CarO Car CarO OO+ CarH

Car CarO OO Car

decompose

Car CarO

carotene epoxide

Odecompose

CarO OCar + CarC C

O

Car

+ O2

dicarbonyls

ROOH

O

CarH

ROO

OCar

or

Car CarO O

Figure 12 --- Reaction of β-carotene and lipid peroxy radicals.

(Car) and produce carotene peroxy radical (ROO–Car �), espe-cially at oxygen pressure higher than 150 mm Hg (Burton andIngold 1984). β-Carotene peroxy radical reacts with tripletoxygen to form peroxy radical of carotene peroxide (ROO–Car–OO �), which then abstracts hydrogen from another lipidmolecule and produces lipid radicals (R′ �). The resulting lipidradicals propagate the chain reaction of lipid oxidation (Iannoneand others 1998), thus β-carotene acts as a prooxidant:

Car + ROO � → ROO − Car �

ROO − Car � + 3O2 → ROO − Car − OO �

ROO − Car − OO � + R′H → ROO − Car − OOH + R′ �

β-Carotene may donate electrons to free radicals and becomeβ-carotene radical cation (Liebler 1993; Mortensen and others2001). β-Carotene radical cation is stable due to resonance, andthe reaction rate with oxygen is very low (Edge and Truscott 2000;Decker 2002). However, β-carotene radical cation can easilyoxidize tocopherols and ubiquinones (Liebler 1993) as well astyrosine and cystein (Burke and others 2001). Hydrogen or elec-tron transfers from carotenoids to food radicals depend on thereduction potentials of food radicals and chemical structures ofcarotenoids, especially the presence of oxygen-containing func-tional groups (Edge and others 1997). Electron-transfer reactionfrom carotenoids to free radicals is favored when the alkyl peroxyradicals contain electron withdrawing R groups (Edge and others1998).

Ascorbic acid and glutathione scavenge free radicals by do-nating hydrogen to food radicals, producing more stable ascor-bic acid and glutathione radicals than food radicals (Buettner1993). Ascorbic acid radicals become dehydroascorbic acid byloss of proton (Decker 2002). Amino acids containing sulfhydrylor hydroxy groups such as cystein, tyrosine, phenylalanine, andproline also inactivate free radicals (Gebicki and Gebicki 1993).Inactivation of food radicals by proteinaceous compounds mightbe a result of competition between proteinaceous compoundsand lipid for high-energy food radicals, rather than an actualchain breaker (Decker 2002).

Metal chelatingMetals reduce the activation energy of the oxidation, especially

in the initiation step, to accelerate oil oxidation (Jadhav and oth-ers 1996). The activation energies for the autoxidation of refinedbleached and deodorized soybean, sunflower, and olive oils were17.6, 19.0, and 12.5 kcal/mol, respectively (Lee and others 2007).

Metals catalyze food radical formation by abstracting hydrogen.They also produce hydroxy radicals by catalyzing decomposi-tion of hydrogen peroxide (Andersson 1998) or hydroperoxides(Benjelloun and others 1991). Ferric ions decrease the oxidativestability of olive oil by decomposing phenolic antioxidants suchas caffeic acid (Keceli and Gordon 2002).

Crude oil contains transition metals such as iron or copper,often existing in chelated form rather than in a free form (Decker2002). Oil refining decreases metal contents. Edible oils manu-factured without refining, such as extra virgin olive oil (9.8 ppbcopper and 0.73 ppm iron) and roasted sesame oil (16 ppb copperand 1.16 ppm iron), contain relatively high amounts of transitionmetals (Choe and others 2005).

Metal chelators decrease oxidation by preventing metal re-dox cycling, forming insoluble metal complexes, or providingsteric hindrance between metals and food components or theiroxidation intermediates (Graf and Eaton 1990). EDTA and citricacid are the most common metal chelators in foods. Most chela-tors are water-soluble, but citric acid can be dissolved in oilswith some limitation to chelate metals in the oil system. Phos-pholipids also act as metal chelators (Koidis and Boskou 2006).Flavonoids can also bind the metal ions (Rice-Evans and others1996) and the activity is closely related with the structural fea-tures: 3′, 4′-dihydroxy group in the B ring, the 4-carbonyl and3-hydroxy group in the C ring, or the 4-carbonyl group in theC ring together with the 5-hydroxy group in the A ring (Hudsonand Lewis 1983; Feralli and others 1997). Lignans, polyphenols,ascorbic acid, and amino acids such as carnosine and histidinecan also chelate metals (Decker and others 2001).

Singlet oxygen quenchingSinglet oxygen having high energy of 93.6 kJ above the ground

state triplet oxygen (Korycka-Dahl and Richardson 1978; Girotti1998) reacts with lipids at a higher rate than triplet oxygen. Toco-pherols, carotenoids, curcumin, phenolics, urate, and ascorbatecan quench singlet oxygen (Lee and Min 1992; Das and Das2002; Choe and Min 2005). Singlet oxygen quenching includesboth physical and chemical quenching. Physical quenching leadsto deactivation of singlet oxygen to the ground state triplet oxy-gen by energy transfer or charge transfer (Min and others 1989).There is neither oxygen consumption nor product formation. Sin-glet oxygen quenching by energy transfer occurs when the en-ergy level of a quencher (Q) is very near or below that of singletoxygen:

1O2 + Q → 3O2 + 3Q3Q → 1Q (no radiation)



OO

OHHO

+ 1O2

OO

OOH

OO

HOOHOO OOH

+

OH

HO

OH

HO

OH

HO

Figure 13 --- Formationof ascorbic acidhydroperoxides bysinglet oxygen.

Carotenoids with 9 or more conjugated double bonds are goodsinglet oxygen quenchers by energy transfer. The singlet oxy-gen quenching activity of carotenoids depends on the numberof conjugated double bonds in the structure (Beutner and oth-ers 2001; Min and Boff 2002; Foss and others 2004) and thesubstituents in the β-ionone ring (Di Mascio and others 1989). β-Carotene and lycopene which have 11 conjugated double bondsare more effective singlet oxygen quenchers than lutein whichhas 10 conjugated double bonds (Viljanen and others 2002). Thepresence of oxo and conjugated keto groups, or cyclopentanering in the structure increases the singlet oxygen quenching ability(Di Mascio and others 1989); however, β-ionone ring substitutedwith hydroxy, epoxy, or methoxy groups is less effective (Viljanenand others 2002). The rate constants for singlet oxygen quenchingby canthaxanthin, β-apo-8′-carotenal, all trans β-carotene, andethyl β-apo-8′-carotenate are 1.45 × 1010, 1.38 × 1010, 1.25 ×1010, and 1.20 × 1010 /M/s, respectively (Min and others 1989).

When a quencher has high reduction potential and low tripletenergy, it quenches singlet oxygen by a charge transfer mecha-nism. These types of quenchers are amines, phenols (includingtocopherols), sulfides, iodide, and azides, which all have manyelectrons (Min and others 1989). The quencher donates electronto singlet oxygen to form a singlet state charge transfer complexand then changes the complex to the triplet state by intersys-tem crossing. Finally, the triplet state charge transfer complex isdissociated into triplet oxygen and a quencher:

1O2 + Q → [O2−− − − Q+]1 → [O2

−− − − Q+]3 → 3O2 + Q

Chemical quenching of singlet oxygen is a reaction in-volving the oxidation of a quencher rather than a quench-ing, thus producing breakdown or oxidation products of aquencher. β-Carotene, tocopherols, ascorbic acid, amino acids(such as histidine, tryptophan, cysteine, and methionine), pep-tides, and phenolics are oxidized by singlet oxygen, and theyare all chemical quenchers of singlet oxygen (Foote 1976;Michaeli and Feitelson 1994; Halliwell and Gutteridge 2001).β-Carotene reacts with singlet oxygen at a rate of 5.0 ×109/M/s (Devasagayam and others 1992) and produces 5,8-endoperoxides of β-carotene (Stratton and others 1993). Re-action of ascorbic acid with singlet oxygen produces anunstable hydroperoxide of ascorbic acid as shown in Figure 13.Tocopherol reacts irreversibly with singlet oxygen and pro-duces tocopherol hydroperoxydienone, tocopherylquinone, andquinone epoxide (Decker 2002). The reaction rates of tocopherolswith singlet oxygen are different among isomers: α-tocopherolshows the highest reaction rate of 2.1 × 108/M/s, followed by β-tocopherol with 1.5 × 108/M/s, γ -tocopherol with 1.4 × 108/M/s,and δ-tocopherol with 5.3 × 107/M/s (Mukai and others 1991).

Photosensitizer inactivationFoods contain sensitizers such as chlorophylls and riboflavin

(Jung and others 1989; Salvador and others 2001), which areactivated by light. Photoactivated sensitizers transfer the energy totriplet atmospheric oxygen to form singlet oxygen, or transfer an

electron to the triplet oxygen to form a superoxide anion radical,and these reactive oxygen species react with food componentsto produce free radicals (Min and Lee 1988). Carotenoids havingfewer than 9 conjugated double bonds prefer the inactivationof photosensitizers instead of singlet oxygen quenching; singletoxygen quenching is preferable by carotenoids with 9 or moreconjugated double bonds (Viljanen and others 2002). Energy ofthe photosensitizer is transferred to the singlet state of carotenoidsto become a triplet state of carotenoids, which is changed tothe singlet state by transferring the energy to the surrounding oremitting phosphorescence (Stahl and Sies 1992). The edge-to-edge distance for a direct quenching of triplet state of chlorophyllby carotenoids must be less than the van der Waals distance(0.36 nm), which enables some overlap between electron orbitalsof these 2 pigments (Edge and Truscott 1999).

Inactivating lipoxygenaseLipoxygenase is a catalytic enzyme in the oxidation of lipids

and is inactivated by tempering, which is heat treatment withmoisture. Steaming of ground soybeans at 100 ◦C for 2 min or116 ◦C under 44.5 N for 1 min decreases the lipoxygenase activityby 80% to 100%, with a decrease in peroxide values, whichimproves the sensory quality of crude soybean oil (Engeseth andothers 1987).

Interactions of Antioxidants in the Oxidation of FoodsInteractions among antioxidants can be synergistic, antagonis-

tic, or merely additive. Synergism is a phenomenon in which anet interactive antioxidant effect is higher than the sum of theindividual effects. A typical example of antioxidant synergism isbetween α-tocopherol and ascorbic acid in autoxidation (Liebler1993) and photooxidation of lipids (Van Aardt and others 2005).Antagonism is a phenomenon in which a net interactive antiox-idant effect is lower than the sum of the individual antioxidanteffects, and the additive interaction means that a net interac-tive antioxidant effect is the same as the sum of individual ef-fects. Polyphenolic compounds such as epigallocatechin gallate,quercetin, epicatechin gallate, epicatechin, and cyanidin showedadditive effects on free radical scavenging activity with ascorbicacid or α-tocopherol (Murakami and others 2003).

SynergismSeveral mechanisms are involved in synergism among antioxi-

dants: a combination of 2 or more different free radical scavengersin which one antioxidant is regenerated by others, a sacrificialoxidation of an antioxidant to protect another antioxidant, and acombination of 2 or more antioxidants whose antioxidant mech-anisms are different (Decker 2002). Regeneration of a more effec-tive free radical scavenger (primary antioxidant) by a less effectivefree radical scavenger (coantioxidant, synergist) occurs mostlywhen one free radical scavenger has a higher reduction poten-tial than the other. The free radical scavenger having a higherreduction potential acts as a primary antioxidant. Regenerationof primary antioxidants contributes to a higher net interactive



antioxidant effect than the simple sum of individual effects(Decker 2002). The antioxidant system of ascorbic acid and to-copherols is an example, in which tocopherols (E◦ = 500 mV)are primary antioxidants and ascorbic acid (E◦ = 330 mV) is asynergist (Liebler 1993). Tocopherols (TH) act as antioxidant bydonating hydrogen to alkyl (R �) or alkyl peroxy (ROO �) radi-cals in foods and become tocopherol radical (T �) which doesnot have antioxidant activity. Ascorbic acid (AsH) gives hydrogento tocopherol radical to regenerate tocopherols and it becomessemihydroascorbyl radical (As �), and then dehydroascorbic acid(DHAs; Buettner 1993):

TH + R � → T � + RH

TH + ROO � → T � + ROOH

T � + AsH → TH + As �

As � → DHAs + H �

Interaction between tocopherols and carotenoids for theirregeneration is another example of synergism, although it ismore complicated. Carotenoids are regenerated by tocopherolsand tocopherols are regenerated by carotenoids (Mortensen andSkibsted 1997b, 1997c). But the carotenoid regeneration by to-copherols is more preferable (Kago and Terao 1995; Thiyam andothers 2006), partly because of the higher standard reductionpotential of the carotenoid radical cation (700 to 1000 mV;Jeevarajan and Kispert 1996; Liu and others 2000; Niedzwiedzkiand others 2005; Han and others 2006) than that of the to-copherol radical (500 mV). β-Carotene (0.75 M) was shownto sharply disappear from the beginning of oleic acid oxida-tion and then mostly disappeared within 100 h in the absenceof tocopherols. But the co-presence of α-tocopherol at 3.8 ×10−3 M increased the time from 100 to 1500 h (Przybylski2001). α-Tocopherol regenerates the carotenoid by reducing thecarotenoid radical cation (Mortensen and Skibsted 1997c; Edgeand others 1998; Mortensen and others 1998). However, Henryand others (1998) showed that there was no cooperative interac-tion between α-tocopherol and β-carotene in delaying the onsetof safflower seed oil oxidation at 75 ◦C.

Two antioxidants whose bond dissociation energy differenceis high exert a synergistic antioxidant effect (Decker 2002). Re-generation of the antioxidant is fast when a synergist has a higherbond dissociation energy than the primary antioxidant (Pedrielliand Skibsted 2002). Also, the primary antioxidant can be regen-erated when the rate constant for regeneration of the primaryantioxidant is at least 103/M/s and the reaction constant of alkylperoxy radicals with that of the antioxidant radicals is similar(Amorati and others 2002a, 2002b). Regeneration of the antiox-idant can be accomplished by electron transfer from a synergistto a primary antioxidant (Jovanovic and others 1995).

Synergistic antioxidant effects can be achieved by the protec-tive action of one antioxidant by means of its sacrificial oxidation(Decker 2002). The less effective antioxidant traps alkyl or alkylperoxy radicals in foods, resulting in protecting more an effectiveantioxidant from the oxidation due to antioxidant action. Or theantioxidant radical produced from the oxidation of the less ef-fective antioxidant competes with more effective antioxidant fortrapping alkyl peroxy radicals to decrease the oxidation of themore effective antioxidant. The interaction between tocopherolsand carotenoids partly results from this mechanism (Haila andothers 1996).

When there are 2 or more antioxidants whose antioxidantmechanisms are different, the antioxidation can also show a

synergism (Decker 2002). A combination of metal chelators andfree radical scavengers is a good example. They show synergismin inhibiting the oxidation of food components, mainly due tothe sparing action of free radical scavengers by chelators. Metalchelators such as phospholipids inhibit metal-catalyzed oxidation(Koidis and Boskou 2006), producing lower levels of radicals tobe reduced by the antioxidants acting as free radical scavengers.Metal chelators mainly act during the initiation step of lipid oxi-dation and free radical scavengers do so at the propagation step(Choe and Min 2006). Phosphatidylinositol acts as a synergistwith tocopherols in decreasing lipid oxidation, mainly by in-active complex formation with prooxidative metals (Servili andMontedoro 2002). Quercetin and α-tocopherol show a synergismin decreasing the oxidation of lard by the mechanism in whichα-tocopherol acts as a free radical scavenger while quercetin actsas a metal chelator (Hudson and Lewis 1983).

AntagonismAntagonism has been observed between α-tocopherol and ros-

marinic acid or caffeic acid (Peyrat-Maillard and others 2003;Samotyja and Malecka 2007), between catechin and caffeic acid,and between caffeic acid and quercetin (Peyrat-Maillard and oth-ers 2003). Plant extracts rich in polyphenols showed strong an-tagonism with α-tocopherol in lard (Banias and others 1992) orsunflower oil (Hras and others 2000).

Antagonism among antioxidants in the oxidation of food com-ponents can arise by regeneration of the less effective antioxi-dant by the more effective antioxidant (Peyrat-Maillard and others2003), oxidation of the more effective antioxidant by the radicalsof the less effective antioxidant, competition between formationof antioxidant radical adducts and regeneration of the antioxi-dant (Mortensen and Skibsted 1997a, 1997c), and alteration ofmicroenvironment of one antioxidant by another antioxidant. An-tagonism of antioxidants in the oxidation of foods has not yet beendescribed in detail.

ConclusionsReaction mechanisms and the type of natural antioxidants in

foods, tocopherols, ascorbic acid, carotenoids, flavonoids, aminoacids, phospholipids, and sterols were reviewed kinetically andthermodynamically. They inhibit the oxidation of useful foodcomponents by inactivating free radicals, chelating prooxida-tive metals, and quenching singlet oxygen and photosensitizers.When there are 2 or more antioxidants together, interaction oc-curs such as synergism, antagonism, and simple addition.

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