A Touch of Tartare

• Mixing tuna with lime or lemon juice triggers a similar reaction to cooking, but without the need to heat the fish. The acids in the citrus fruit act on the tuna flesh, turning it from bright red and translucent, to slightly darker red and opaque. This reaction happens fast, so there's no need to marinate the tuna overnight. If you do so, the acid will continue to react with the flesh and turn it soft and mushy.

Mixing the Marinade

• Tuna tartare marinade ingredients vary, but often include salt, pepper, chili flakes, onion and chopped herbs such as cilantro. The citrus comes from lime, lemon or occasionally orange juice. It's a fast marinade to mix. Indeed, Canadian chef and restaurateur John Bishop suggests marinating tuna tartare for only 10 minutes and never more than half an hour. Longer than that, and the flesh becomes discolored.

Non-Tartare Marinating

• You can marinate the tuna, minus the citrus, overnight. Cut the fresh tuna into little cubes of around 1/4-inch. Mix these with your chosen marinade ingredients, except for the lime. Add 2 tablespoons of olive oil to keep the tuna moist and help the marinade soak into the fish. Without the acids from the citrus, the tuna tartare won't start to "cook." The next day, around 15 minutes before serving, add the juice of two limes and stir well.

Safety First

• Because tuna tartare uses uncooked fish, it's important to choose fresh tuna and store it safely. Look for fish with firm, red flesh and a fresh smell. Dull flesh and a sour or ammonia-like smell suggest the fish is turning bad. To prevent bacteria from growing in the fish, keep it refrigerated at temperatures below 40 degrees Fahrenheit before preparing, according to the FDA. If the fish is left out for more than two hours at temperatures between 40 and 140 degrees F, discard.