84607_11.pdf

Antioxidants: Science,

Technology, and

Applications

P. K. J. P. D. Wanasundara 1 and F. Shahidi 2

1 Agriculture and Agri-Food Canada Saskatoon Research Center

Saskatoon, Saskatchewan, Canada

2 Memorial University of Newfoundland,

St. John’s, Newfoundland, Canada

1. AN ANTIOXIDANT—DEFINITION

In a biological system, an antioxidant can be deﬁned as ‘‘any substance that when

present at low concentrations compared to that of an oxidizable substrate would sig-

niﬁcantly delay or prevent oxidation of that substrate’’ (1). The oxidizable substrate

may be any molecule that is found in foods or biological materials, including car-

bohydrates, DNA, lipids, and proteins. Food is a multicomponent system composed

of a variety of biomolecules, and therefore, this deﬁnition describes well an antiox-

idant. However, regulatory bodies that overlook the food-supply categorize antiox-

idants under food additives and deﬁne them as ‘‘substances used to preserve food by

retarding deterioration, rancidity, or discoloration due to oxidation’’ (Code of

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.

Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

431

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ANTIOXIDANTS: SCIENCE, TECHNOLOGY, AND APPLICATIONS

Federal Regulations, Food and Drug Administration). In foods, much of the work

on antioxidants has emphasized retardation of lipid oxidation, which eventually

triggers and transforms to the oxidation of other macromolecules such as proteins.

It is the intention of this chapter to summarize the available information on the

chemistry, technology, and regulatory aspects of compounds that can delay oxida-

tion of unsaturated fats and lipids in food.

2. HISTORY OF ANTIOXIDANTS AND THEIR USE

Antioxidants may occur as natural constituents of foods, and may intentionally be

added to products or formed during processing. Use of substances to enhance qual-

ity of food by means of delaying lipid oxidation has been in practice for centuries,

although it was not chemically deﬁned or understood. The ﬁrst recorded scientiﬁc

observation on oxidation inhibitors came from Berthollet in 1797 (2) and later from

Davy (3). Their theory was described as ‘‘catalyst poisoning’’ in oxidative reactors,

and this was well before the free radical theory of peroxidation had been proposed.

Duclaux (4) ﬁrst demonstrated participation of atmospheric oxygen in oxidation of

free fatty acids. Later, it was found that oxidation of unsaturated acylglycerols can

generate rancid odors in ﬁsh oils (5).

The earliest reported work on the use of antioxidants to retard lipid oxidation

appeared in 1843, in which Deschamps showed that an ointment made of fresh

lard containing gum benzoin (contains vanillin) or populin (from polar buds, con-

tains saligenin and derivatives) did not become rancid as did the one with pure lard

(2). Interestingly, the ﬁrst reports on antioxidants employed for food lipids were

about using natural sources; in 1852, Wright (6) reported that elm bark was effec-

tive in preserving butterfat and lard. Chevreul (7) showed that wood of oak, poplar,

and pine (in the order of decreasing efﬁcacy) retarded the drying of linseed oil ﬁlms

applied on them, and on all three, it took much longer time to dry than on glass.

Moureu and Dufraise (8–11) ﬁrst reported the possibility of using synthetic chemi-

cals, especially phenolic compounds, to retard oxidative decomposition of food

lipids. Their work provided the basic information leading to theories of lipid oxida-

tion and antioxidants, which they referred to as ‘‘inverse catalysis.’’ Systematic

investigation of antioxidant activity based on the chemistry of radical chain perox-

idation of ‘‘model’’ chemicals was reported by Lowry and his collegues (12) and

Bolland and tenHave (13) of the British Rubber Producers Research Association.

Antioxidant synergism in food was ﬁrst reported by Olcott and Mattill (14), and

this was signiﬁcant in achieving oxidative stability in food by using a combination

of antioxidants found in the unsaponiﬁable fraction of oils. They described the anti-

oxidants as inhibitors and grouped them into acid type, inhibitols, and hydroqui-

none and phenolics. Bailey (15) and Scott (16) have provided the history and a

descriptive analysis of the development of antioxidants in their books, ‘‘The Retar-

dation of Chemical Reactions’’ and ‘‘Antioxidants and Autoxidation’’, respectively.

Since the early 1960s, the understanding of autoxidation of unsaturated lipids and

antioxidative mechanisms have advanced signiﬁcantly as a result of development of

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SCOPE OF USING ANTIOXIDANTS IN FOOD

effective analytical tools. The last two decades have been very important to the anti-

oxidant research. Around the world a revival is seen in studying the natural antiox-

idants in foods and the potential health beneﬁts of natural antioxidants in relation to

prevention and therapy of oxidative stress and related diseases. The emphasis has

largely been on their implications on vital biological reactions that have a direct

relationship to tissue injury and degenerative diseases. Enough scientiﬁc evidences

have already been accumulated in relation to these conditions with free radicals and

reactive oxygen species. Therefore, not only enhancing the shelf life stability of

foods has been examined, but also control of lipid oxidation by suppressing free

radical formation in foods to prevent their deleterious health effects has become

important. The quest for understanding the oxidation of lipids and its prevention

and control has continued since historical times and is still on.

3. SCOPE OF USING ANTIOXIDANTS IN FOOD

The function of an antioxidant is to retard the oxidation of an organic substance,

thus increasing the useful life or shelf life of that material. In fats and oils, anti-

oxidants delay the onset of oxidation or slow the rate of oxidizing reactions. Oxida-

tion of lipids chemically produces compounds with different odors and taste and

continues to affect other molecules in the food. The main purpose of using an anti-

oxidant as a food additive is to maintain the quality of that food and to extend its

shelf life rather than improving the quality of the food. Figure 1 illustrates how anti-

oxidants can affect the quality maintenance of food in terms of oxidative rancidity

Figure 1. Typical curves for oxidation of lipids (a) No antioxidant added; (b) and (c) represent

added or endogenous antioxidants. Antioxidant activity of (c) is higher than (b). IP1, IP2, and IP3

are induction period in hours or days.

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ANTIOXIDANTS: SCIENCE, TECHNOLOGY, AND APPLICATIONS

development. Use of antioxidants reduces raw material wastage and nutrition loss

and widens the range of fats that can be used in speciﬁc products. Thus, antioxi-

dants are useful additives that allow food processors to use fats and oils economic-

ally in their product formulation.

4. OXIDATION OF FATS AND OILS AND MECHANISM

OF ANTIOXIDANTS

In fats and oils, the process of oxidation is similar to that oxidation of any other

unsaturated organic material and requires an initiation process, in order to generate

free radicals from the substrate. As antioxidants inhibit oxidation or autoxidation

process, the mechanism(s) involved need(s) to be discussed. Figure 1 explains

the relationship of antioxidant activity and oxidation of a lipid as examined by a

typical evaluation method.

Autoxidation is the oxidative deterioration of unsaturated fatty acids via an auto-

catalytic process consisting of a free radical chain mechanism. This chain includes

initiation, propagation, and termination reactions that could be cyclical once

started. The initiation process generates free radicals from the substrate. The a-

methylenic H atom is abstracted from the unsaturated lipid molecule to form a lipid

(alkyl) radical (R ) (Scheme 1, Equation [1]). The lipid radical is highly reactive

and can react with atmospheric oxygen ( 3 O 2 ), a facile reaction resulting from the

diradical nature of the oxygen molecule, and it produces a peroxy radical (ROO )

(Scheme 1, Equation [2]). In the propagation reactions, the peroxy radical reacts

with another unsaturated lipid molecule to form a hydroperoxide and a new

unstable lipid radical (Scheme 1, Equation [3]). As a new free radical is generated

at each step, more oxygen is incorporated into the system. The newly propagated

lipid radical will then react with oxygen to produce another peroxy radical, result-

ing in a self-catalyzed, cyclical mechanism (Scheme 1, Equation [4]).

[1]

R• + 3 O 2

[2]

ROO• + RH

[3]

ROOH

[4]

RO•+ RH

[5]

Scheme 1. Possible reactions of the autoxidation process. ‘‘R’’ is an alkyl group of an

unsaturated lipid molecule. ‘‘H’’ is an a -methylenic hydrogen atom easily detachable because of

the activating inﬂuence of the neighboring double bond or bonds. ‘‘ RO ’’ is alkoxy radical,

‘‘ROO ’’ is peroxy radical, and ‘‘ I ’’ is an initiator.

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OXIDATION OF FATS AND OILS AND MECHANISM OF ANTIOXIDANTS

Hydroperoxides are unstable and may degrade to radicals that accelerate propaga-

tion reactions. These are branching steps of lipid autoxidation process (Scheme 1,

Equations [5] and [6]). This chain reaction proceeds, and termination occurs only

when two free radicals combine to form a nonradical product. Autoxidation can

break down the substrate molecules as well as forming new molecules causing

gross changes in the chemical and physical properties of the oxidizing substrate

(17–19). Degradation of hydroperoxides may generate new molecules that have

undesirable odors and ﬂavors, associated with oxidative rancidity of unsaturated

lipids. Such sensory perceivable changes are noted when oxidation of unsaturated

lipids has been progressed to advanced stages. This is only a brief description of

autoxidation process.

A lipid that contains double bonds undergoes autoxidation induced by various

ways. It is now clear that metal-catalyzed decomposition of preformed hydroperox-

ides is the most likely cause for the initiation process. The direct oxidation of unsa-

turated lipids by triplet oxygen ( 3 O 2 ) is spin forbidden. This is because of the

opposite spin direction of ground state lipid of single multiplicity and oxygen of

triplet multiplicity, which does not match. When initiators are present, this spin bar-

rier between lipids and oxygen can readily be overcome and produce radicals by

different mechanisms. Ground state oxygen may be activated in the presence of

metal or metal complexes and can initiate oxidation either by formation of free radi-

cals or singlet oxygen. Exposure of lipids to light, metals, singlet oxygen and sen-

sitizers (chlorophyll, hemoproteins, and riboﬂavin), or preformed hydroperoxide

decomposition products causes generation of primary hydroperoxides. Photosensi-

tized oxidation or lipoxygenase-catalyzed oxidation also produces hydroperoxides.

Thermal oxidation is also autocatalytic and considered as metal-catalyzed

because it is very difﬁcult to eliminate trace metals (from fats and oils or food)

that act as catalysts and may occur as proposed in Equation 4. Redox metals of

variable valency may also catalyze decomposition of hydroperoxides (Scheme 2,

Equations [6] and [7]). Direct photooxidation is caused by free radicals produced

by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and per-

oxides. This oxidation proceeds as a free radical chain reaction. Although there

should be direct irradiation from ultraviolet light for the lipid substrate, which is

usually uncommon under normal practices, the presence of metals and metal com-

plexes of oxygen can become activated and generate free radicals or singlet oxygen.

ROOH + M 2+

+ M 3+

[6]

ROOH + M 3+

H + + M 2+

[7]

Scheme 2. Possible reactions of generating hydroperoxides (M nþ

is the metal ion with

transitional valency).

Photosensitized oxidation is a direct reaction of light-activated, singlet oxygen with

unsaturated fatty acids, and subsequently hydroperoxides are formed. Photosensitized

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