Food Lipids: Chemistry, Nutrition, and Biotechnology

oxygen [22]. Nonenzymatic production of singlet oxygen, in turn, involves two different
pathways. The type 1 pathway is characterized by hydrogen atom transfer or
electron transfer between an excited triplet sensitizer and a substrate, resulting in the
production of free radicals or free radical ions. These free radicals may then react
with triplet oxygen to produce oxidized compounds, which readily break down to
form free radicals that can initiate free radical chain reactions. In the second pathway
for production of nonenzymatic production of singlet oxygen (type II), the excited
triplet sensitizer reacts with triplet oxygen via a triplet–triplet annihilation mechanism.
Enzymatic production of singlet oxygen, on the other hand, has been shown
to be a direct or indirect consequence of the action of certain microsomal oxidases,
lipoxygenase, and prostaglandin synthetase.
Evidence that singlet oxygen can initiate lipid oxidation can be obtained from
analysis of the oxidative products. In the reaction of singlet oxygen with unsaturated
fatty acids, one end of the singlet oxygen molecule reacts with the �-olefinic carbon,
while the other end abstracts the �-allylic hydrogen. As a result of this six-membered
ring transition state, both conjugated and nonconjugated hydroperoxides are formed
[154], whereas free radical autoxidation of lipid produces only nonconjugated
hydroperoxides.
Pigments present in muscle foods that may act as photosensitizers (because
their conjugated double-bond system easily absorbs visible light energy) include
hematoporphyrins and riboflavin. In model systems containing myoglobin and its
derivatives, Whang and Peng [155] demonstrated through electron paramagnetic resonance
spectroscopy coupled with a spin trapping technique that dissociated hematin
and especially the protoporphyrin IX ring exerted a photosensitizing function. The
participation of photosensitization in meat systems is supported by several storage
studies: (1) for ground turkey, ground pork, and shrimp, samples exposed to light
had higher levels of oxidative products than samples stored in the dark [156–158];
(2) incorporation of a UV light absorber in the packaging of pork patties prevented
light-induced lipid oxidation [157]; and (3) incorporation of a singlet oxygen
quencher (2,2,6,6-tetramethyl-4-piperidone) reduced the prooxidant effect of light in
turkey meat [156].
3. Enzymic Initiation Systems
Several enzyme systems capable of initiating lipid oxidation have been identified in
muscle foods. Among these, lipoxygenase stereoselectively absorbs a hydrogen atom
from an active methylene group in 1,4-pentadiene structures of PUFA and releases
a stereospecific conjugated diene hydroperoxy fatty acid product. Although it has
greater recognition for its off-flavor development in vegetables and legumes, lipoxygenase
has also been found in fish gill tissue [159,160], chicken muscle [161], and
sardine skin [162]. However, according to Kanner et al. [163], these enzymes are
not true initiators, since preformed hydroperoxides are necessary for their activation.
Despite this requirement, products of lipoxygenase have been associated with ‘‘fresh
flavors’’ of fish [160]. Their contribution to off-flavor generation in muscle foods
during storage, on the other hand, remains debatable. While Grossman et al. [161]
detected little loss of lipoxygenase activity in chicken muscle stored at �20�C for
12 months, German et al. [160] noted that lipoxygenases were unstable, being inactivated
by 50% within 3 hours at 0�C or completely inactivated with a single
freeze–thaw cycle.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.