One of the kitchen's most striking transformations occurs when cooking crustaceans. Like autumn leaves emerging in a green forest, the mostly drab, green-gray shells of lobsters, crabs and shrimp take on vivid pink and crimson hues. The flesh inside colors as well, an effect seen best on shrimp. Even when already peeled and deveined, shrimp gain an appealingly rosy hue almost as soon as they're heated. The color change has its roots in their natural diet.
You Are What You Eat
The adage that "you are what you eat" explains the color of shrimp and other crustaceans. The base of the oceanic food chain is microscropic plants called plankton, which are rich in color compounds such as astaxanthin and beta carotene. When shrimp eat the plankton, these compounds accumulate in their bodies and shells. They cause vivid shades of pink, orange and red, all tones that would be life-shorteningly obvious in the shrimps' aquatic environment; therefore, as a survival trait crustaceans bind these coloring agents into protein molecules that mask their color.
Movin' On Up
As you move up the food chain, these color compounds become more concentrated. The tiniest species of shrimp are small enough to be considered plankton in their own right, and are eaten by their larger kin. In the course of a normal life cycle shrimp acquire a high concentration of color compounds, which in turn are passed on to larger predators. That's why salmon -- voracious predators of small crustaceans -- have naturally pink flesh. The concentration of astaxanthins in a shrimp isn't as high, which is why its flesh doesn't become uniformly pink.
Letting It All Out
When you cook shrimp, the heat you apply causes physical changes to its proteins at the molecular level. The protein molecules are usually tightly wound, but heat causes their internal bonds to weaken and the molecules to unfold. This frees the color compounds and makes them visible, whether the shrimp are cooked quickly at high heat or slowly in gently simmering water. The end result is that the shrimp turn pink, taking on a distinctive and appealing coloration.
The Shell Game
As with lobsters and crabs, shrimps' shells act as a storehouse for many of the color compounds -- and flavor compounds -- they derive from their food. Cooking shrimp in their shells is slightly more work, but it gives them additional flavor and color. Alternatively, save the shells and cook them separately. Crush them and cook them in butter, then strain the butter and use it as an ingredient in sauces and seafood stews. You can also simmer the shells in water, extracting a flavorful broth to use in soups or sauces.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- On Cooking: A Textbook of Culinary Fundamentals; Sarah Labensky, et al.
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