While whole crabs can be purchased live, most crab legs are sold already cooked. Snow crabs and King crabs have large meaty legs that are removed from the crab on the boat, cooked, and flash frozen to enable them to be transported and stored without degrading in quality. So, if you see crab legs in the fish market, they are probably already cooked. Since crabs, like lobster, change color from greenish-brown to orange or red when cooked, you can be absolutely sure that the crab legs are cooked by checking the color.
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Check the color of the crab. If it is already orange or red, it is cooked and just needs to be reheated. If it is greenish or brown, you'll want to make sure that you steam or boil it until it turns orange or red.
Reheat the crab for the appropriate length of time. Crab legs will take from 5 to 7 minutes to steam or boil, 7 to 10 minutes to bake (if you include 1/8 inch of water in the baking pan) or 2 to 4 minutes to microwave. Dry methods of preparation, such as baking without water or grilling, may take up to 16 minutes.
The crab is done when it is heated through.
Pay attention to the odors in your kitchen. The crab will begin to smell like crab as it heats through.