What accounts for different colors of fat in steamed Dungeness crabs?

Question

During crab season, we buy Dungeness crabs that are steamed our boiled and then stored on ice, for eating cold or for using in recipes. One of the culinarily valuable parts of the crab, used for flavoring soups and sauces, is the "crab fat"* which is attached to the base of the leg clusters and to the inside of the main shell.

Thing is, sometimes this fat is good and sometimes it isn't. Just this week, I got two crabs from the same fishmarket, and one had beautiful golden-yellow crab fat, and the other had horrible grey-green fat I couldn't use. The meat on the crab with the repulsive fat was fine.

What causes the crab fat to sometimes have a hideous appearance? Is it cooking time, time held on ice, gender, stage in life of the crab, an infection, or something else? Is the awful grey-green fat OK to eat even though it looks horrible?

(* the "crab fat" is actually some kind of crab organ, rather than fat as we think of it)

Answer 1

The organ you're referring to is the hepatopancreas (an organ similar to the liver and pancreas (combined) in humans). It is the primary site for digestion and nutrient storage.

Many things could affect its color and taste, including species, age and diet of crab, health of crab, and age and storage of the crab after it's caught and killed.

Are the crabs you're getting alive (and kept alive until cooking)? Once the crab is killed, autolysis sets in, and the enzymes in the organ can start to break down the phospholipids, fats, and proteins in the organ. This will definitely affect its taste and consistency.

More info: If you are boiling the crabs right after killing them then autolysis wouldn't be a factor (as the enzymes should be denatured/deactivated in cooking). The color and taste can definitely be affected by autolysis, but they are also affected by the crab's diet. For example, if the crab eats foods with a lot of carotenoids or astaxanthins (common food pigments — the name astaxanthins comes from the Greek for lobster, where this pigment was first isolated), then it is likely to be more yellow/orange.

Sometimes when enzymes act on molecules, they release nutrients or antinutrients (e.g. many, many plants create cyanogenic glycosides along with enzymes that break the cyanide portion free when the plant is damaged, e.g. by an insect or human eating it). Crustaceans turn a bright red color during cooking because the astaxanthins that you see are bound to protein molecules that appear gray/green until the pigment molecules are separated (to be clear, it is one larger molecule that becomes two or more smaller molecules).

Autolysis is particularly important for shrimp, and it sets in very shortly after their death (from enzymes in their brain), which is why you should either purchase live shrimp or they should be de-headed immediately after killing (as they often are before freezing). I can't answer the specifics for your particular crabs, but if I were to guess based on what you've said I would say the color is likely from diet and/or where it was at in its molting cycle.