The shells of crustaceans such as crabs and lobsters contain astaxanthin (a type of naturally occurring chemical compound). It is complexed -- a type of bonding -- with other proteins, giving raw crustaceans a dark green tinge.
The astaxanthin-proteins complex is, however, not stable when heated. It breaks down into just astaxanthin, which on its own is a fat soluble pigment called a carotenoid, and is stable to heating.
It is also red in color. This is why crabs, lobsters, shrimps and crayfish turn red when they are cooked, changing from their dark green tinge to beautiful reds on a serving platter.
While crustaceans are mostly dark green when raw, many of us might have seen brown or even blue lobsters and crabs. When these are cooked, they turn red too, which indicates the same astaxanthin phenomenon is at work.
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