At least we now know the origins of lutefisk.
Lutefisk is dried whitefish (normally cod, but ling and burbot is also used) treated with lye. The first step is soaking 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) is caustic, with a pH of 11-12. 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 marinate the fish too long in the lye because saponification of the fish fats may occur.
Source: Wikipedia