Food Biochemistry and Food Processing

16 Biochemistry of Seafood Processing 359

ity of endogenous urease after death. This problem
renders shark meat with ammonia odor unaccept-
able. 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 proce-
dure has been successfully practiced to provide ac-
ceptable shark meat for consumers.
After death, the fish muscle also produces large
amounts of ammonia due to degradation of aden-
osine triphosphate to adenosine monophosphate
(AMP), followed by deamination of AMP.
It should be noted that both ammonia production
and degradation of TMAO can be either endogenous
or contributed by bacteria. Ammonia, TMA, small
amounts of DMA, and methyamine constitute the
“total volatile base,” an indicator of freshness com-
monly 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 mono-
phosphate (IMP) at a certain concentrations in dried
fish product can enhance the flavor (Murata and Sa-
kaguchi 1989). Hypoxanthine contributes to bitter-
ness and may add undesirable notes to the product
(Lindsay 1991).

LIPIDS IN SEAFOODS

LIPIDCOMPOSITION

Lipids play an important nutritional role in seafoods,

but at the same time they contribute greatly to quali-

ty 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 varying amounts of lipids and the presence (or

absence) of certain fatty acids can lead to significant-

ly different effects on quality 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, Atlantic 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., her-

ring, mackerel, eel, and sablefish) are classified as

having more than 8% total fat. This classification is,

however, not valid for many species, since they can

fluctuate widely in composition between seasons. For

example, Atlantic mackerel can have less than 4% fat

Figure 16.1.Seasonal variation in the fat content in different parts of Atlantic mackerel. (Adapted from Leu et al.
1981.)