How Come Fish Stinks But Chicken Doesn't?

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A big haul from the grocery store requires some careful triage: You should probably cook the fresh veggies before the frozen ones, eat the fresh fruit before the packaged snacks, and prepare a fish dinner before one that uses the chicken or beef. What is it about seafood that makes it so easy to go bad? It's because refrigeration is no match for the meat of a cold-water animal.

Enzymes Gonna Enzyme

All living cells are full of different types of enzymes, which trigger the chemical reactions that keep the animal living. That includes the reactions that break down old cells to leave room for new ones. Animals of all types, humans included, are also full of bacteria, which help break down certain chemicals and produce others. While animals are alive, all of their biological processes keep these functions in a delicate balance.

Those functions don't stop when an animal is killed. Enzymes keep on breaking down cells and bacteria go on reproducing, but now they don't have a living, breathing biological organism to keep things in check. Luckily, cold temperatures make chemical reactions slow down, which is why it's super important to keep meat and seafood in the fridge, at a temperature below 40 degrees Fahrenheit (4 degrees Celsius). The fridge slows bacterial growth and enzyme activity to a crawl, which lengthens the amount of time you can wait to make that chicken curry or steak dinner without getting sick.

Chickens and cows are warm-blooded creatures with bodies to match, so putting their meat in the fridge slows their biological processes considerably. Fish, on the other hand, are cold-blooded creatures that live in water temperatures that aren't much different than your refrigerator. "Some species of Arctic or deepwater fish spend most of their lives in water that is just barely above freezing," writes Kenji López-Alt in "The Food Lab."* "Compared to that, the 40º F of an average refrigerator is positively balmy. So fish-related enzymatic action will occur just fine in your fridge or the fish market's display case."

What's worse is that fish contain high levels of a chemical called trimethylamine oxide (TMAO). TMAO is odorless, but once the cold-loving bacteria in fish flesh get ahold of it, they break it down into trimethylamine (TMA). TMA is what gives fish such a stench. At low concentrations, it smells slightly fishy; at high concentrations, it takes on a caustic, ammonia-like scent. That TMAO-to-TMA breakdown is exponential — it happens slowly at first, but faster and faster over the next few days — which is why you need to cook fish so soon after you buy it.

What You Should Do About It

So if your fish smells fishy, should you throw it out? That all depends. According to the FDA, the best fish should have flesh that's vibrantly colored and springy, not darkened or mushy, and a fresh, mild scent — a little brine is okay, the smell of ammonia is not.

But even if you use it before the FDA-recommended deadline of two days, you may notice a fishier smell than when you first bought it. That doesn't necessarily mean it's gone bad, but it could make it a little less appetizing than you'd like. In that case, fight chemistry with chemistry: If you soak the fish in milk for 20 minutes, the casein in the milk will bind with those fishy-smelling compounds and draw them out of the flesh. (Just make sure to throw the milk out when you're done because, ew, fish milk).

For a quicker fix, just spritz some lemon juice over the filet (and on your now fishy-smelling hands, for good measure!). The acids will neutralize the smell, and you'll be left with a citrusy seafood dish. Who doesn't want that?

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For more on how science can improve your cooking, check out "The Food Lab" by Kenji López-Alt. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer December 14, 2017

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