We've found three important variables to consider when purchasing shrimp: variety, size, and fresh vs. frozen status.
Shrimp Varieties
Not all shrimp are the same—far from it. The Gulf of Mexico supplies about 200 million pounds of shrimp annually to the United States, but three times that amount is imported, mostly from Asia and Central and South America.
After tasting all of the commonly available varieties of shrimp several times, we had little trouble declaring two winners: Mexican whites (Panaeus vannamei), from the Pacific coast, are usually the best. A close second, and often just as good, are Gulf whites (P. setiferus). Either of these may be wild or farm-raised.
Unfortunately, these are rarely the shrimp you’re offered in supermarkets. The shrimp most commonly found in supermarkets is Black Tiger, a farmed shrimp from Asia. Its quality is inconsistent, but it can be quite flavorful and firm. And even if you go to a fishmonger and ask for white shrimp, you may get a farm-raised, less expensive, and decidedly inferior shrimp from China (P. chinensis). (There are more than 300 species of shrimp in the world and not nearly as many common names.)
Size
Despite the popularity of shrimp, there are no standards for size. Small, medium, large, extra-large, jumbo, and other size classifications are subjective and relative. Small shrimp of 70 or so to the pound are frequently labeled “medium,” as are those twice that size and even larger.
It pays, then, to judge shrimp size by the number it takes to make a pound, as retailers do. Shrimp labeled “16/20,” for example, require 16 to 20 (usually closer to 20) individual specimens to make a pound. Those labeled “U-20” require fewer than 20 to make a pound. Extra-large shrimp (21 to 25 per pound) usually yield the best combination of flavor, ease of preparation, and value (really big shrimp usually cost more).
And what about prawns? prawns and shrimp (both salt and freshwater) are the same creatures. The words can be used interchangeably, though in the US, the word prawn is rarely seen or heard. Occasionally, it serves to describe what most Americans commonly know as jumbo shrimp, with the word prawn used as a marketing tactic to boost the mystique and interest of the larger crustaceans. In Canada and other English-speaking countries, the word shrimp is not used at all; all shrimp—jumbo, large, and medium—are known as prawns. Additional confusion may arise if you come across Dublin Bay Prawn on a menu. These so-called prawns, also called Norway Lobsters, lobsterettes, or scampi, resemble large shrimp but are really members of the lobster family. Unlike most common shrimp, Dublin Bay Prawns have hard, orange-red shells, bodies up to ten inches in length, and miniature pincer claws. And like lobster, they are usually cooked with heads and claws still attached for presentation -- the claws are too small to yield any meat.
One more note about size: Larger shrimp generally have larger veins, which should be removed. The veins in smaller shrimp are often so negligible it’s not worth removing them. Either way, we find the issue of removing the vein to be one of aesthetics. It neither harms nor improves the flavor of the shrimp. We tested several shrimp deveiners and found that while some models work better than others, none beats a regular paring knife. We recommend that you save money (and drawer space) and live without this gadget.
Shopping
Whichever size you choose, we recommend buying still-frozen shrimp. Virtually all shrimp are frozen at sea (unless otherwise noted), so the fresh-looking shrimp at the fish counter have simply been defrosted and may have been sitting around for days losing flavor and turning rubbery. Still-frozen shrimp are usually cheaper too—it’s worth buying in bulk and storing them in the freezer for future use.
To defrost frozen shrimp, simply place them under cold running water. They will be ready in minutes.