Treatment by a food approved acid of fish makes the flesh more firm. Examples of these acids are: acetic acid (vinegar), citric acid (part of lemon juice), maleic acid (in apple vinegar) or tartaric acid (wine). The exterior flesh looks more whitish, as if it were partially heat treated. The acid denatures the proteins. Fish treated this way needs less time to cook. Alternatively, one can eat without further cooking. But it will also taste more acid. In addition, the acid environment will reduce the spoiling bacteria. Thus essentially preserving the food in a natural manner.
In many cured or marinated fish foods this process takes place. Examples are herring, sardines, mackerel, anchovies, salmon and halibut. Salmon tartar is a good example. But also in making ceviche, a south American marinade of fresh fish pieces cured in lemon juice, this process takes place. The commonly known gravlax as we currently make it and sushi have a preserving effect in common. Namely small amounts of acid, originating from carbohydrates (sugar and rice, respectively) cause this preserving effect.
However, fish treated this way still perishes rather quickly. Only if the treatment is thorough enough -such as in pickling- the fish will store a lot longer. This treatment must take place at refrigerator temperatures.
Parasites are common in several species of fish. This treatment does not kill these parasites. For this reason, use frozen fish (-20 C for 48 h or more) as a base to make these delicious raw dishes. The parasites do not survive the freezing process.
The same treatment by a food approved acid works for meats. For example steak tartar, raw minced beef fillet, is usually treated with lemon juice to ‘cook’ the meat. Meat is also vulnerable to acids, therefore food acids are usually part of meat marinades (vinegar, lemon or orange juice, wine). However, the meat proteins will usually not denature as fast as the fish proteins. So therefore fish is most sensitive to the “cooking” process with acid and is therefore made advantage of.