BLBS102-c19 BLBS102-Simpson March 21, 2012 13:33 Trim: 276mm X 219mm Printer Name: Yet to Come
350 Part 3: Meat, Poultry and Seafoodsexample, in freshwater prawn, the development of mushy texture
downgrades its quality. In tuna, the development of honeycomb-
ing is an undesirable defect. In the postcooking examination
of tuna before filling into the cans, the appearance of honey-
combing is a sign of mishandling of the raw material. In more
extensive cases, it may even cause the rejection of the precooked
fish for further processing into canned tuna.BIOCHEMICAL CHANGES IN
NONPROTEIN NITROGENOUS
COMPOUNDSReviews on the postmortem degradation of nonprotein nitroge-
nous (NPN) compounds are available (Sikorski et al. 1990a,
Haard et al. 1994, Sikorski and Pan 1994). Such compounds
in the meat of marine animals vary among species, with the
habitat, and with life cycle; more importantly, they play a role
in the postmortem handling processes (Sikorski et al. 1990a,
Sikorski 1994a). Bykowski and Kolodziejski (1983) reported
that white meat generally contains less NPN compounds than
dark meat. For example, in the meat of white fish, the NPN
generally made up 9–15% of the total nitrogen, in clupeids
16–18%, in muscles of mollusks and crustaceans 20–50%, and
in some sharks up to 55%. Ikeda (1979) showed that about 95%
of the total amount of NPN in the muscle of marine fish and
shellfish is composed of free amino acids, imidazole dipeptides,
trimethylamine oxide (TMAO) and its degradation products,
urea, guanidine compounds, nucleotides and the products of
their postmortem changes, and betaines.
The content of free amino acids in the body of oysters is higher
in the winter than it is in the summer (Sakaguchi and Murata
1989).
The endogenous enzymatic breakdown of TMAO to DMA and
then to formaldehyde and the bacterial reduction of TMAO to
TMA have been most extensively studied. It should be noted that
the production of DMA and formaldehyde takes place mainly in
anaerobic conditions (Lundstrom et al. 1982).
Shark muscles contain fairly high amounts of urea, and am-
monia may accumulate due to the activity of endogenous urease
after death. This problem renders shark meat with ammonia
odor unacceptable. Fortunately, this problem can be overcome
by complete bleeding of the shark at the tail, gutting, filleting,
and thorough washing right after catch to reduce the amount of
substrate (urea). This procedure has been successfully practiced
to provide acceptable shark meat for consumers.
After death, fish muscle also produces large amounts of
ammonia due to degradation of adenosine triphosphate (ATP)
to adenosine monophosphate (AMP), followed by deamination
of AMP.
It should be noted that both ammonia production and degra-
dation of TMAO can be either endogenous or contributed by
bacteria. Ammonia, TMA, small amounts of DMA, and methy-
lamine constitute the “total volatile base (TVB),” an indicator of
freshness commonly used for seafood.
Postmortem enzymatic breakdown of nucleotides in fish may
have a positive or negative impact on the flavor of seafood.
The production of inosine monophosphate (IMP) at a cer-tain concentrations in dried fish product can enhance the fla-
vor (Murata and Sakaguchi 1989). Hypoxanthine contributes
to bitterness and may add undesirable notes to the product
(Lindsay 1991).LIPIDS IN SEAFOODS
Lipid CompositionLipids play an important nutritional role in seafoods, but at the
same time, they contribute greatly to quality changes in many
species. Just as aquatic species vary widely biologically, the
lipid content and fatty acid types are greatly variable between
species and sometimes within the same species. This can create
a challenge for the seafood harvester and processor since vary-
ing amounts of lipids and the presence (or absence) of certain
fatty acids can lead to significantly different effects on qual-
ity and shelf life. Seafood can be roughly classified into four
categories based on fat content (O’Keefe 2000). Lean species
(e.g., cod, halibut, pollock, snapper, shrimp, and scallops) have
fat contents below 2%; low-fat species (e.g., yellowfin tuna, At-
lantic sturgeon, and smelt) have fat contents between 2% and
4%; medium-fat species (e.g., catfish, mullet, trout, and salmon)
are classified as having 4–8% total fat; and fatty species (e.g.,
herring, mackerel, eel, and sablefish) are classified as having
more than 8% total fat. However, this classification is not valid
for many species, since they can fluctuate widely in composi-
tion between seasons. For example, Atlantic mackerel can have
less than 4% fat during late spring shortly after spawning, while
it can have about 24% fat during late November due to active
feeding (Leu et al. 1981; Fig. 19.1).
Different parts of the aquatic animal will also have differ-
ent fat contents. Ohshima et al. (1993a) reported the following
descending order for the fat content of mackerel and sardines:
Skin (including subcutaneous lipids),
Viscera,
Dark muscle, and
White muscle.Some species, most notably salmonids, have larger fat de-
posits in the belly flaps than in other parts of the fish. Distri-
bution of fat in many fish seems to be in descending fat levels
from the head to the tail (Icekson et al. 1998, Kolstad et al.
2004). Diet will have a significant impact on the fat content
and fatty acid profiles of aquatic animals. Aquacultured fish on
a controlled diet, for example, do not show the same seasonal
variation in fat content as wild species. It is also possible to mod-
ify the fat content and fat type via diet. For example, adding fish
oil to fish feed will increase the fat content of fish (Lovell and
Mohammed 1988), usually proportionately to the level of fat/oil
in the diet (Solberg 2004). It has also been shown with many
species (salmon being the most investigated) that the fatty acid
composition is a reflection of the fatty acid composition in the
fish diet (Jobling 2004). Therefore, it is possible to selectively
increase the nutritional value of aquacultured fish by increas-
ing the level and improving the ratio of nutritionally beneficial
fatty acids (Lovell and Mohammed 1988). Conversely, one can