Mechanisms of
Antioxidants in
the Oxidation
of Foods
Eunok 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 inhibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and
photosensitizers, and inactivating lipoxygenase. Antioxidants show interactions, such as synergism (tocopherols
and ascorbic acids), antagonism (␣-tocopherol and caffeic acid), and simple addition. Synergism occurs when one
antioxidant is regenerated by others, when one antioxidant protects another antioxidant by its sacrificial oxidation,
and when 2 or more antioxidants show different antioxidant mechanisms.
Introduction
kinetic characteristics depending on their surroundings during
Oxidation decreases consumer acceptability of foods by pro- the oxidation of foods.
ducing low-molecular-weight off-flavor compounds, as well as by
destroying essential nutrients, and it produces toxic compounds Major Antioxidants in Foods
and dimers or polymers of lipids and proteins (Aruoma 1998).
Extensive research has been done on the isolation, purificaOxidation of foods can be minimized by removing prooxidants tion, and identification of the various antioxidants. Phenolic
such as free fatty acids, metals, and oxidized compounds, and by compounds and ascorbic acid are the most important natural
protecting foods from light. Air evacuation by reduced pressure antioxidants. Carotenoids, protein-related compounds, Maillard
or adding oxygen scavengers can also reduce oxidation. Since it reaction products, phospholipids, and sterols also show natural
is very difficult to completely remove all the prooxidants and air, antioxidant activities in foods.
antioxidants are now increasingly added to foods to slow down
the process of oxidation.
Phenolic compounds
Antioxidants significantly delay or inhibit oxidation of oxidizPhenolic compounds such as tocopherols, polyphenols, pheable substrates at low concentration, compared to the higher
nolic acids, and lignans are widely distributed in plants (Dicko
contents of lipids and proteins in foods (Halliwell and Gutteridge
and others 2006; Wang and Ballington 2007).
2001). Antioxidants in foods do not necessarily protect biological
Tocopherols. Tocopherols are monophenolic compounds and
tissues from free radical oxidative damage because they have to
derivatives of chromanol as shown in Figure 1. They are very solbe converted into usable forms in tissues and interact with other
uble in oil and thus are the most important antioxidants in edible
substances, in addition to effective concentration differences, and
fats and oils. Tocopherols are more frequently found in vegetable
they must display difficulty in absorption from the diet (Azzi and
oils than animal fats, especially soybean, canola, sunflower, corn,
others 2004). The antioxidants are naturally present in foods,
and palm oils. Most vegetable oils contain tocopherols at conor can be added or formed during processing. Antioxidants for
centrations higher than 500 ppm; beef tallow and lard contain
foods should be reasonable in cost, nontoxic, stable, effective at
less than 40 ppm (Choe and others 2005). Palm oil contains tocolow concentration, have carry-through, and should not change
pherols at 100 to 150 ppm, and also 620 to 650 ppm tocotrienols
flavor, color, and texture of the food matrix (Schuler 1990). The
(Al-Saqer and others 2004). The refining process, especially deeffects of antioxidants on the oxidation of foods are dependent
odorization, reduces tocopherol contents in oils (Jung and others
on their concentration (Frankel and others 1996), polarity, and
1989; Reische and others 2002; Eidhin and others 2003). Tothe medium (Cuvelier and others 2000; Samotyja and Malecka
copherols in crude soybean oil (1670 ppm) were decreased to
2007), and also the presence of other antioxidants (Decker 2002).
1138 ppm during deodorization (Jung and others 1989).
The objective of this article was to discuss the reaction mechaPolyphenols. Olive oil is oxidation-resistant due to the presnisms of antioxidants by focusing on their thermodynamic and
ence of tyrosol (4-hydroxyphenylethanol; 34.9 ppm), hydroxytyrosol (3,4-dihydroxyphenylethanol; 37.8 ppm), and catechol
MS 20090169 Submitted 2/26/2009, Accepted 4/15/2009 . Author Choe is (Keceli and Gordon 2002; Servili and Montedoro 2002). Hydroxwith Dept. of Food and Nutrition, Inha Univ., Incheon, Korea. Author Min ytyrosol is the most effective antioxidant in olive oil (Papadopouis with Dept. of Food Science and Technology, The Ohio State Univ., 2015 los and Boskou 1991; Tsimidou and others 1992; Baldioli and
Fyffe Rd., Columbus, Ohio, U.S.A. Direct inquiries to author Min (E-mail: others 1996). Most of these antioxidants are removed during [email protected]).
kaline refining and deodorization (Garcia and others 2006).
C
R
2009 Institute of Food Technologists
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CRFSFS: Comprehensive Reviews in Food Science and Food Safety
HO
HO
CH2[CH2CH2CH(CH3)CH2]3H
CH2[CH2CH2CH(CH3)CH2]3H
O
O
α-tocopherol
β-tocopherol
HO
HO
CH2[CH2CH2CH(CH3)CH2]3H
CH2[CH2CH2CH(CH3)CH2]3H
O
O
γ-tocopherol
δ-tocopherol
HO
HO
CH2[CH2CH=C(CH3)CH2]3H
CH2[CH2CH=C(CH3)CH2]3H
O
O
α-tocotrienol
β-tocotrienol
HO
HO
CH2[CH2CH=C(CH3)CH2]3H
O
γ-tocotrienol
Flavonoids are major plant polyphenols and are derivatives
of diphenylpropanes and a heterocyclic 6-membered ring with
oxygen. They include flavanols (catechins, naringin), flavanones
(hesperidin, naringenin), flavones (apigenin, luteolin), flavonols
(kaempferol, quercitrin, myricetin, quercetin), anthocyanins, and
leucoanthocyanidins. The glycosylation of flavonoids results
in lower antioxidant activity than the corresponding aglycons
(Shahidi and Wanasundara 1992). The solubility of flavonoids in
fats and oils is very low and their role in the oxidation of oil is
not significant; however, they can contribute to decreasing the
oxidation 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, and
vanillic acids). Chlorogenic and caffeic acids are present in sunflower oil, and sinapic and ferulic acids are present in rapeseed
(Leonardis and others 2003) and defatted rice bran oils (Devi and
others 2007), respectively. Olive oil contains vanillic, syringic,
caffeic, and cinnamic acids (Servili and Montedoro 2002). Phenolic acids as antioxidants in oils are also limited due to solubility
problems.
Lignans. Lignans are phenylpropanoids derived from phenylalanine as shown in Figure 2. They include sesamol, sesamin,
sesamolin, sesaminol, sesamolinol, pinoresinol, and secoisolariciresinol. The major lignans in unroasted sesame oil are sesamin
(474 ppm), sesamolin (159 ppm), and sesamol (<7 ppm) (Fukuda
346
Figure 1 --Structures of
tocopherols and
tocotrienols.
CH2[CH2CH=C(CH3)CH2]3H
O
δ-tocotrienol
and others 1986; Dachtler and others 2003). Concentration of
sesamol is increased to higher than 36 ppm by roasting the
sesame seeds due to hydrolysis of sesamolin to sesamol (Kim
and Choe 2005). Sesamin and sesamolin extracted from roasted
sesame oil and sesaminol in bleached sesame oil are more
heat-resistant than α-tocopherol (Fukuda and others 1986; Lee
and others 2007). Secoisolariciresinol and secoisolariciresinol
diglucoside (14.1 to 30.9 mg/g, dry basis) are found in flaxseed
(Eliasson and others 2003).
Ascorbic acid
Ascorbic acid, sodium ascorbate, and calcium ascorbate are
water soluble and have a limitation as antioxidants for fats and
oils. Ascorbyl palmitate is used in fat-containing foods to decrease
their oxidation.
Carotenoids
Carotenoids are polyenoic terpenoids having conjugated trans
double bonds. They include carotenes (β-carotene and lycopene), which are polyene hydrocarbons, and xanthophylls
(lutein, zeaxanthin, capsanthin, canthaxanthin, astaxanthin, and
violaxanthin) having oxygen in the form of hydroxy, oxo, or epoxy
groups (Figure 3). Carotenoids are fat soluble and play an important role in the oxidation of fats and oils.
Carotene is the major carotenoid in oils, and β-carotene is the
most studied. Palm oil is one of the richest sources of carotenoids.
Crude palm oil and red palm olein contain 500 to 700 ppm
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009
Reaction mechanism of antioxidants . . .
O
Figure 2 --- Structures of lignans.
O
HO
O
O
O
O
sesamol
O
O
O
O
O
sesamin
O
O
O
HO
O
O
O
O
O
O
O
sesamolin
O
O
O
sesaminol
O
HO
O
O
O
O
OH
sesamolinol
OH
O
OH
OCH3
pinoresinol
H3CO
OH
HO
OCH3
OH
secoisolariciresinol
carotenoids (Bonnie and Choo 2000), but refined plam oil is not
a good source of carotenoids. Virgin olive oil contains 1.0 to
2.7 ppm β-carotene, as well as 0.9 to 2.3 ppm lutein (Psomiadou
and Tsimidou 2002). Corn, soybean, and peanut oils contain
lower amounts of β-carotene at 1.2, 0.28, and 0.13 ppm, respectively (Parry and others 2006).
Maillard reaction products
Maillard reaction products from amines and reducing sugars or
carbonyl compounds from lipid oxidation slow down lipid oxidation (Kumari and Waller 1987; Saito and Ishihara 1997). There
are a number of Maillard reaction products, but the responsible compounds for the antioxidant activity have not been clearly
determined to date.
Protein-related compounds
Hypoxanthine, xanthine, glycine, methionine, histidine, tryptophan, proline, lysine, ferritin, transferritin, and carnosine show
their antioxidant activities in the oxidation of lipid-containing
foods (Reische and others 2002). Enzymes such as glucose oxidase, superoxide dismutase, catalase, and glutathione peroxidase
are known to decrease the oxidation of foods (Yuan and Kitts
1997). Application of enzymes and proteins as antioxidants is
limited to unprocessed oil because oil processing denatures the
enzymes and proteins.
Phospholipids
Crude oil contains phospholipids such as phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, and phosphatidylserine, but most of them are removed by oil processing such as degumming (Jung and others 1989). Oils that are
consumed without refining contain higher amounts of phospholipids. Crude soybean oil contains phosphatidylcholine and
phosphatidylethanolamine at 501 and 214 ppm, respectively;
however, RBD soybean oil contains only 0.86 and 0.12 ppm
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Figure 3 --- Structures
of carotenoids.
β-carotene
lycopene
OH
lutein
O
OH
HO
capsanthin
O
OH
HO
astaxanthin
O
OH
O
O
HO
violaxanthin
phosphatidylcholine and phosphatidylethanolamine, respectively (Yoon and others 1987). Unroasted sesame oil contains
690 ppm phospholipids (Yen 1990). Extra virgin olive oil contains 34 to 156 ppm phospholipids and filtration of the oil lowers
the contents to 21 to 124 ppm (Koidis and Boskou 2006).
Phosphatidylcholine decreased the oxidation of docosahexaenoic acid (DHA) and soybean oil in the dark (Koo and
Kim 2005; Lyberg and others 2005). Egg yolk phospholipids
at 0.031% to 0.097% decreased the autoxidation of DHArich oil and squalene, and the antioxidant activity of egg
yolk phosphatidylethanolamine was higher than that of
348
phosphatidylcholine (Sugino and others 1997). Although phospholipids are generally known as antioxidants, they can increase
lipid oxidation depending on the environment such as presence
of iron (Yoon and Min 1987). Lee (2007) reported that phosphatidylcholine and phosphatidylethanolamine increased the oxidation of tocopherol-stripped canola oil with added chlorophyll
b under light.
Sterols
Sterols are steroid alcohols with an aliphatic hydrocarbon side
chain of 8 to 10 carbons at the C17-position and a hydroxy
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009
Reaction mechanism of antioxidants . . .
Autoxidation
HO
β-sitosterol
stigmasterol
HO
Fats and oils should be in radical forms to react with triplet
oxygen in autoxidation. Lipids are normally in nonradical singlet state and heat, metals, or light accelerates their radical formation. Allylic hydrogen, especially hydrogen attached to the
carbon between 2 double bonds, is easily removed due to low
bond dissociation energy (Min and Boff 2002; Choe and Min
2005). The carbon and hydrogen dissociation energies are the
lowest at the bis-allylic methylene position (Wagner and others
1994). Bis-allylic hydrogen at C11 of linoleic acid is removed
at 75 to 80 kcal/mol. The energy required to remove allylic hydrogen in C8 or C14 of linoleic acid is 88 kcal/mol, and 101
kcal/mol is necessary to remove alkyl hydrogen from C17 or C18
(Wagner and others 1994; Min and Boff 2002; Choe and Min
2005). Upon formation of lipid radicals by hydrogen removal,
the double bond adjacent to the carbon radical in linoleic and
linolenic acids shifts to the more stable next carbon, resulting
in conjugated diene structures. The shifted double bond mostly
takes the more thermodynamically stable trans form.
The lipid radical reacts with triplet oxygen very quickly at normal oxygen pressure (2 to 8 × 109 /M/s; Zhu and Sevilla 1990)
and forms lipid peroxy radical. The lipid peroxy radical abstracts
hydrogen from other lipid molecules to form lipid hydroperoxide
and another lipid radical. The radicals automatically catalyze the
reaction and the autoxidation is called free radical chain reaction. When radicals react with each other, nonradical species are
produced to stop the reaction.
Photosensitized oxidation
sitostanol
HO
Figure 4 --- Structures of sterols.
group at the C3-position (Figure 4). β-Sitosterol, stigmasterol, and
sitostanol are present in edible oils, with the highest amount of
β-sitosterol. Corn and rapeseed oils have 8000 ppm sterols, and
palm and coconut oils have 600 to 1000 ppm sterols (Verhe and
others 2006). Virgin and refined olive oils contain β-sitosterol at
667 and 898 ppm, respectively (Canabate-Diaz and others 2007).
Antioxidant activity of β-sitosterol was lower than those of ferulic
acid and tocopherol in the autoxidation of soybean oil (Devi and
others 2007). Solubility of plant sterols in corn oil is 2% to 3% at
25 ◦ C (Vaikousi and others 2007).
Oxidation Mechanisms of Fats and Oils
Different chemical mechanisms are responsible for the oxidation of fats and oils during processing, storage, and cooking.
Two types of oxygen, atmospheric triplet oxygen and singlet oxygen, can react with fats and oils. Triplet oxygen, having a radical
character, reacts with radicals and causes autoxidation. The nonradical electrophilic singlet oxygen does not require radicals to
react with; it directly reacts with the double bonds of unsaturated
fats and oils with high electron densities, which is called type II
photosensitized oxidation (Choe and Min 2005).
Light accelerates lipid oxidation, especially in the presence
of photosensitizers such as chlorophylls. Chlorophylls in singlet
state become excited upon absorption of light energy in pico
second (Choe and Min 2006). Excited singlet state chlorophylls
become excited triplet state via intersystem crossing (k = 1 to
20 × 108 /s; Min and Boff 2002). Excited triplet state chlorophylls
react with triplet oxygen and produce singlet oxygen by energy
transfer, returning to their ground singlet state. Singlet oxygen
is able to diffuse over larger distances, about 270 nm (Skovsen
and others 2005), to react with electron-rich compounds. Since
singlet oxygen is electrophilic due to a completely vacant 2 pπ
orbital, it directly reacts with high-electron-density double bonds
via 6-membered ring without lipid radical formation (Gollnick
1978; Choe and Min 2005). The resulting hydroperoxides by
singlet oxygen are both conjugated and nonconjugated (Frankel
1985; Figure 5). Production of nonconjugated hydroperoxides
does not occur in autoxidation. The oxidation of linoleic acid by
singlet oxygen produces C9- and C13-hydroperoxides, as well as
C10- and C12-hydroperoxides (Frankel 1985).
The reaction rate of lipid with singlet oxygen is much higher
than that with triplet oxygen; the reaction rates of linoleic acid
with singlet oxygen and triplet oxygen are 1.3 × 105 and 8.9 ×
101 /M/s, respectively (Rawls and Van Santen 1970).
Thermal oxidation
Heating of oil produces various chemical changes including
oxidation. The chemical mechanism of thermal oxidation is basically the same as the autoxidation mechanism. The rate of thermal
oxidation is faster than the autoxidation, and the unstable primary
oxidation products, hydroperoxides, are decomposed rapidly into
secondary oxidation products such as aldehydes and ketones
(Choe and Min 2007). Specific and detailed scientific information and comparisons of the oxidation rates between thermal
oxidation and autoxidation are not yet available.
Thermal oxidation of oil produces many volatiles and nonvolatiles. Volatiles such as aldehydes, ketones, short-chain hydrocarbons, lactones, alcohols, and esters are produced from
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light
Chlorophyll
Chlorophyll*
3
O2
1
Chlorophyll
O2 +
OOH
13
10
12
9
1
11
CH3(CH2)4
13
12
O2
(CH2)7COOH
9
10
13
12
11
O
CH3(CH2)4
10
9
11
H
(CH2)7COOH
CH3(CH2)4
(CH2)7COOH
O
Conjugated
OOH
13
10
12
1
O2
9
13
10
12
9
11
11
O
CH3(CH2)4
O
CH3(CH2)4
(CH2)7COOH
H
CH(CH2)6COOH
Nonconjugated
OOH
1
13
12
O2
10
9
13
12
10
CH3(CH2)4
H
9
11
11
O
(CH2)7COOH
O
(CH2)7COOH
CH3(CH2)3CH
Nonconjugated
OOH
1
13
O2
10
12
13
9
10
12
9
11
11
O
CH3(CH2)4
O
H
(CH2)7COOH
(CH2)7COOH
CH3(CH2)4
Conjugated
Figure 5 --- Formation of lipid hydroperoxides by photosensitized oxidation.
decomposition of hydroperoxides by the same mechanisms as
the autoxidation. Many nonvolatile polar compounds and triacylglycerol dimers and polymers are produced in thermally oxidized
oil by radical reactions. Dimerization and polymerization are major reactions in the thermal oxidation in oil. Dimers and polymers
are large molecules with a molecular weight range of 692 to 1600
Daltons and formed by a combination of –C–C–, –C–O–C–, and
–C–O–O–C– bonds (Kim and others 1999). Polymerization occurs more easily in oil with high linoleic acid than in high oleic
acid oil contents (Bastida and Sanchez-Muniz 2001). C–C bonds
are formed between 2 acyl groups to produce acyclic dimers in
heated oil under low oxygen (Nawar 1996). The Diels-Alder reaction produces cyclic dimers of tetrasusbtituted cyclohexene,
and radical reactions within or between triacylglycerols also produce cyclic polymers (Choe and Min 2007). Polymers are rich
in oxygen and highly conjugated dienes and produce a brown,
resin-like residue (Moreira and others 1999).
Enzymatic oxidation
Lipid oxidation is catalyzed by lipoxygenase in a nonradical
mechanism (Niki 2004). Lipoxygenase is an iron-bound enzyme
with Fe in its active center. Lipoxygenase oxidizes unsaturated
fatty acids having a 1-cis, 4-cis-pentadiene system resulting in
oil deterioration (Engeseth and others 1987), and oils containing
linoleic, linolenic, and arachidonic acids are favored substrates
(Hsieh and Kinsella 1986). Eicosapentaenoic acid (EPA) and DHA
can also be oxidized by lipoxygenase (Wang and others 1991).
350
LOX(Fe3+)
ROOH
RH
H
H
LOX(Fe3+)
LOX(Fe2+)
ROO
R
O2
LOX(Fe2+)
ROO
Figure 6 --- Oxidation of linoleic acid by lipoxygenase
(LOX).
Lipoxygenase with iron in the ferric state (LOX-Fe3+ ) forms
a stereospecific complex with the unsaturated fatty acid having
a 1,4-pentadienyl system (RH), and it abstracts hydrogens from
interrupted methylenes in the fatty acids (Figure 6). It binds to
pentadienyl radical which is rearranged into a conjugated diene
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009
Reaction mechanism of antioxidants . . .
OH
O
HO
S
Figure 7 --- Structure of thiocremonone.
system, followed by the reaction with oxygen to produce lipid
peroxy radicals (ROO r). The iron in the enzyme is reduced to
the ferrous state (LOX-Fe2+ ). Lipid peroxy radicals are reduced to
ROO− by lipoxygenase with iron in a ferric state again, and the
attachment of a proton, which is produced by the oxidation of
hydrogen abstracted from fats and oils by lipoxygenase, results in
release of hydroperoxides (Belitz and Grosch 1999).
Mechanisms of Antioxidants in the Oxidation of Foods
Antioxidants slow down the oxidation rates of foods by a
combination of scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and photosensitizers, and
inactivating lipoxygenase.
Free radical scavenging
Antioxidants scavenge free radicals of foods by donating hydrogen to them, and they produce relatively stable antioxidant radicals with low standard reduction potential, less than
500 mV (Choe and Min 2005). Rates of hydrogen abstraction
from lipids and antioxidants are in the order of 10◦ /M/s and 105
to 106 /M/s, respectively (Burton and others 1985; Mukai and
others 1993; Amorati and others 2007). The higher stability of
antioxidant radicals than that of food radicals is due to resonance
delocalization throughout the phenolic ring structure (Choe and
Min 2006). Examples of antioxidants to scavenge free radicals
are phenolic compounds (tocopherols, butylated hydroxytoluene
(BHT), butylated hydroxyanisole (BHA), tert-butylhydroquinone
(TBHQ), propyl gallate (PG), lignans, flavonoids, and phenolic
acids), ubiquinone (coenzyme Q), carotenoids, ascorbic acids,
and amino acids. Thiacremonone (Figure 7) extracted from
heated garlic at 130 ◦ C has higher radical scavenging activity
than ascorbic acid, α-tocopherol, or BHA (Hwang and others
2007).
The effectiveness of antioxidants to scavenge free radicals of
foods depends on the bond dissociation energy between oxygen
and a phenolic hydrogen, pH related to the acid dissociation
constant, and reduction potential and delocalization of the antioxidant radicals (Litwinienko and Ingold 2003; Choe and Min
2006; Cao and others 2007). Hydrogen transfer from antioxidants
to the peroxy or alkyl radicals of foods is more thermodynamically favorable when the bond dissociation energy for O–H in
the antioxidants is low (Cao and others 2007). Bond dissociation
energy for O–H of phenolic antioxidants corresponds to 70 to
80 kcal/mol (Berkowits and others 1994; Lucarini and others
1996; Wright and others 2001), and decreases in the order of δ
> γ > β > α-tocopherol (Wright and others 2001). Bond dissociation energy for O–H of phenolic antioxidants is affected by
surrounding solvents; it is higher in polar solvents such as acetonitrile and tert-butyl alcohol than nonpolar benzene (Lucarini
and others 2002; Zhang and Wang 2005). Thus, polar solvents
decrease the radical scavenging activity of the antioxidants due
to the intermolecular hydrogen bonding between oxygen or nitrogen in a polar solvent and OH group in phenolic antioxidants
(Amorati and others 2007).
The bond dissociation energy for O–H of the phenolic antioxidants also predicts the stabilization of antioxidant radicals.
The lower the bond dissociation energy for the O–H group of
the antioxidants, the more stable the antioxidant radical. The
antioxidants with low bond dissociation energy are thus more efficient hydrogen donors and better antioxidants. The O–H bond
strength of phenolic antioxidants is affected by substitution of hydrogen in a benzene ring. The antioxidant activity of the phenolic
antioxidants is dependent on the balance between the electrondonating effect of the substituents and the steric crowding around
the phenolic OH groups which is related to the position of the
substituents (Amorati and others 2007). Any substituent destabilizing the ground-state phenolic antioxidants, and/or stabilizing the phenoxy radical form of the antioxidants, reduces the
O–H bond strength. Substituents such as an alkyl or a 2nd hydroxy group improve stabilization of the antioxidant radicals and
increase radical scavenging activity (Shahidi and Wanasundara
1992). A single substitution of methyl, tert-methyl, or methoxy
group at the ortho-position decreased the O–H bond strength by
1.75, 1.75, and 0.2 kcal/mol, and the O–H bond strength decrease by the same substituent at the meta-position was about
0.5 kcal/mol (Brigati and others 2002).
An intramolecular hydrogen bond between phenolic hydrogen
and the oxygen-containing substituent, such as a methoxy group
at the ortho-position, stabilizes ground-state phenol to cancel the
O–H bond strength decrease by the methoxy group, and there is
a negligible change in the bond dissociation energy (0.2 kcal/mol
decrease; Brigati and others 2002). Double substitution interactively (additively or synergistically) contributes to the O–H bond
strength. Electron-withdrawing substituents such as COOR and
COOH at the para-position stabilize the phenol form of antioxidants, and destabilize the phenoxy radical form of the antioxidants, to increase the O–H bond strength and make the antioxidants less efficient (Rice-Evans and others 1996). However, the
substituent such as methyl, tert-butyl, methoxy, or phenyl group
decreases the O–H bond strength (Brigati and others 2002). When
the substituent at the para-position is an unsaturated hydrocarbon in which the unpaired electron is highly delocalized, the
phenoxy radical is strongly stabilized and the bond dissociation
energy for the O–H is decreased (Brigati and others 2002). The
hydrogen-donating ability decreases in the order of hydroxytyrosol, oleuropein, caffeic acid, chlorogenic acid, and ferulic acid
in olive oil (Roche and others 2005).
The antioxidant activity of phenolic acids such as caffeic, protocatechuic, and chlorogenic acids is dependent on the pH;
they are not efficient radical scavengers under acidic pH, but
very good scavengers above pH 7 to 8 (Mukai and others 1997;
Amorati and others 2006). At the basic pH, phenolic acids are
ionized to a phenolated form. The phenolated antioxidant has a
higher electron-donating capacity than the parent species and activates the phenolic group to give higher free radical scavenging
activity (Amorati and others 2006). The higher radical scavenging
activity of the phenolated form of phenolic acids was suggested
to be due to a rapid electron transfer to lipid peroxy radicals from
the anion of the phenolic acids (Amorati and others 2006).
The reduction potential of antioxidant radicals can predict the
ease of a compound to donate hydrogen to food radicals; the
lower the reduction potential of the antioxidant radicals, the
greater the hydrogen donating ability of the antioxidants (Choe
and Min 2005). Any compound whose radical has a reduction
potential lower than food radicals or oxygen-related radicals can
donate hydrogen to them, and can act as an antioxidant (Choe
and Min 2005). The reduction potentials of hydroxy, alkyl, alkoxy,
alkyl peroxy, and superoxide anion radicals are approximately
2300, 600, 1600, 1000, and 940 mV (Choe and Min 2005),
respectively. Tocopherol, ascorbic acid, and quercetin radicals
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CRFSFS: Comprehensive Reviews in Food Science and Food Safety
Figure 8 --- Reaction of α-tocopherol with
lipid peroxy radical (R, R = alkyl group).
HO
+
O
ROO
C16H33
(T)
O
O
+
O
C16H33
O
ROOH
C16H33
(T )
+ T
+ R'OO
tocopherol dimer
O
CH2
O
+
R'OOH
C16H33
tocopherol semiquinone
have reduction potentials of 500, 330, and 330 mV (Steenken
and Neta 1982; Jovanovic and others 1996), respectively, which
are lower than peroxy, alkoxy, and alkyl radicals. This enables for
tocopherol and ascorbic acid to donate hydrogen to the peroxy,
alkoxy, and alkyl radicals to slow down the formation of food
radicals. Phenolic compounds can donate hydrogen to alkyl peroxy radicals and the resulting phenolic radicals do not catalyze
the oxidation of other molecules due to the low reduction potential (Shahidi and Wanasundara 1992). The phenolic radicals
react with each other to form hydroquinone with regeneration of
phenolic antioxidants or to form phenolic dimers. The phenolic
radical can react with lipid peroxy radicals to form phenolicperoxy species adducts that undergo the degradation reactions
(Reische and others 2002).
α-Tocopherol reacts with alkyl peroxy radicals more rapidly
than alkyl radicals since the difference in reduction potential between tocopherol radicals and alkyl peroxy radicals (500 mV) is
higher than that between tocopherol radicals and alkyl radicals
(100 mV). Tocopherol donates hydrogen at the 6-hydroxy group
on a chromanol ring to alkyl peroxy radical, and alkyl hydroperoxide and tocopherol radical are formed. Tocopherol radical is
relatively stable due to a resonance structure (Figure 8). Tocopherol radical can react with lipid peroxy radical to produce
tocopherol semiquinone having no vitamin E activity, or react
with each other for the formation of tocopherol dimer (Reische
352
and others 2002). Reaction rates of peroxy radical of unsaturated
fatty acids with α-tocopherol are 1.85 × 106 /M/s (Kamal-Eldin
and others 2008). Tocopherols slowly and irreversibly react with
superoxide anion radicals in organic solvents and produce tocopherol radical, but the reaction is insignificant in aqueous solution (Arudi and others 1983; Halliwell and Gutteridge 2001).
Tocopherol radical sometimes reacts with lipid peroxy radicals at their very high concentration and produces tocopherol
peroxide. Tocopherol peroxide produces 2 isomers of epoxy8α-hydroperoxytocopherone by elimination of an alkoxy radical
followed by oxygen addition and hydrogen abstraction. Epoxy8α-hydroperoxytocopherone becomes epoxyquinones upon hydrolysis (Liebler and others 1990). This reaction produces alkoxy
radicals, 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, reducing
agents such as ascorbic acid can regenerate tocopherols from
tocopherylquinone.
Tocopherol radical at high concentration sometimes abstracts
hydrogen from lipids having very low concentration of peroxy radical and produces tocopherol and lipid radical; however, the rate is very low (Kamal-Eldin and others 2008). The
resulting lipid radical can increase the lipid oxidation by reacting with triplet oxygen, and tocopherol acts as prooxidant
instead of antioxidant (Bowry and Stocker 1993; Yamamoto
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009
Reaction mechanism of antioxidants . . .
3'
2'
1
8
9
7
A
O
2
1'
6'
C
6
10
5
B
Figure 9 --Molecular
4' structure
of
flavonoid
5' backbone.
3
4
O
2001). Tocopherol-mediated peroxidation is prevented by ascorbic acid since ascorbic acid quickly reduces tocopherol radicals
to tocopherols (Yamamoto 2001).
Tyrosol and hydroxytyrosol in olive oil (Chimi and others 1991)
and sesamol and sesaminol in sesame oil (Dachtler and others
2003; Suja and others 2005) scavenge free radicals by a similar mechanism as tocopherols due to the presence of phenolic
hydrogen. Phenolic hydrogens of tyrosol and hydroxytyrosol are
transferred to food radicals with the production of semiquinone
radicals. The semiquinone radical of tyrosol or hydroxytyrosol
may scavenge another radical to give a quinone, disproportionate with another semiquinone radical to give the parent compound and quinone, or react with oxygen to produce quinone
and hydroperoxy radical (Niki and Noguchi 2000).
Flavonoids should have special structural features for scavenging free radicals as shown in Figure 9: the ortho-dihydroxy or
catechol group in the B-ring, the conjugation of the B-ring to the
4-oxo group (Rice-Evans and others 1995; Van Acker and others 1996; Pietta 2000; Silva and others 2002). Quercetin, rutin,
and luteolin satisfy the requirements and are known as some of
the most efficient radical scavengers among the nonvitamin plant
phenols (Rice-Evans and others 1995). Catechin, an efficient radical scavenger, does not have a 2,3-double bond and 4-carbonyl
group, but it has many hydroxy groups to donate hydrogen (RiceEvans and others 1996). Catechol-structured flavonoids scavenge
lipid peroxy radicals by donating hydrogen and become more stable phenoxy radicals. Phenoxy radicals undergo disproportionation and produce phenolic quinone and a dihydroxy phenolic
compound, as shown in Figure 10 (Shahidi and Wanasundara
1992).
Carotenoids can give electrons and then donate hydrogen as
shown in Figure 11. Two electrons rather than 1 are transferred
per carotenoid with 2 reduction potentials, E1 and E2. Ease
of electron donation of carotenoids depends on the nature of
substituents on the carotenoids (Jeevarajan and Kispert 1996).
CarH
E1
Car
X
OH
O
OH
ROO
R
X
X
CarH2+
+
2e
+
H
+
e-
Figure 11 --- Hydrogen release from carotenoids (CarH) via
electron donation (E; reduction potential).
Reduction potential for sequential transferring 2 electrons are
different in canthaxanthin and astaxanthin, generally E1 < E2,
while lycopene, β-carotene, and zeaxanthin have similar E1 and
E2 values (Jeevarajan and Kispert 1996; Liu and others 2000).
Electron donation of carotenoids containing terminal electron
acceptor group is difficult and the 2nd electron donation occurs at quite a different potential to the 1st oxidation step. As the
electron-accepting strength of the end groups decreases, E (E1 –
E2) decreases or cation radical can be reduced to carotenoid
radical with a reduction potential E3 which is generally much
lower than E1 (Jeevarajan and Kispert 1996). The standard reduction potential of carotenoid radical cation (700 to 1000 mV;
Jeevarajan and Kispert 1996; Liu and others 2000; Niedzwiedzki
and others 2005; Han and others 2006) is not low enough that
carotenoid cation donates hydrogen to alkyl (E◦ = 600 mV)
or peroxy radicals of polyunsaturated fatty acids (E◦ = 770 to
1440 mV). It is easier for carotenoids to give hydrogen to hydroxy radicals having a high reduction potential (2310 mV) than
to alkyl peroxy radicals. The energy required to remove hydrogen
from carbons in carotene cation is about 65 kcal/mol (Zhou and
others 2000). Lycopene radical cation has the lowest reduction
potential (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 antioxidant than zeaxanthin (Mortensen and Skibsted 1997a; Edge and
others 1998).
β-Carotene may donate hydrogen to lipid peroxy radical with
some limitations and produce carotene radical (Edge and others
1998). Carotene radical is a fairly stable species due to delocalization of unpaired electrons in its conjugated polyene, and has
enough lifetime for a reaction with lipid peroxy radicals at low
oxygen concentration and forms nonradical carotene peroxides
(Burton and Ingold 1984; Beutner and others 2001). Carotene radical can also undergo oxygen addition, and subsequent reaction
with another carotene molecule, and produce carotene epoxides
and carbonyl compounds of carotene (Beutner and others 2001)
as shown in Figure 12.
In addition to the radical scavenging activity of carotenoids by
donating hydrogen to lipid peroxy radicals, carotenoids can enhance lipid oxidation (Lee and others 2003). Lipid peroxy radicals
(ROO r) from the oxidation of oils may be added to β-carotene
OH
OH
OH
OH
E2
e
E3
O
OH
+
CarH
O
X
O
OH
Figure 10 --Reactions of
catecholstructured
flavonoid with
lipid peroxy
radicals.
X
Vol. 8, 2009—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY
353
CRFSFS: Comprehensive Reviews in Food Science and Food Safety
ROO
Car O
ROOH
CarH
Car
+ O2
+ CarH
Car OO
Car O O Car
carotene epoxide
Car O
+ CarH
Car O Car
+ O2
Car O Car OO
+ CarH
Car O Car OO Car
decompose
Car O Car O
or
carotene epoxide
Car O Car O
decompose
O
Car
+
O Car O
O
C Car C
dicarbonyls
Figure 12 --- Reaction of β-carotene and lipid peroxy radicals.
(Car) and produce carotene peroxy radical (ROO–Car r), especially at oxygen pressure higher than 150 mm Hg (Burton and
Ingold 1984). β-Carotene peroxy radical reacts with triplet
oxygen to form peroxy radical of carotene peroxide (ROO–
Car–OO r), which then abstracts hydrogen from another lipid
molecule and produces lipid radicals (R r). The resulting lipid
radicals propagate the chain reaction of lipid oxidation (Iannone
and others 1998), thus β-carotene acts as a prooxidant:
Car + ROO
r → ROO − Car r
ROO − Car r + 3 O2 → ROO − Car − OO
r
ROO − Car − OO r + R H → ROO − Car − OOH + R
r
β-Carotene may donate electrons to free radicals and become
β-carotene radical cation (Liebler 1993; Mortensen and others
2001). β-Carotene radical cation is stable due to resonance, and
the reaction rate with oxygen is very low (Edge and Truscott 2000;
Decker 2002). However, β-carotene radical cation can easily
oxidize tocopherols and ubiquinones (Liebler 1993) as well as
tyrosine and cystein (Burke and others 2001). Hydrogen or electron transfers from carotenoids to food radicals depend on the
reduction potentials of food radicals and chemical structures of
carotenoids, especially the presence of oxygen-containing functional groups (Edge and others 1997). Electron-transfer reaction
from carotenoids to free radicals is favored when the alkyl peroxy
radicals contain electron withdrawing R groups (Edge and others
1998).
Ascorbic acid and glutathione scavenge free radicals by donating hydrogen to food radicals, producing more stable ascorbic acid and glutathione radicals than food radicals (Buettner
1993). Ascorbic acid radicals become dehydroascorbic acid by
loss of proton (Decker 2002). Amino acids containing sulfhydryl
or hydroxy groups such as cystein, tyrosine, phenylalanine, and
proline also inactivate free radicals (Gebicki and Gebicki 1993).
Inactivation of food radicals by proteinaceous compounds might
be a result of competition between proteinaceous compounds
and lipid for high-energy food radicals, rather than an actual
chain breaker (Decker 2002).
Metals catalyze food radical formation by abstracting hydrogen.
They also produce hydroxy radicals by catalyzing decomposition of hydrogen peroxide (Andersson 1998) or hydroperoxides
(Benjelloun and others 1991). Ferric ions decrease the oxidative
stability of olive oil by decomposing phenolic antioxidants such
as 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 (Decker
2002). Oil refining decreases metal contents. Edible oils manufactured without refining, such as extra virgin olive oil (9.8 ppb
copper and 0.73 ppm iron) and roasted sesame oil (16 ppb copper
and 1.16 ppm iron), contain relatively high amounts of transition
metals (Choe and others 2005).
Metal chelators decrease oxidation by preventing metal redox cycling, forming insoluble metal complexes, or providing
steric hindrance between metals and food components or their
oxidation intermediates (Graf and Eaton 1990). EDTA and citric
acid are the most common metal chelators in foods. Most chelators are water-soluble, but citric acid can be dissolved in oils
with some limitation to chelate metals in the oil system. Phospholipids also act as metal chelators (Koidis and Boskou 2006).
Flavonoids can also bind the metal ions (Rice-Evans and others
1996) and the activity is closely related with the structural features: 3 , 4 -dihydroxy group in the B ring, the 4-carbonyl and
3-hydroxy group in the C ring, or the 4-carbonyl group in the
C ring together with the 5-hydroxy group in the A ring (Hudson
and Lewis 1983; Feralli and others 1997). Lignans, polyphenols,
ascorbic acid, and amino acids such as carnosine and histidine
can also chelate metals (Decker and others 2001).
Singlet oxygen quenching
Singlet oxygen having high energy of 93.6 kJ above the ground
state triplet oxygen (Korycka-Dahl and Richardson 1978; Girotti
1998) reacts with lipids at a higher rate than triplet oxygen. Tocopherols, carotenoids, curcumin, phenolics, urate, and ascorbate
can quench singlet oxygen (Lee and Min 1992; Das and Das
2002; Choe and Min 2005). Singlet oxygen quenching includes
both physical and chemical quenching. Physical quenching leads
to deactivation of singlet oxygen to the ground state triplet oxygen by energy transfer or charge transfer (Min and others 1989).
There is neither oxygen consumption nor product formation. Singlet oxygen quenching by energy transfer occurs when the enMetal chelating
ergy level of a quencher (Q) is very near or below that of singlet
Metals reduce the activation energy of the oxidation, especially oxygen:
in the initiation step, to accelerate oil oxidation (Jadhav and oth1
ers 1996). The activation energies for the autoxidation of refined
O2 + Q → 3 O2 + 3 Q
bleached and deodorized soybean, sunflower, and olive oils were
3
17.6, 19.0, and 12.5 kcal/mol, respectively (Lee and others 2007).
Q → 1 Q (no radiation)
354
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 8, 2009
Reaction mechanism of antioxidants . . .
OH
OH
O
HO
HO
O
+
HO
OH
O
1
OH
O
HO
O
+
O2
HOO
OH
Carotenoids with 9 or more conjugated double bonds are good
singlet oxygen quenchers by energy transfer. The singlet oxygen quenching activity of carotenoids depends on the number
of conjugated double bonds in the structure (Beutner and others 2001; Min and Boff 2002; Foss and others 2004) and the
substituents in the β-ionone ring (Di Mascio and others 1989). βCarotene and lycopene which have 11 conjugated double bonds
are more effective singlet oxygen quenchers than lutein which
has 10 conjugated double bonds (Viljanen and others 2002). The
presence of oxo and conjugated keto groups, or cyclopentane
ring in the structure increases the singlet oxygen quenching ability
(Di Mascio and others 1989); however, β-ionone ring substituted
with hydroxy, epoxy, or methoxy groups is less effective (Viljanen
and others 2002). The rate constants for singlet oxygen quenching
by canthaxanthin, β-apo-8 -carotenal, all trans β-carotene, and
ethyl 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 triplet
energy, it quenches singlet oxygen by a charge transfer mechanism. These types of quenchers are amines, phenols (including
tocopherols), sulfides, iodide, and azides, which all have many
electrons (Min and others 1989). The quencher donates electron
to singlet oxygen to form a singlet state charge transfer complex
and then changes the complex to the triplet state by intersystem crossing. Finally, the triplet state charge transfer complex is
dissociated into triplet oxygen and a quencher:
1
O
Figure 13 --- Formation
of ascorbic acid
hydroperoxides by
singlet oxygen.
O2 + Q → [O2 − − − − Q+ ]1 → [O2 − − − − Q+ ]3 → 3 O2 + Q
Chemical quenching of singlet oxygen is a reaction involving the oxidation of a quencher rather than a quenching, thus producing breakdown or oxidation products of a
quencher. β-Carotene, tocopherols, ascorbic acid, amino acids
(such as histidine, tryptophan, cysteine, and methionine), peptides, and phenolics are oxidized by singlet oxygen, and they
are 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 5,8endoperoxides of β-carotene (Stratton and others 1993). Reaction of ascorbic acid with singlet oxygen produces an
unstable hydroperoxide of ascorbic acid as shown in Figure 13.
Tocopherol reacts irreversibly with singlet oxygen and produces tocopherol hydroperoxydienone, tocopherylquinone, and
quinone epoxide (Decker 2002). The reaction rates of tocopherols
with singlet oxygen are different among isomers: α-tocopherol
shows the highest reaction rate of 2.1 × 108 /M/s, followed by βtocopherol 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 inactivation
Foods contain sensitizers such as chlorophylls and riboflavin
(Jung and others 1989; Salvador and others 2001), which are
activated by light. Photoactivated sensitizers transfer the energy to
triplet atmospheric oxygen to form singlet oxygen, or transfer an
O
O
HO
OOH
electron to the triplet oxygen to form a superoxide anion radical,
and these reactive oxygen species react with food components
to produce free radicals (Min and Lee 1988). Carotenoids having
fewer than 9 conjugated double bonds prefer the inactivation
of photosensitizers instead of singlet oxygen quenching; singlet
oxygen quenching is preferable by carotenoids with 9 or more
conjugated double bonds (Viljanen and others 2002). Energy of
the photosensitizer is transferred to the singlet state of carotenoids
to become a triplet state of carotenoids, which is changed to
the singlet state by transferring the energy to the surrounding or
emitting phosphorescence (Stahl and Sies 1992). The edge-toedge distance for a direct quenching of triplet state of chlorophyll
by carotenoids must be less than the van der Waals distance
(0.36 nm), which enables some overlap between electron orbitals
of these 2 pigments (Edge and Truscott 1999).
Inactivating lipoxygenase
Lipoxygenase is a catalytic enzyme in the oxidation of lipids
and is inactivated by tempering, which is heat treatment with
moisture. Steaming of ground soybeans at 100 ◦ C for 2 min or
116 ◦ C under 44.5 N for 1 min decreases the lipoxygenase activity
by 80% to 100%, with a decrease in peroxide values, which
improves the sensory quality of crude soybean oil (Engeseth and
others 1987).
Interactions of Antioxidants in the Oxidation of Foods
Interactions among antioxidants can be synergistic, antagonistic, or merely additive. Synergism is a phenomenon in which a
net interactive antioxidant effect is higher than the sum of the
individual effects. A typical example of antioxidant synergism is
between α-tocopherol and ascorbic acid in autoxidation (Liebler
1993) and photooxidation of lipids (Van Aardt and others 2005).
Antagonism is a phenomenon in which a net interactive antioxidant effect is lower than the sum of the individual antioxidant
effects, and the additive interaction means that a net interactive antioxidant effect is the same as the sum of individual effects. Polyphenolic compounds such as epigallocatechin gallate,
quercetin, epicatechin gallate, epicatechin, and cyanidin showed
additive effects on free radical scavenging activity with ascorbic
acid or α-tocopherol (Murakami and others 2003).
Synergism
Several mechanisms are involved in synergism among antioxidants: a combination of 2 or more different free radical scavengers
in which one antioxidant is regenerated by others, a sacrificial
oxidation of an antioxidant to protect another antioxidant, and a
combination of 2 or more antioxidants whose antioxidant mechanisms are different (Decker 2002). Regeneration of a more effective free radical scavenger (primary antioxidant) by a less effective
free radical scavenger (coantioxidant, synergist) occurs mostly
when one free radical scavenger has a higher reduction potential than the other. The free radical scavenger having a higher
reduction potential acts as a primary antioxidant. Regeneration
of primary antioxidants contributes to a higher net interactive
Vol. 8, 2009—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY
355
CRFSFS: Comprehensive Reviews in Food Science and Food Safety
antioxidant effect than the simple sum of individual effects
(Decker 2002). The antioxidant system of ascorbic acid and tocopherols is an example, in which tocopherols (E◦ = 500 mV)
are primary antioxidants and ascorbic acid (E◦ = 330 mV) is a
synergist (Liebler 1993). Tocopherols (TH) act as antioxidant by
donating hydrogen to alkyl (R r) or alkyl peroxy (ROO r) radicals in foods and become tocopherol radical (T r) which does
not have antioxidant activity. Ascorbic acid (AsH) gives hydrogen
to tocopherol radical to regenerate tocopherols and it becomes
semihydroascorbyl radical (As r), and then dehydroascorbic acid
(DHAs; Buettner 1993):
TH + R
TH + ROO
r → T r + RH
r → T r + ROOH
T r + AsH → TH + As
As
r
r → DHAs + H r
Interaction between tocopherols and carotenoids for their
regeneration is another example of synergism, although it is
more complicated. Carotenoids are regenerated by tocopherols
and tocopherols are regenerated by carotenoids (Mortensen and
Skibsted 1997b, 1997c). But the carotenoid regeneration by tocopherols is more preferable (Kago and Terao 1995; Thiyam and
others 2006), partly because of the higher standard reduction
potential of the carotenoid radical cation (700 to 1000 mV;
Jeevarajan and Kispert 1996; Liu and others 2000; Niedzwiedzki
and others 2005; Han and others 2006) than that of the tocopherol radical (500 mV). β-Carotene (0.75 M) was shown
to sharply disappear from the beginning of oleic acid oxidation and then mostly disappeared within 100 h in the absence
of tocopherols. But the co-presence of α-tocopherol at 3.8 ×
10−3 M increased the time from 100 to 1500 h (Przybylski
2001). α-Tocopherol regenerates the carotenoid by reducing the
carotenoid radical cation (Mortensen and Skibsted 1997c; Edge
and others 1998; Mortensen and others 1998). However, Henry
and others (1998) showed that there was no cooperative interaction between α-tocopherol and β-carotene in delaying the onset
of safflower seed oil oxidation at 75 ◦ C.
Two antioxidants whose bond dissociation energy difference
is high exert a synergistic antioxidant effect (Decker 2002). Regeneration of the antioxidant is fast when a synergist has a higher
bond dissociation energy than the primary antioxidant (Pedrielli
and Skibsted 2002). Also, the primary antioxidant can be regenerated when the rate constant for regeneration of the primary
antioxidant is at least 103 /M/s and the reaction constant of alkyl
peroxy radicals with that of the antioxidant radicals is similar
(Amorati and others 2002a, 2002b). Regeneration of the antioxidant can be accomplished by electron transfer from a synergist
to a primary antioxidant (Jovanovic and others 1995).
Synergistic antioxidant effects can be achieved by the protective action of one antioxidant by means of its sacrificial oxidation
(Decker 2002). The less effective antioxidant traps alkyl or alkyl
peroxy radicals in foods, resulting in protecting more an effective
antioxidant from the oxidation due to antioxidant action. Or the
antioxidant radical produced from the oxidation of the less effective antioxidant competes with more effective antioxidant for
trapping alkyl peroxy radicals to decrease the oxidation of the
more effective antioxidant. The interaction between tocopherols
and carotenoids partly results from this mechanism (Haila and
others 1996).
When there are 2 or more antioxidants whose antioxidant
mechanisms are different, the antioxidation can also show a
356
synergism (Decker 2002). A combination of metal chelators and
free radical scavengers is a good example. They show synergism
in inhibiting the oxidation of food components, mainly due to
the sparing action of free radical scavengers by chelators. Metal
chelators such as phospholipids inhibit metal-catalyzed oxidation
(Koidis and Boskou 2006), producing lower levels of radicals to
be reduced by the antioxidants acting as free radical scavengers.
Metal chelators mainly act during the initiation step of lipid oxidation and free radical scavengers do so at the propagation step
(Choe and Min 2006). Phosphatidylinositol acts as a synergist
with tocopherols in decreasing lipid oxidation, mainly by inactive complex formation with prooxidative metals (Servili and
Montedoro 2002). Quercetin and α-tocopherol show a synergism
in decreasing the oxidation of lard by the mechanism in which
α-tocopherol acts as a free radical scavenger while quercetin acts
as a metal chelator (Hudson and Lewis 1983).
Antagonism
Antagonism has been observed between α-tocopherol and rosmarinic 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 others 2003). Plant extracts rich in polyphenols showed strong antagonism with α-tocopherol in lard (Banias and others 1992) or
sunflower oil (Hras and others 2000).
Antagonism among antioxidants in the oxidation of food components can arise by regeneration of the less effective antioxidant by the more effective antioxidant (Peyrat-Maillard and others
2003), oxidation of the more effective antioxidant by the radicals
of the less effective antioxidant, competition between formation
of antioxidant radical adducts and regeneration of the antioxidant (Mortensen and Skibsted 1997a, 1997c), and alteration of
microenvironment of one antioxidant by another antioxidant. Antagonism of antioxidants in the oxidation of foods has not yet been
described in detail.
Conclusions
Reaction mechanisms and the type of natural antioxidants in
foods, tocopherols, ascorbic acid, carotenoids, flavonoids, amino
acids, phospholipids, and sterols were reviewed kinetically and
thermodynamically. They inhibit the oxidation of useful food
components by inactivating free radicals, chelating prooxidative metals, and quenching singlet oxygen and photosensitizers.
When there are 2 or more antioxidants together, interaction occurs such as synergism, antagonism, and simple addition.
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