364 Part III: Muscle Foods
and carotenoids in some important fish and shellfish
products. The changes during handling, storage, and
processing greatly affect the quality of these sea-
foods. Epithelial pigments and carotenoids also un-
dergo changes during postmortem ice-chilled stor-
age.
EPITHELIALDISCOLORATION
The market value of squid is related to the contrac-
tion state of its epidermal chromatophores, called
ommochromes. In recently harvested, prerigor
squid, the pigment is dispersed throughout the chro-
matophores: hence the dark red-brown appearance
of the epithelial tissue. Following rigor mortis, the
pigment cells contract, giving the skin a pale, light
coloration dotted with dark flecks. The continued
storage of squid results in a structural deterioration
of the ommochrome membrane, leading to bleeding
of pigment and downgrading of quality. The dark
brown coloration characteristic of very fresh squid
can be retained during processing, for example, by
freezing or dehydration. However, improper storage
of frozen or dried squid will result in chromatophore
disruption and red discoloration of the meat (Hink
and Stanley 1985).
The frozen storage of some fish may result in sub-
cutaneous yellowing of flesh below the pigmented
skin (Thompson and Thompson 1972). Apparently,
freezing or other processes that disrupt chroma-
tophores can lead to the release of carotenoids and
their migration to the subcutaneous fat layer. Sub-
cutaneous yellowing that occurs during the pro-
longed storage of frozen fish can originate from other
causes, that is, yellowing associated with lipid oxi-
dation and carbonyl-amine reactions.
HEMOGLOBIN
Normally, hemoglobin contributes less to the ap-
pearance of seafood than myoglobin because it is
lost easily during handling and storage, while myo-
globin is retained in the intracellular structure. The
amount of hemoglobin present also affects to a
greater extent the color appearance of light red and
dark red muscles of red-meat fish flesh (Wang and
Amiro 1979). For example, in yellowfin tuna Neo-
thunnus macropterus, hemoglobin concentrations
ranged from 12 to 50 mg% in light-red muscle and
50 to 380 mg% in dark-red muscle (Livingston and
Brown 1981). The amount of residual hemoglobin
in fish muscle is obviously influenced by the bleed-
ing efficacy at the time of catch. Method of catch
also affects the residual hemoglobin in the fish mus-
cle. For example, the percent of total heme as hemo-
globin in the meat of yellowfin caught by bait boat
and purse seine was 24 and 32%, respectively (Bar-
rett et al. 1965).
The green meat of raw or frozen broadbill sword-
fish (Xiphias gladus)is believed to be due to the
combination of hemoglobin with hydrogen sulfide
generated from the fairly extensive decomposition
of the meat (Amano and Tomiya 1953).HEMOCYANINHemocyanin, not hemoglobin, is present in the
blood of shellfish, that is, crustaceans and mollusks.
Hemocyanins are copper-containing proteins, as com-
pared with iron-containing proteins in hemoglobins,
and they combine reversely with oxygen. The contri-
bution of hemocyanins to seafood quality is not very
well understood. It is suspected that the blue discol-
oration of canned crabmeat is associated with a high
content of hemocyanin. The average copper content
of blue meat (e.g., 2.8mg%) is higher than meat of
normal color (e.g., 0.5mg%) (Ghiretti 1956).MYOGLOBINMyoglobin in fish muscle is retained in the intracel-
lular structure. In fish muscle, the red, white, and
intermediate fibers tend to be more distinctively seg-
regated than they are in muscle from land animals.
The myoglobin content in muscle of yellowfin tuna
was found to range from 37 to 128 mg% in the light-
colored muscle and from 530 to 22,400 mg% in the
dark-colored muscle (Wolfe et al. 1978). In cod, the
deep-seated, dark-colored muscle is richer in myo-
globin than superficial dark-colored muscle (Brown
1962, Love et al. 1977).
Myoglobin in fish is easily oxidized to a brown-
colored metmyoglobin. The discoloration of tuna
during frozen storage is associated with the forma-
tion of metmyoglobin, depending on temperature
and location (Tichivangana and Morrissey 1985).
Greening is a discoloration problem associated
with cooking various tunas. The problem arises
from the formation of a sulfhydryl adduct of myo-
globin in the presence of an oxidizing agent (e.g.,
trimethylamine oxide, TMAO). This greening prob-
lem can be effectively prevented by the use of a re-