Food Biochemistry and Food Processing

16 Biochemistry of Seafood Processing 357

they have had more time to accumulate it. These
large fish (swordfish, shark, king mackerel, and tile-
fish) pose the greatest risk. Other types of fish and
shellfish may be eaten in the amounts recommended
by the FDA and EPA. (www.FDA.gov)
By following these recommendations for select-
ing and eating fish or shellfish, women and young
children will receive the benefits of eating fish and
shellfish and be confident that they have reduced
their exposure to the harmful effects of mercury.

• Avoid eating fish that contain high levels of
  mercury: shark, swordfish, king mackerel, or
  tilefish.


• Eat up to 12 ounces (two average meals) a week
  of a variety of fish lower in mercury: shrimp,
  canned light tuna, salmon, pollock, and catfish.
  However, limit intakes of albacore (“white”) tuna
  to 6 ounces (one average meal) per week because
  albacore has more mercury than canned light
  tuna.


• Check local advisories about the safety of fish
  caught by family and friends in your local lakes,
  rivers, and coastal areas. Some kinds of fish and
  shellfish caught in local waters may have higher
  or much lower than average levels of mercury.
  This depends on the levels of mercury in the
  water in which the fish are caught. Those fish
  with much lower levels may be eaten more
  frequently and in larger amounts. If no advice is
  available, eat up to 6 ounces (one average meal)
  per week of fish you catch from local waters, but
  don’t consume any other fish during that week.

For young children, these same recommendations
can be followed, except that a serving size for chil-
dren is smaller than for adults, 2–3 ounces for chil-
dren instead of 6 ounces.

BIOCHEMISTRY OF GLYCOGEN
DEGRADATION

When the fish or crustacean animals are being
caught, they struggle vigorously in the fishing gear
and on board, causing antemortem exhaustion of
energy reserves, mainly glycogen, and high-energy
phosphates. Asphyxia phenomena set in, with grad-
ual formation of anoxial conditions in the muscle.
Tissue enzymes continue to metabolize energy re-
serves. Degradation of high-energy phosphates even-
tually produces hypoxanthine, followed by forma-

tion of formaldehyde, ammonia, inorganic phos-

phate, and ribose phosphates. The degradation of gly-

cogen in fish follows the Embden-Meyerhof-Parnas

pathway via the amylolytic route, catalyzed by en-

dogenous enzymes. This results in an accumulation

of lactic acid and a reduction of pH (
7.2 to
5.5),

contracting the tissues and inducing rigor mortis

(Hobbs 1982, Hultin 1992a, Sikorski et al. 1990a).

The rate of glycolysis is temperature dependent and

is slowed by a lower storage temperature.

BIOCHEMISTRY OF PROTEIN

DEGRADATION

Approximately 11–27% of seafood (fish, crusta-

ceans, and mollusks) consists of crude proteins. The

types of seafood proteins, similar to other muscle

foods, may be classified as sarcoplasmic, myofibril-

lar, and stromal. The sarcoplasmic proteins, mainly

albumins, account for approximately 30% of the

total muscle proteins. A large proportion of sarco-

plasmic proteins are composed of hemoproteins.

The myofibrillar proteins are myosin, actin, acto-

mysin, and troponin; these account for 40–60% of

the total crude protein in fish. The rest of the muscle

proteins, classified as stromal, consist mainly of col-

lagenous material (Shahidi 1994).

SARCOPLASMICPROTEINS

Sarcoplasmic proteins are soluble proteins in the

muscle sarcoplasm. They include a large number of

proteins such as myoglobin, enzymes, and other al-

bumins. The enzymatic degradation of myoglobin is

discussed in the section on pigment degradation.

Sarcoplasmic enzymes are responsible for quality

deterioration of fish after death and before bacterial

spoilage. The significant enzyme groups are hydro-

lases, oxidoreductases, and transferases. Other sar-

coplasmic proteins are the heme pigments, parval-

bumins, and antifreeze proteins (Haard et al. 1994).

Glycolytic deterioration in seafood was discussed in

Chapter 1 in this book and will not be repeated here.

Changes in the heme pigments will be covered be-

low in the section on Biochemical Changes in Pig-

ments during Handling, Storage, and Processing in

this chapter. Hydrolytic deterioration of seafood

myofibrillar and collagenous proteins are discussed

below.