Not Quite Cooking

• When you cook a piece of fish conventionally, heat causes the proteins in its muscle tissues to change. The molecules first unwind, then contract and become firm under the impact of the heat. Acidic substances have a similar effect, tightening the proteins and firming their texture. If you drop a piece of fish into lime juice or vinegar, you'll see its surface quickly turn opaque and become firm. This effect is the basis of ceviche making. The citrus juice and aromatic ingredients give the fish a bright, fresh flavor and a texture similar to that of cooked fish.

Vetting the Variables

• Recipes for ceviche give marinating times ranging from as low as five minutes to as long as several hours, so it's little wonder if you're hesitant to take any of them at their word. The problem is that there are many, many variables to take into consideration. For one thing, limes aren't always consistent in their level of acidity. The type of fish or shellfish you select is also important because relatively firm fish such as sea bass or grouper can stand up to the marinade much longer than relatively delicate fish such as sole. The size and thickness of your cuts is also important, since thin cuts require less marinating time than thick ones.

Thick and Thin

• If you're just beginning to experiment with ceviche, start with thinly sliced fish that will marinade quickly. For example, a lobster tail sliced very thinly, like carpaccio, might only require five to eight minutes of marinating time and the thin slices with their pink edges will look very appealing on a plate. If you'd rather have pieces 1/4 inch to 3/8 inch in thickness that can be mounded on the plate, your marinating time might be as little as 10 to 15 minutes for delicate fish or four to six hours for firmer fish. If you have the opportunity, it's best to make a test batch ahead of time and taste it at regular intervals to find the optimal balance of flavor and texture.

A Few Further Pointers

• Successful ceviche requires the best and freshest fish you can find, so it's never a bad idea to cultivate your local fishmonger's goodwill. The best fish has just the faintest hint of brine in its aroma, and no fishiness at all. If the fish is whole when you first see it, its eyes should be dark, glossy and well-rounded and its gills red and fresh-looking rather than brown and dull. Have the fishmonger skin it and bone it for you if you're uncertain of your own filleting skills. Use a glass or stainless steel bowl for the marinade, or a good quality food-grade plastic container. Other metals, or inexpensive plastics, can often leave an unpleasant flavor on the fish and the marinade.