BLBS102-c19 BLBS102-Simpson March 21, 2012 13:33 Trim: 276mm X 219mm Printer Name: Yet to Come
19 Biochemistry of Seafood Processing 349pollock, and catfish. However, limit intakes of albacore
(“white”) tuna to 6 oz (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 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 ad-
vice is available, eat up to 6 oz (one average meal) per week
of fish you catch from local waters, but do not consume any
other fish during that week.For young children, these same recommendations can be fol-
lowed, except that a serving size for children is smaller than for
adults, 2–3 oz for children instead of 6 oz.BIOCHEMISTRY OF GLYCOGEN
DEGRADATIONWhen 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
gradual formation of anoxial conditions in the muscle. Tissue
enzymes continue to metabolize energy reserves. Degradation
of high-energy phosphates eventually produces hypoxanthine,
followed by formation of formaldehyde, ammonia, inorganic
phosphate, and ribose phosphates. The degradation of glycogen
in fish follows the Embden-Meyerhof-Parnas pathway via the
amylolytic route, catalyzed by endogenous enzymes. This re-
sults 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
DEGRADATIONApproximately 11–27% of seafood (fish, crustaceans, and mol-
lusks) consists of crude proteins. The types of seafood proteins,
similar to other muscle foods, may be classified as sarcoplasmic,
myofibrillar, and stromal. The sarcoplasmic proteins, mainly al-
bumins, account for approximately 30% of the total muscle pro-
teins. A large proportion of sarcoplasmic proteins are composed
of hemoproteins. The myofibrillar proteins are myosin, actin,
actomysin, 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 collagenous material (Shahidi 1994).Sarcoplasmic ProteinsSarcoplasmic proteins are soluble proteins in the muscle sar-
coplasm. They include a large number of proteins such as myo-
globin, enzymes, and other albumins. The enzymatic degrada-tion of myoglobin is discussed in the section on pigment degra-
dation. Sarcoplasmic enzymes are responsible for quality dete-
rioration of fish after death and before bacterial spoilage. The
significant enzyme groups are hydrolases, oxidoreductases, and
transferases. Other sarcoplasmic proteins are the heme pigments,
parvalbumins, and antifreeze proteins (Haard et al. 1994). Gly-
colytic deterioration in seafood was discussed in Chapter 1 of
this book and will not be repeated here. Changes in the heme pig-
ments will be covered in the Section on “Biochemical Changes
in Pigments during Handling, Storage, and Processing”. Hy-
drolytic deterioration of seafood myofibrillar and collagenous
proteins are discussed in the Section on “Myofibrillar Protein
Deterioration.”Myofibrillar Protein DeteriorationThe most common myofibrillar proteins in the muscles of aquatic
animals are myosin, actin, tropomyosin, and troponins C, I, and
T (Suzuki 1981). Myofibrillar proteins undergo changes during
rigor mortis, resolution of rigor mortis, and long-term frozen
storage. The integrity of the myofibrillar protein molecules and
the texture of fish products are affected by these changes. These
changes have been demonstrated in various research reports and
reviews (Kye et al. 1988, Sikorski et al. 1990a, Martinez 1992,
Haard 1992a, 1992b, 1994, Sikorski and Pan 1994, Sikorski
1994a, 1994b, Jiang 2000). The degradation of myofibrillar pro-
teins in seafood causes these proteins to lose their integrity and
gelation power in ice-stored seafood. The cooked seafood will
no longer possess the characteristic firm texture of very fresh
seafood; it will show a mushy or soft (sometimes mislabeled
as tender) mouthfeel. For frozen seafoods, these degradations
are accompanied by a loss in the functional characteristics of
muscle proteins, mainly solubility, water retention, gelling abil-
ity, and lipid emulsifying properties. This situation gets even
worse when the proteins are cross-linked due to the presence
of formaldehyde formed from trimethylamine degradation. The
cooked products become tough, chewy, and stringy or fibrous.
Repeated freezing and thawing make the situation even worse.
Readers should refer to the review by Sikorski and Kolakowska
(1994) for a detailed discussion of the topic.Stromal Protein DeteriorationThe residue remaining after extraction of sarcoplasmic and
myofibrillar proteins is known as stromal protein. It is com-
posed of collagen and elastin from connective tissues (Sikorski
and Borderias 1994). Degradation causes textural changes in
these seafoods (honeycombing in skipjack tuna and mackerel
and mushiness in freshwater prawn) (Frank et al. 1984, Nip
et al. 1985, Pan et al. 1986). Bremmer (1992) reviewed the
role of collagen in fish flesh structure, postmortem aspects,
and the implications for fish processing, using electron mi-
croscopic illustrations. Jiang (2000) reviewed the proteinases
involved in the textural changes of postmortem muscle and
surimi-based products. Microstructural changes in ice-stored
freshwater prawns have been revealed (Nip and Moy 1988).
These textural changes are due to the degradation of collage-
nous matter and definitely influence the quality of seafood. For