16 Biochemistry of Seafood Processing 361hexanoic acid (22:63) (Shewfelt 1981). These fatty
acids have been found to be highly active physio-
logically, for example, leading to decreased plasma
triacylglycreols, cholesterol, and blood pressure (Wi-
jendran and Hayes 2004) and modulating immun-
ological activities (e.g., inflammation) (Klurfeld
2002), to name a few important functions.
LIPIDS ANDQUALITYPROBLEMS
Even though the phospholipids are at very low levels
compared to neutral fats, they are believed to lead
to more quality problems such as lipid oxidation,
which is a major cause of quality deterioration in
many aquatic foods. This is in part because of their
higher level of unsaturation, but also is due to their
significantly larger surface area compared with neu-
tral lipids (Hultin and Kelleher 2000). Undeland et
al. (2002) tested this hypothesis in a model system
with cod membrane lipids and varying levels of neu-
tral menhaden lipids and hemoglobin as a prooxi-
dant. The oxidation was found to be independent of
the amount of neutral lipids in the system. Their
larger surface area also makes the unstable phospho-
lipids more exposed to a variety of prooxidants (and
antioxidants) such as heme proteins, radicals, iron,
and copper. These prooxidants are especially rich in
the metabolically more active dark muscle, which
is in turn more susceptible to lipid oxidation than
white muscle. These prooxidants, especially the
heme proteins hemoglobin and myoglobin, can lead
to high levels of lipid oxidation products, especially
as pH of the muscle is reduced (which occurs post-
mortem) or when muscle is minced and these com-
ponents are mixed in with lipids and oxygen (Hultin
1994, Undeland et al. 2004). The secondary prod-
ucts formed as a result of lipid oxidation have highly
unpleasant off-odors and flavors, and they can also
adversely affect the texture and color of the muscle.
Several studies have suggested a link between lipid
oxidation and protein oxidation (e.g., Srinivasan et
al. 1996, Tironi et al. 2002, King and Li 1999). Ox-
idation of proteins can lead to protein cross-linking
and thus textural defects, including muscle toughen-
ing and loss of water-holding capacity. King and Li
(1999) proposed that lipid oxidation products may
also induce protein denaturation, which in turn
could cause textural problems.
Although oxidation is often delayed during frozen
storage of fish muscle, thawed fish can oxidize more
rapidly than fresh fish since muscle cells are disrupt-
ed, leading to an increase in free iron and copper,
among other changes (Hultin 1994, Benjakul and
Bauer 2001). Much frozen seafood has also been
found to accumulate high levels of free fatty acids
over time as a result of lipase and phospholipase
activity (Undeland 1997, Reddy et al. 1992). These
free fatty acids may or may not be more susceptible
to lipid oxidation (Shewfelt 1981), but they can lead
to protein denaturation and cross-linking and thus
adverse effects on texture and water-holding capaci-
ty (Reddy et al. 1992).
Conflicting results have been reported on the ef-
fect of thermal processing on lipids. Canning has
been reported to increase oxidation (Auborg 2001).
Short-term heating to 80°C, in contrast to long-term
heating, has been reported to reduce oxidation due
to inactivation of lipoxygenases, while long-term
heating may accelerate nonenzymatic oxidation re-
actions (Wang et al. 1991). Shahidi and Spurvey
(1996) reported that cooked dark muscle of macker-
el oxidized more than raw dark muscle, while inter-
estingly, the opposite was found for white muscle.
In a study where herring was cooked by various
methods (microwave, boiling, grilling, and frying),
there was essentially no effect on lipid oxidation
(Regulska-Ilow and Ilow 2002). That same study
also demonstrated that fish lipids, including the nu-
tritionally important omega-3 fatty acids, were high-
ly stable under the different cooking methods (Fig.
16.2). Very high temperatures (e.g., 550°C) can,
however, lead to decomposition of fish fatty acids
and thus to decreased nutritional value (Sathivel et
al. 2003). Changes in seafood fatty acid profiles can
occur with frying. It has been reported that there
may be an exchange of fatty acids from the frying
medium and the muscle (Sebredio et al. 1993). This
can either lower or increase the nutritional quality
and stability of the seafood, depending on the type
of frying oil and the type of seafood.
Novel processing methods such as irradiation and
high-pressure processing have been reported to lead
to increased levels of lipid oxidation. High-pressure
treatment has been reported to have relatively small
effects on purified lipids from fish, while high-pres-
sure treatment of fish muscle leads to high levels of
lipid oxidation products (Ohshima et al. 1993a). This
increase in oxidation is hypothesized, in part, to be
via pressure-induced denaturation of heme proteins,
which are potent prooxidants (Yagiz, Kristinsson,