Discoloration of dark-fleshed fish species during storage and
processing was mainly due to the reaction of myoglobin, a monomeric
globular heme protein contributed to the pigmentation of red meat fish,
alone and with other muscle components. Metmyoglobin formation caused
by the autooxidation of myoglobin was a major factor influencing the
darkening of dark-fleshed fish meat during iced and frozen storage. The
increase in metmyoglobin content concomitant with a decrease in redness
index of meat usually occurred during storage of dark-fleshed fish. Lipid
oxidation was positively correlated with myoglobin oxidation and related to
the discoloration of fish flesh. Aldehydic lipid oxidation products such as
hexanal and hexenal potently induced the formation of metmyoglobin and
the lowered whiteness of fish meat. Furthermore, a green pigment could be
produced during heat processing in some dark muscle fish species such as
tuna. Greening was generated when tuna myoglobin was reacted with other
muscle components including trimethylamine oxide (TMAO) and cysteine.
After heating, a single type of green pigment was formed and resulted in
discoloration of cooked tuna meat. Furthermore, green pigmentation of tuna
meat could be formed by the reaction of myoglobin with hydrogen peroxide,
a by-product of lipid and myoglobin oxidations.