16 Biochemistry of Seafood Processing 363removes not only a large fraction of the heme pro-
teins, but also a large amount of oxidatively unstable
lipids present in the dark muscle itself and below the
skin. Protection from oxygen is another effective
means for reducing oxidation. This can be achieved
through modified atmosphere packaging, where oxy-
gen is either reduced (for lean species) or completely
removed (for fatty species). Vacuum packaging of
seafood is also highly effective for reducing oxida-
tion (Flick et al. 1992). Filleting fish under water
(where O 2 is low) has also been reported to lead to
less oxidation on storage than filleting in air (Rich-
ards et al. 1998). Special gases such as carbon mon-
oxide have been found to significantly reduce oxida-
tion of several species, even after fillets are removed
from the gas, most likely since the gas reduces the
prooxidative activities of heme proteins (Kristinsson
et al. 2003).
Various antioxidative components can be highly
effective in delaying lipid oxidation. All aquatic ani-
mals have a number of different indigenous antioxi-
dants, and it is important to avoid conditions that can
negatively effect or destroy these (Hultin 1994). It
has been well researched and is well known that
antioxidants can be added to seafood during pro-
cessing to increase their oxidative stability (Fig.
16.3). Addition of antioxidants early during process-
ing is also important. Since many of the prooxidants
in seafood are in the aqueous phase and the lipids
constitute a nonpolar phase, a combination of polar
and nonpolar antioxidants has been found to be very
effective. Tocopherol (vitamin E), a nonpolar anti-
oxidant, and ascorbate (vitamin C), a polar antioxi-
dant, are among the most popular added antioxidants.
Sometimes metal chelators such as EDTA are added.
It is worth mentioning, however, that under certain
conditions antioxidants can act as prooxidants. For
example, this is true for ascorbic acid and EDTA,
both of which can stimulate iron-mediated lipid oxi-
dation. For example, when the ratio of EDTA: iron is
1, it is prooxidative, but when the ratio is1, it is
antioxidative (Halliwell and Gutteridge 1998). Since
ascorbate is a reducing agent, it can also reduce iron
under certain conditions and thus make it more pro-
oxidative (Halliwell and Gutteridge 1988).BIOCHEMICAL CHANGES IN
PIGMENTS DURING HANDLING,
STORAGE, AND PROCESSINGThe main pigments in seafood can be classified as
heme proteins (hemoglobin and myoglobin) in red-
meat (warm-blooded) fish, hemocyanin in cold-
blooded shellfish such as crustaceans and mollusks,Figure 16.3.The effect of absence or presence of antioxidants on the oxidative stability of washed dark and white
mackerel muscle during frozen storage (20°C). The antioxidant TBHQ was added during grinding while ascorbate
and EDTA were added during grinding and washing. (Adapted from Kelleher et al. 1992.)