BLBS102-c19 BLBS102-Simpson March 21, 2012 13:33 Trim: 276mm X 219mm Printer Name: Yet to Come
352 Part 3: Meat, Poultry and Seafoodsdeterioration in many aquatic foods. This is in part because of
their higher level of unsaturation, but also is due to their signif-
icantly larger surface area compared with neutral lipids (Hultin
and Kelleher 2000). Undeland et al. (2002) tested this hypoth-
esis in a model system with cod membrane lipids and varying
levels of neutral 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 phospholipids 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 prooxi-
dants, 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 postmortem) or
when muscle is minced and these components are mixed in with
lipids and oxygen (Hultin 1994, Undeland et al. 2004). The sec-
ondary products formed as a result of lipid oxidation have highly
unpleasant off-odors and flavors, and they can also adversely af-
fect 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, King and Li 1999, Tironi et al.
2002). Oxidation of proteins can lead to protein cross-linking
and thus textural defects, including muscle toughening 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 disrupted, leading to an increase in freeiron and copper, among other changes (Hultin 1994, Benjakul
and Bauer 2001). Much frozen seafood has also been found
to accumulate high levels of FFAs over time as a result of li-
pase and phospholipase activity (Reddy et al. 1992, Undeland
1997). These FFAs may or may not be more susceptible to lipid
oxidation (Shewfelt 1981), but they can lead to protein denatu-
ration and cross-linking and thus adverse effects on texture and
water-holding capacity (Reddy et al. 1992).
Conflicting results have been reported on the effect of thermal
processing on lipids. Canning has been reported to increase ox-
idation (Aubourg 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 reactions (Wang et al. 1991).
Shahidi and Spurvey (1996) reported that cooked dark muscle
of mackerel oxidized more than raw dark muscle, while interest-
ingly, 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 ox-
idation (Regulska-Ilow and Ilow 2002). That same study also
demonstrated that fish lipids, including the nutritionally impor-
tant omega-3 fatty acids, were highly stable under the differ-
ent cooking methods (Fig. 19.2). However, very high tempera-
tures (e.g., 550◦C) can 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 (Sebedio et al. 1993).
This can either lower or increase the nutritional quality and sta-
bility of the seafood, depending on the type of frying oil and the
type of seafood.Figure 19.2.Effect of different thermal treatments on fatty acids in herring. (Adapted from Regulska-Ilow and Ilow 2002.)