If you have ever caught fresh fish, you know that it doesn’t have a particularly strong odor, maybe a hint of ocean or lake water. But sometimes the fish you get from the store can have a pungent “fishy” odor.What causes that smell?
What makes fish smell “fishy”?
The answer has to do with some interesting physiology unique to sea creatures.Water in the open ocean is about 3% salt by weight, but the optimal levels of dissolved minerals inside an animal cell is less than 1%.In order to maintain fluid balance, ocean creatures must fill their cells with amino acids and amines to counter the saltiness of seawater. Ocean fish tend to rely on trimethylamine oxide (TMAO) for this purpose.
The problem is that when fish are killed, bacteria and fish enzymes convert TMAO into trimethylamine (TMA), which gives off the characteristic “fishy” odor.This smell can be reduced in two ways. TMA on the surface of the fish can be rinsed off with tap water.Treating the fish with acidic ingredients such as lemon, vinegar, or tomato can also cause TMA to bind to water and become less volatile.Thus the odor compounds do not reach the nose.
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Freshwater fish generally do not accumulate TMAO because their environment is less salty than their cells.As a result their flesh tends to be milder, and they do not get as “fishy” as ocean fish.However, freshwater fish sometimes suffer from an unpleasant “muddy” aroma.This often occurs in bottom-feeders such as catfish, and is caused by two compounds produced by blue-green algae (geosmin and methylisoborneol).These chemicals concentrate in the skin and dark muscle tissue of the fish.Acidic conditions will cause these compounds to break down, so there is good reason for the inclusion of acidic ingredients in traditional recipes.
Next time you have fish be sure to give it a squeeze of lemon or a splash of vinegar!
Reference: McGee, Harold.On Food and Cooking: The Science and Lore of the Kitchen.New York: Scribner, 2004.