362 Part III: Muscle Foods
Marshall, and Balaban, unpublished data). Increased
pressure and time also appears to lead to increased
oxidation. Lipid hydrolysis has been found to be
increased by medium levels of pressure, while it is
reduced at high pressures, likely due to lipase inacti-
vation (Ohshima et al. 1993b). Several studies have
been conducted on the effect irradiation has on sea-
food quality, although this process is not allowed in
many countries. It has been observed that low doses
of gamma irradiation (5 kGy) may have an accel-
erating effect on the oxidative stability of some fish
species and not others during post-irradiation refrig-
erated storage. Al-Kahtani et al. (1996) reported that
irradiation (1.5–10 kGy) led to more oxidation in
tilapia and Spanish mackerel during refrigerated
storage than in untreated fish. Fatty acid changes
were also observed in that study. The likely cause for
increased lipid oxidation on irradiation is the forma-
tion of radicals, more at higher levels of irradiation.
MINIMIZATION OFLIPID-DERIVEDQUALITY
PROBLEMS
To maintain or extend the sensory and nutritional
qualities of seafood, it is important to minimize or
delay the undesirable reactions of lipids, such as
oxidation and hydrolysis. Time is of the essence
with these reactions, and thus interventions should
be done as early as possible to be able to extend
quality as much as possible. The simplest means of
controlling oxidation is maintaining low tempera-
tures, since enzymatic and nonenzymatic oxidation
reactions are greatly influenced by temperature
(Hultin 1994). As mentioned before, very low tem-
peratures such as freezing will accelerate lipid
hydrolysis. Hydrolysis as well as oxidation will,
however, be reduced if products are kept at ex-
tremely low frozen storage temperatures (e.g., below
40°C) compared with conventional frozen storage
(about 20°C or higher).
At the harvest level, bleeding fish can lead to sig-
nificantly lower levels of oxidation since heme pro-
teins are reduced (Richards et al. 1998). Special care
should be taken to prevent tissue disruption in stor-
age since both oxidation and hydrolysis will be in-
creased. Washing fish fillets and fish mince will also
remove significant amounts of heme proteins (Rich-
ards et al. 1998, Kelleher et al. 1992). Another effec-
tive way to reduce oxidation of unstable species is
the removal of dark muscle, or deep skinning. ThisFigure 16.2.Effect of different thermal treatments on fatty acids in herring. (Adapted from Regulska-Ilow and Ilow
2002.)