Sometimes when you buy raw shrimp you will notice a thin, black "string" down its back. Although removing that "string" is called deveining, it is actually not a vein at all--in the circulatory sense of the word. It is the shrimp's digestive tract, and when it is dark in color it means it is filled with grit. So when you see this, should you devein the shrimp? And if there is no dark thread visible, is it necessary to devein anyway?
To Devein or Not to Devein
The decision to devein shrimp is basically a matter of personal preference and aesthetics. The vein is visible through the shell and meat, and if you find the digestive tract unappealing and unattractive, then it makes sense to remove it. (In some countries, like Japan, they serve the shrimp with the visible vein.) With large shrimp, it is fairly easy to devein them by slitting along the back and lifting out the vein with a knife.
Most cooks will not bother deveining medium-sized or smaller shrimp unless they look particularly dirty. Unless you are particularly patient and fussy, do not even bother with small shrimp--it could take literally hours to go through the deveining process. To make cooks' lives easier, many suppliers are now offering deveined shrimp, which is sometimes done without splitting the flesh or even removing the shell. So if deveining is not for you, try to find prepacked frozen deveined shrimp.
And What About That Tail
And while we are speaking of removing things from shrimp, what about the shell on the tail? It is simple enough to peel the shell off of the body of the shrimp, but removing the shell over the tail takes a little bit of finesse--too forceful and you will pull that tail right off, losing precious meat.
You can leave the tail on in dishes where it is obvious and conspicuous, so diners can remove it themselves. However, in recipes where a heavy sauce may disguise the tail, you should remove it rather than surprise a guest with an unexpected crunchy bite. If you are frying the shrimp leave it on--some people actually enjoy eating the tail when it is deep-fried and crunchy.
More about Shrimp and Shrimp Recipes
Now that you know what that little black thread is, you can move past it and dig deeper into learning about shrimp as well as cook up some delicious shrimp recipes.