84607_11.pdf
(
502 KB
)
Pobierz
11
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
432
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
433
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.
434
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]).
I
RH
[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.
435
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
Plik z chomika:
kasatka
Inne pliki z tego folderu:
84607_01.pdf
(316 KB)
84607_01_2.pdf
(316 KB)
84607_02.pdf
(238 KB)
84607_03.pdf
(527 KB)
84607_04.pdf
(1262 KB)
Inne foldery tego chomika:
html
Sandie Agar - Small Animal Nutrition
Volume 18 [2002]
Zgłoś jeśli
naruszono regulamin