Selection of Finfish

Selection of Finfish
The criteria for selection of vertebrate and invertebrate fish are very different. Retailers providing consumers with nutrition information must abide by the nutrition labeling values provided by the FDA for fish and shellfish.

Vertebrate Fish

Fish can be purchased fresh or frozen as whole, drawn, dressed, steaks, fillets, and sticks.

• Whole fish. The body is entirely intact.
• Drawn fish. Whole fish that have had their entrails (inner organs) removed.
• Dressed fish. The head, tail, fins, and scales have been removed in addition to the entrails.
• Steaks. Cut from dressed fish by slicing from the top fin to the bottom fin at a 90 degree angle at varying thicknesses. Steaks contain a portion of the backbone and other bones.
• Fillets. Made by slicing the fish lengthwise from front to back to avoid the bones.
• Fish sticks. Uniform portions cut from fillets or steaks. They can also be made from fish that has been minced, which is then shaped, breaded, and frozen.

Sniffing for aroma may be the safest, easiest method of determining whether or not fish is fresh, but other criteria can be applied in addition to the “sniff test”. When selecting whole fish, look for skin that is bright and shiny and eyes that bulge, are jet black, and have translucent corneas (the part surrounding the pupil). They should have a “fresh fish” aroma, tight scales, firm flesh, a stiff body, red gills, and a belly free of swelling or gas. The same criteria hold true for drawn fish with the exception of the potential gas-filled belly, which, of course, has been removed.

A stiff body is preferred when selecting a finfish because it is an indication that it is still in rigor mortis, which occurs after slaughter. Flesh that is allowed to go through rigor mortis (stiff to relaxed muscles) has a better texture and flavor. The water-holding capacity of the proteins is increased, which makes the flesh juicier than fish that have not undergone rigor mortis. For these reasons, it is better that handling, packing, processing, and freezing be avoided while fish are in the rigor state. it is also recommended that fish, prior to slaughter, not be subjected to excessive stress if possible, because the resulting stronger rigor mortis is detrimental to texture.

Rigor mortis in fish can last anywhere from several hours to days, depending on the species, temperature, and condition of the fish when caught. Stiffness is delayed if caught fish are immediately placed on ice and kept chilled. Freshness is extended under these conditions because bacterial spoilage does not occur until after rigor mortis has passed. Freezing fish immediately after capture, rather than chilling them on ice and allowing rigor mortis to proceed until the muscles relax again, results in a tough-textured flesh. Cooking fish prior to rigor mortis also results in tough texture.

The meat of the fish should not be slimy, but these can be tricky to judge, because any slime present may have been produced by the fish having been soaked in a special phosphate-containing solution to prevent moisture loss. This solution increases the pH of the tissue, which denatures the proteins and makes them more capable of binding water. Fishermen frequently treat fish with this solution to cut down on the water loss, which might endanger their weight-based profits. Without this solution, fish that is refrigerated may lose up to 80 percent of its water-binding capacity within five days after harvest. The phosphate-containing solution restores the binding capacity of the muscle proteins and prevents the flesh from becoming dry and stringy. Treatment with phosphates also partially inhibits the oxidation of the natural fats in fish, which can result in “fishy” smells when the phosphates bind with the metal ions that promote oxidation.

Other changes that occur in a fish after death is that the eyes flatten and become concave (although this may also be a result of the fish having been picked up by the eye sockets), the pupil turns gray or creamy brown, and the cornea becomes opaque and discolored. In addition, the bright red gills turn a paler brown and as a result are sometimes removed. When the gills turn brown and the eyes lose their bright look, the fish may be cut up as steaks, fillets, or fish sticks. Steaks and fillets should have a shiny, smooth surface that has no signs of curling at the edges. The pieces should be cut clean with no signs of blood, skin fragments, or loose bone, and they should be firm and free of gaping. Although gaping is a sign of aging, it may also be result of rough handling, processing before rigor mortis is complete, the fish having been caught after spawning, or even genetics. Certain fish, like blue fish, and hake, are known to gape more easily.

Part of selecting finfish is knowing how much to buy. A few general guidelines exist. About one-third pound of steaks, fillets, or sticks make an appropriate portion per person. Purchases of half pound for each person will be required when buying dressed fish, and three quarter pound per serving for whole or drawn fish.