What people in the mid-Atlantic call “mustard,” also known as tomalley, is found in all kinds of crabs and lobsters (which are more greenish in color). Many sources claim that mustard is simply crab fat, but this is not true; it’s actually an internal organ called the hepatopancreas. In crustaceans, it plays a similar role as the liver and pancreas in humans.
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The reason many people mistake crab mustard for fat is because the hepatopancreas is a fatty organ, and in Chesapeake Bay blue crabs especially, it stores extra fat during hibernation. This is why Maryland crabs are buttery tender and delicious. How the crabs are cooked also makes a difference: Marylanders steam their crabs instead of boiling them, so more of the creamy mucus remains. Many consider the mustard to be the tastiest part of the crab, which is why mustard, and Maryland blue crabs in general, have such rabid fans. But the hepatopancreas’ role in the crab’s body raises some serious concerns about eating it.
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