Biomedical Engineering Reference
In-Depth Information
mechanism of lipid oxidation is a three-stage process: (1) initiation, (2) propagation,
and (3) termination.
3
During the initiation stage, lipid radicals are formed directly
from both unsaturated fatty acids in the presence of light, heat, other radicals, and
catalyzers including metals. At the propagation stage, lipid radicals react with oxygen
to form peroxy radicals (LOO
·
), which in turn abstract a hydrogen atom from another
lipid molecule to form hydroperoxides (LOOH) and another lipid radical
(
Figure 6.1
).
These generated radicals cause this process to become autocatalytic.
Duringthe termination phase, free radicals interact with each other to form nonrad-
ical products. Any components that prevent or interfere with the propagation of
oxidation by deactivating free radicals in the system play a key role in the termination
mechanism. Chain breaking antioxidants, such as phenolic compounds, react with
lipid radicals by donating a hydrogen atom to the lipid radicals, thereby stopping
propagation by forming inactive components.
4
Examples of phenolic antioxidants
include tocopherols, butylated hydroxyanisole, butylated hydroxytoluene, and propyl
gallate.
The LOOH formed are unstable and decompose into a wide range of volatile
and non-volatile products (Figure 6.1). These volatile and non-volatile products are
themselves unstable and undergo further oxidation and/or decomposition to a range
of oxidized products responsible for the off-flavors associated with rancid oils.
5
The susceptibility of fatty acids to oxidation depends on their ability to donate
a hydrogen atom. In
Table 6.1
dissociation energies for hydrogen bonds are pre-
sented. Unsaturated fatty acids with more than one double bond are particularly
susceptible to oxidation due to the presence of methylene interrupted bond config-
urations, where a methylene carbon atom is located between two double bonds. The
relative oxidation rates for oleic, linoleic, and linolenic acids were reported to be in
the order of 1:12:25 based on the peroxides formed.
6
P
HOTOOXIDATION
Photooxidation is far more detrimental to the stability of vegetable oils than free-
radical oxidation. Most oils contain photosensitizers, natural pigments such as chlo-
rophyll and its degradation products, heme and related compounds, methylene blue,
fluorescein derivatives, erythrosine, and polycyclic aromatic hydrocarbons capable
of transferring energy from light to chemical molecules.
7,8
The following reactions
outline the process of photooxidation:
Sensitizer
Ground
+ hv
→
Sensitizer
Excited
(6.1)
Sensitizer
H
+
L
·
Sensitizer
Excited
+
LH
→
(6.2)
Sensitizer
Excited
+
3
O
2
→
Sensitizer
Ground
+
1
O
2
(6.3)
LOO
·
+
1
O
2
+ LH
→
3
O
2
(6.4)
Energy (hv) is transferred from light to the sensitizer (Sensitizer
Excited
), which
may react directly with lipid (LH) forming radicals (L
·
), thereby initiating autoxi-
dation (Equation 6.2). The direct formation of lipid radicals is less likely to occur
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