Can lemon 'cook' fish?
Fish 'meat' consists of large, complex protein molecules. All proteins have a specific 3D structure, which is necessary for them to function. The structure is fragile, and particularly heat can change the 3D structure. When a fish is fried, the proteins unfold, and the consistency of the fish is altered. The structure of the proteins and so the fish can also be affected by acid. Lemon and lime juice contain citric acid, and the low pH of the juice can change the structure of the proteins. Although the fish is not fried, the structure changes anyway, so the fish is prepared – just as if it had been fried.
Ceviche is a cold dish with fish and shellfish that have only been prepared with lemon juice.