How to Cold-Cook a Fish

When people think about cooking, they mostly think about heat. But there’s one sort of dish that is “cooked” without any heat. The food comes out looking and feeling boiled, but it’s never been near a flame.

The dish is ceviche, and the secret is acidity. Soak fish or shellfish in vinegar or lemon or lime juice, and the meat turns opaque and the texture firms up, almost as if it has been cooked.

What’s going on here?

It’s a neat little piece of food science, but it’s actually far from trivial. In fact, the same process is an integral part of how some cheeses are made.

Meat is mainly proteins, which are like microscopic threads. At ordinary temperatures, each thread kinks into a tiny, distinct ball, but when they’re heated, the balls unravel. This process is known as denaturing.

The more unraveled threads there are, the more likely they are to bump into each other. And when they do, they stick together--they coagulate. Coagulation is the reason translucent foods such as fish and shellfish turn opaque when they’re cooked. The new coagulated protein structure is denser and light can’t pass through it as easily.

Now, here’s the odd thing: Denaturing, and therefore coagulation, also takes place when proteins are exposed to strongly acidic substances. This most often happens with dairy products; it’s why milk curdles when you put orange juice in it. That may not sound like such a good thing to you, but if you substitute vinegar for the orange juice and then drain off the liquid, that’s how you make a fresh cheese like ricotta.

The same thing also happens if protein is exposed to an extremely alkaline substance--though I can’t think of any that tastes good--or even just to air. Air-induced coagulation takes the longest, but it’s one of the reasons that meat stored in the refrigerator darkens and turns hard if it’s not wrapped carefully (outside the refrigerator, of course, it would spoil before it ever firmed).

Though chemical denaturing applies to both animal meat and seafood, in practice, only fish and shellfish are “cooked” this way (meat and poultry do show up in vinegary dishes, called escabeches in Spain and carpione in Italy, but they are cooked--er, heated--first).

That’s because all this protein denaturing and coagulating are not the only things that happen when meat is cooked. Not by a long shot.

Animal meats and poultry are also full of fat and connective tissues. Heat renders the fat, softening it and distributing it deliciously through the meat. At a certain point (beginning at an internal temperature of 130 degrees, to be exact), it also turns tough, stringy connective tissue pleasantly gelatin-like. Soaking a steak in lemon juice is not going to perform either of these essential services.

But fish are built differently. Their fat is almost always already distributed through the meat. They also have hardly any connective tissue to worry about (living in a virtually gravity-free environment, they don’t need it).

All of which makes them the perfect candidates for ceviche, which is, in turn, the perfect dish for this time of year, when the weather has turned warm.

CEVICHE WITH SHRIMP AND AVOCADO

Lime juice has a sprightly tartness and a rich herbaceous flavor that brings out the best in seafood. The avocado is nontraditional, but its creaminess is a good foil for the slightly chewy shrimp. One taster missed the tomatoes that are normally in ceviche, but they aren’t any good yet, so why include them?

1 1/2 pounds peeled raw shrimp

Juice of 5 limes

2 tablespoons seeded, minced jalapeno

1/4 cup minced red onion

Salt

1 cucumber, peeled, seeded and diced

2 avocados, seeded, peeled and diced

1 bunch cilantro, chopped

Fried tortilla chips, optional

Rinse shrimp under cold running water and pat dry. Place in mixing bowl with lime juice, 1 tablespoon jalapeno, 2 tablespoons red onion and sprinkling of salt to taste. Toss well to coat. Spoon into plastic bag, press out air and seal tightly. Refrigerate, turning from time to time to distribute lime juice, until shrimp are opaque, about 1 hour.

When ready to serve, toss shrimp with remaining 1 tablespoon jalapeno and remaining 2 tablespoons red onion, cucumber, avocados and cilantro. Taste and adjust salt. Serve with tortilla chips alongside.

8 to 10 servings. Each of 10 servings, without tortilla chips:

124 calories; 137 mg sodium; 74 mg cholesterol; 7 grams fat; 6 grams carbohydrates; 11 grams protein; 1.05 grams fiber.