Tilapia, or Nile Perch, was little known to American cooks until recent decades. It's a fast-growing fish, not unlike the crappies, perch and sunfish loved by generations of Midwestern anglers, and it's easily farmed even in relatively small ponds and tanks. That makes it widely available, and its firm, delicately flavored white flesh is suited to many cooking techniques. Commercially it's usually sold as fillets, but some companies also offer tilapia "loins."
Tilapia are herbivores, and farmed tilapia are usually fed a diet high in commodity feeds such as corn and soybeans. They can grow to a market weight of 2 pounds in just nine months, making them ideal for fish farming. A 2-pound tilapia yields fillets averaging about 12 ounces, or three-quarters of a pound. It's a relatively flat fish, compared to other common species such as trout, cod or salmon. That means their fillets are relatively broad and flat.
The thickest portion of a tilapia fillet is at the top and front, the area directly behind the fish's head where the head meets the backbone. From the backbone to the beginning of the ribs the fillet remains thick, tapering toward the tail and also becoming thinner over the rib cage. For cooks, preparing the fillet in a single piece presents a problem of timing. The thin areas of the belly and tail cook more quickly than the thicker area behind the head. So typically they're slightly overdone by the time the fillet is fully cooked.
Many cooks address that problem by cutting the fillet into thicker and thinner portions, and allowing longer cooking time for the thin sections. Many fish producers do the same, marketing the thick portion as a tilapia "loin." They're cut from the section behind the head and along the backbone, leaving out the thinner belly and tail meat. Because their shape, size and thickness is consistent, they're easier to cook to a precise degree of doneness. They're also better for retailers and foodservice operators, because producers can cut the loins to specific and consistent portion sizes.
Cooking Loins and Fillets
Tilapia is mild enough to work with almost any combination of flavors, and has enough fat to stand up to dry-heat cooking methods such as baking, grilling and pan-frying. With loins, you can simply cook them until the thickest portion is barely translucent. The flesh will be moist and delicate, and have a fine texture. With whole fillets you can coat the fish with breading or batter to protect the thin portions from overcooking, or bake or poach them in a liquid that will help preserve their moisture. For family meals, you can also even out the fillet by folding the thin belly meat underneath the fillet, doubling its thickness and providing more consistent cooking.