Food Lipids: Chemistry, Nutrition, and Biotechnology

in tissue oxidation along with the high degree of polyunsaturation in phospholipids
[76]. Evidence that phospholipids are the major contributors to the development of
warmed-over flavor (WOF) in meat from several different species of animals has
been provided [77–82]. However, levels of total lipid seemed to be the major contributor
to WOF in pork [77].
The method used to measure degree of oxidation may influence a study’s conclusions.
For example, a greater concentration of volatiles produced by the triacylglycerol
fraction often does not have the impact on flavor that a smaller concentration
of volatiles produced by the phospholipid fraction would have. This response arises
because solubility in the lipid and/or flavor threshold of many of the volatiles increases
as the level of fat increases. Hence, Roozen et al. [83,84] demonstrated that
lowering the fat content in model systems increases the chance of flavor defects by
reducing the concentration of volatiles retained in the fat.
The time frame under which an investigator examines the relative contributions
of lipid classes to oxidation may also be a determining factor in the reported results.
At earlier stages of oxidation, the peroxide value of raw sardine fillets was attributed
to preferential oxidation of phospholipids and in later stages to oxidation of triacylglycerols
[85]. Igene et al. [86] also showed that triacylglycerols in model meat
systems were slow to oxidize and as such did not serve as a source of oxidative
products until late in storage. In contrast, Erickson [51] suggested that free fatty
acids released from the triacylglycerols served as the major site of oxidation in early
stages of frozen storage of channel catfish and that phospholipids were only major
contributors in later stages of storage. Thus, accessibility of lipid to hydrolytic enzymes
could be an important factor for determining the oxidative susceptibility of a
lipid class.
4. Susceptibility to Oxidation of Membrane Lipids
Variations in oxidative susceptibility within the individual phospholipid classes can
be ascribed to the nature of the polar head group (choline, ethanolamine, serine, or
inositol) and the degree of fatty acid unsaturation of the individual phospholipid
[44,87]. With chicken meat, Pikul and Kummerow [88] identified phosphatidylinositol
as containing the highest malonaldehyde levels and largest percentage of PUFAs,
followed by phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylcholine
(PC), cardiolipin, lysophosphatidylcholine, and sphingomyelin. In model
meat systems, PE consistently changed more than PC in polyunsaturation during
frozen storage, reflecting the higher initial levels of unsaturated fatty acids in PE
[86]. Even when fatty acid composition was held constant in a liposome model
system, PE-based liposomes exhibited a greater increase in both lipid and oxymyoglobin
oxidation than PC-based liposomes [44]. In contrast, saturated PS added to
PC-based liposomes inhibited oxidation through modification of surface charge and
subsequent trapping of iron [89,90]. Similarly, inhibition by plasmalogen phospholipids
(glycerophospholipids that contain a vinyl ether moiety at the sn-1 position)
has been ascribed to their binding of iron [91] and to decreased propagation via
oxidation of the vinyl ether bond [92]. On the other hand, possible causes of the
inhibition of membrane phospholipid oxidation upon incorporation of cholesterol
include an alteration in packing of lipids in the membrane and chemical trapping of
oxygen upon conversion of cholesterol into nonradical oxide derivatives [93–95].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.