What is the chemical reaction with lime and seafood in making ceviche?

Answer 1

The citric acid causes the proteins in the seafood to become denatured, which pickles or "cooks" the fish without heat. The result tastes more like a cooked dish and less like raw fish preparations such as Japanese sashimi. Old style ceviche was left up to 3 hours for marinade. Modern style ceviche usually has a very short marinating period. With the appropriate fish, it will marinate for as long as it takes to mix the ingredients, serve and carry to the table.

And a different explanation that might be clearer and includes a couple of important guidelines:

Let me explain using the example of fish, which consists mostly of protein fibers that resemble coiled springs. When heat is applied, starting at about 130 degrees F., the bonds holding these fibers in place begin to relax-that is called denaturing-and then the protein fibers straighten out and link together (coagulation). The fish is thus cooked. However, this process can be caused by more that just heat. Air drying, a very old culinary practice, has the same effect, and so does acidity. So the citric acid in limes, lemons, grapefruit, and Oranges has precisely the same effect on the protein in fish and other seafood that heat does.

One major difference, however, is that citric acid will not kill any parasites in the fish like heat will. Fortunately, most fish are parasite-free, as any sushi chef will tell you. So the rule of thumb is: if you are comfortable eating sushi or sashimi, go ahead and try ceviche.