Food Lipids: Chemistry, Nutrition, and Biotechnology

fatty acid occurs when Fe 3� at the active site of lipoxygenase is reduced to Fe 2� .
While the majority of lipoxygenases require free fatty acids, there have been reports
of lipoxygenase acting directly on fatty acids in phospholipids [73,74]. Hence, enzymatic
hydrolysis may not always be required prior to lipoxygenase activity.
During propagation, lipid–lipid interactions foster propagation of free radicals
produced during initiation by abstracting hydrogen from adjacent molecules; the
result is a lipid hydroperoxide and a new lipid free radical. Magnification of initiation
by a factor of 10 [75] to 100 [76] may occur through free radical chain propagation.
Further magnification may occur through branching reactions (also known as secondary
initiation) in which Fe 2� interacts with a hydroperoxide to form a lipid
alkoxyl radical and hydroxyl radical, which will then abstract hydrogens from unsaturated
fatty acids.
There are many consequences to phospholipid peroxidation in biological and
membrane systems. On a molecular level, lipid peroxidation has been manifested in
a decreased hydrocarbon core width and molecular volume [77]. In food, the decomposition
of hydroperoxides to aldehydes and ketones is responsible for the characteristic
flavors and aromas that collectively are often described by the terms ‘‘rancid’’
and ‘‘warmed-over.’’ Numerous studies, on the other hand, have shown that specific
oxidation products may be desirable flavor components [78–81], particularly when
formed in more precise (i.e., less random) reactions by the action of lipoxygenase
enzymes [82–87] and/or by the modifying influence of tocopherol on autoxidation
reactions [88].
Through in vitro studies, membrane phospholipids have been shown to oxidize
faster than emulsified triacylglycerols [89], apparently because propagation is facilitated
by the arrangement of phospholipid fatty acids in the membrane. However,
when phospholipids are in an oil state, they are more resistant to oxidation than
triacylglycerols or free fatty acids [90]. Evidence that phospholipids are the major
contributors to the development of warmed-over flavor in meat from different animal
species has been described in several sources [91–94]. Similarly, during frozen storage
of salmon fillets, hydrolysis followed by oxidation of the n-3 fatty acids in
phospholipids was noted [95]. The relative importance of phospholipids in these food
samples has been attributed to the high degree of polyunsaturation in this lipid fraction
and the proximity of the phospholipids to catalytic sites of oxidation (enzymic
lipid peroxidation, heme-containing compounds) [96]. However, the importance of
phospholipids has not been restricted to animal and fish tissues. In an accelerated
storage test of potato granules, both the amounts of phospholipids and their unsaturation
decreased [97]. Moreover, with pecans, a much stronger negative correlation
was found between headspace hexanal and its precursor fatty acid (18:2) from the
phospholipid fraction (R = �0.98) than from the triacylglycerol fraction (R = �0.66)
or free fatty acid fraction (R = �0.79) [98]. These results suggest that despite the
fact that membrane lipid constitutes a small percentage of the total lipid (0.5%),
early stages of oxidation may actually occur primarily within the phospholipids.
The presence of phospholipids does not preclude acceleration of lipid oxidation.
When present as a minor component of oil systems, solubilized phospholipids have
limited the oxidation of the triacylglycerols [99–101]. Order of effectiveness of
individual phospholipids was as follows: SPH = LPC = PC = PE > PS > PI > PG
[102] with both the amino and hydroxy groups in the side chain participating in the
antioxidant activity [103]. It was postulated that antioxidant Maillard reaction prod-
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