Lutefisk is made from air-dried whitefish (normally cod, but ling is also used), prepared with lye, in a sequence of particular treatments. The first treatment is to soak the stockfish in cold water for five to six days (changed daily). The saturated stockfish is then soaked in an unchanged solution of cold water and lye for an additional two days. The fish will swell during this soaking, regaining a size even bigger than the original (undried) fish, but the protein content paradoxically decreases by more than 50 percent, causing its famous 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 (and nights) 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 amount of potassium carbonate and hydrocarbonate, giving the fish more mellow treatment than sodium hydroxide (lyestone). It is important not to incubate the fish too long in the lye, because saponification of the fish fats may occur, effectively rendering the fish fats into soap. The term for such spoiled fish in Finnish is saippuakala (soap fish).