http://en.wikipedia.org/wiki/Lutefisk
Lutefisk is made from dried whitefish (normally cod in Norway, but ling is also used) prepared with lye in a sequence of particular treatments. The watering steps of these treatments differ slightly for salted/dried whitefish because of its high salt content.
The first treatment is to soak the stockfish in cold water for five to six days (with the water changed daily). The saturated stockfish is then soaked in an unchanged solution of cold water and lye for an additional two days. The fish swells during this soaking, and its protein content decreases by more than 50 percent producing a jelly-like consistency. When this treatment is finished, the fish (saturated with lye) has a pH value of 1112 and is therefore caustic. To make the fish edible, a final treatment of yet another four to six days of soaking in cold water (also changed daily) is needed. Eventually, the lutefisk is ready to be cooked.
In Finland, the traditional reagent used is birch ash. It contains high amounts of potassium carbonate and bicarbonate, giving the fish a more mellow treatment than would lye. It is important not to incubate the fish too long in the lye because saponification of the fish fats may occur. The term for such spoiled fish in Finnish is saippuakala (soap fish).
After the preparation, the lutefisk is saturated with water and must therefore be cooked carefully so that it does not fall into pieces. To create a firm consistency in lutefisk, it is common to spread a layer of salt over the fish half an hour before it is cooked. This will "release" some of the water in the fish meat. The salt must be rinsed off before cooking.