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 will swell during this soaking, attaining an even larger size than in its original (undried) state, while its protein content decreases by more than 50 percent, producing its famous jelly-like consistency. When this treatment is finished, the fish (saturated with lye) has a pH value of 11–12, 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.
Sterling silver should never be used in the cooking, serving or eating of lutefisk, which will permanently ruin silver.
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).