How does citrus “cook” raw things, like fish in ceviche?

Technically speaking, cooking requires heat, so ceviche (also known as seviche or cebiche), a dish in which raw fish is marinated in citrus juice, isn’t cooked. But it’s not exactly raw, either. Both heat and citric acid are agents of a chemical process called denaturation. In this process, the heat or citric acid changes the proteins in the fish, unraveling the molecules and altering their chemical and physical properties. When fish is bathed in citrus juices, this process of denaturation turns the flesh firm and opaque, as if it had been cooked with heat.

But how long do you need to marinate fish in citrus juices before denaturation takes place? Well, it depends on the type of fish and how you like it “cooked.” After soaking in citrus juices for just a few minutes, fish develops a firm, opaque exterior but maintains a raw, sashimi-like interior. If you marinate the fish too long, it may seem tough and “overcooked”—and the citrus juices can overpower the flavor of the fish. Whatever type of fish you’re using, it’s important to cut it up into bite-size strips, because the increased surface area will make it easier for the citric acid to do its work. A flakier fillet, like flounder, snapper, or sole, or tender shellfish like scallops may only need to marinate for about 15 minutes. Quarter-inch strips of mahi mahi, a hearty and dense fish, could take closer to 50 minutes or an hour to “cook.”

“Cook” is in quotes because citric acid won’t kill bacteria the way that heat does. So it’s of utmost importance to use fresh, disease-free, and parasite-free fish. According to Mark Bittman’s The Best Recipes in the World: “[I]f you’re cautious you will want to use finfish that has been frozen to -4F for 7 days,” or, if you have a commercial freezer, -31F for 15 or more hours, which will kill parasites like tapeworms and roundworms. If you’re worried about bacteria that might be in the fish, try making mock ceviche by blanching your fish in boiling water for one or two minutes before marinating it. Some believe that preboiling the fish also helps it maintain its texture.

Ceviche originated among the Incas, who seasoned their fish with salt and chile peppers and “cooked” it in the juice of tumbo, or banana passionfruit. To approximate the flavor of pre-Hispanic ceviche, chef and cookbook author Norman Van Aken suggests combining lime and grapefruit juice; other variations include a mixture of lime and passion fruit pulp.

Traditional Peruvian ceviche is spiked with chiles and often served with boiled potatoes, yams, and corn to balance the heat. The dish is now popular throughout South America, the Caribbean, and Mexico—and each country has its own variations.