Composition of Fish

Regardless of their classification, fish are usually tender when they come to the table, and three structural factors contribute to this tenderness. When compared with meat or poultry, fish muscle has lower amounts of collagen. The bodies of land animals average 15 percent connective tissue by weight, while fish are only 3 percent collagen. Another reason fish is tender is that there is less of a certain amino acid (hydroxyproline) in the connective tissue, so when fish is cooked, the collagen breaks down more easily at a lower temperature and converts to gelatin. Also, unlike mammals and birds, whole muscles are arranged in very long bundles of fibers, the muscles of fish are shorter (less than an inch in length) and are arranged into myotomes, which are separated by myocommata. This combination of structure and chemistry contributes to the characteristic flaking of prepared fish as the heat softens the collagen in the myocommata.

Pigments in Fish

When fish flesh is exposed to air during preparation, it will vary in color as a result of the presence of white, pink, or red pigments. The color of a fish’s flesh depends on whether that fish relied predominantly on quick or slow movements to stay alive. “Red” or darker meat flesh, such as that seen in salmon, has a higher concentration of the “slow-twitch fibers” needed for long-distance swimming and endurance. “White” meat, like that of the sole, has more “fast-twitch fibers”, which are designed for quick bursts of speed of brief duration between long periods on “cruise control”, Some fish, such as tuna, are comprised of both fast-twitch and slow-twitch fibers, giving them dark, light, and white meat.

The concentration of myoglobin contributes to the overall color of fish flesh. The more oxygen required by the muscle, the more myoglobin proteins are necessary, because they carry the oxygen. Unfortunately a higher myoglobin concentration results in quicker rancidity because the iron in myoglobin accelerates the oxidation of fat found in the muscle. Sometimes a specific pigment adds a special hue. For example, a carotenoid pigment, astaxanthin, imparts a characteristic orange-pink color to certain salmon and trout that feed on insects and crustaceans containing this pigment. A higher fat content will also darken the color of the flesh, as seen in fatty fish like mackerel and tuna.