Food Biochemistry and Food Processing

358 Part III: Muscle Foods

MYOFIBRILLARPROTEINDETERIORATION

The most common myofibrillar proteins in the mus-
cles of aquatic animals are myosin, actin, tropo-
myosin, and troponins C, I, and T (Suzuki 1981).
Myofibrillar proteins undergo changes during rigor
mortis, resolution of rigor mortis, and long-term fro-
zen storage. The integrity of the myofibrillar protein
molecules and the texture of fish products are affect-
ed by these changes. These changes have been de-
monstrated in various research reports and reviews
(Haard 1992a,b, 1994; Jiang 2000; Kye et al. 1988;
Martinez 1992; Sikorski 1994 a,b; Sikorski and Pan
1994; Sikorski et al. 1990a). The degradation of
myofibrillar proteins in seafood causes these pro-
teins 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 sea-
foods, these degradations are accompanied by a loss
in the functional characteristics of muscular pro-
teins, 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. Re-
peated freezing and thawing make the situation even
worse. Readers should refer to the review by
Sikorski and Kolakowska (1994) for a detailed dis-
cussion of the topic.

STROMALPROTEINDETERIORATION

The residue remaining after extraction of sarcoplas-
mic and myofibrillar proteins is known as stromal
protein. It is composed of collagen and elastin from
connective tissues (Sikorski and Borderias 1994).
Degradation causes textural changes in these sea-
foods (honeycombing in skipjack tuna and mackerel
and mushiness in freshwater prawn) (Frank et al.
1984, Nip et al. 1985, Pan et al. 1986). Bremner
(1992) reviewed the role of collagen in fish flesh
structure, postmortem aspects, and the implications
for fish processing, using electron microscopic illus-
trations. Jiang (2000) reviewed the proteinases in-
volved 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 collagenous matter and defi-

nitely influence the quality of seafood. For example,

in freshwater prawn, the development of mushy tex-

ture downgrades its quality. In tuna, the development

of honeycombing is an undesirable defect. In the

postcooking examination of tuna before filling into

the cans, the appearance of honeycombing is a sign

of mishandling of the raw material. In more exten-

sive cases, it may even cause the rejection of the pre-

cooked fish for further processing into canned tuna.

BIOCHEMICAL CHANGES IN

NONPROTEIN NITROGENOUS

COMPOUNDS

Reviews on the postmortem degradation of nonpro-

tein nitrogenous (NPN) compounds are available

(Haard et al. 1994, Sikorski et al. 1990a, 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 1994a,

Sikorski et al. 1990a). Bykowski and Kolodziejski

(1983) reported that the white meat generally con-

tains less NPN compounds than the dark meat. For

example, in the meat of white fish, the NPN general-

ly made up 9–15% of the total nitrogen, in clupeids

16–18%, in muscles of mollusks and crustaceans 20–

50%, and in some shark 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, trimethy-

lamine oxide (TAMO) 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 sum-

mer (Sakaguchi and Murata 1989).

The endogenous enzymatic breakdown of TMAO

to dimethylamine (DMA) and then 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 ammonia may accumulate due to the activ-