Cooking ingredients with strong scum, such as boiled eggs, konjac, and burdock, can cause discoloration. To remove discoloration, boil citric acid (or vinegar) in a 10:1 ratio (10 parts boiling water to 1 part citric acid) for a few minutes. After leaving it for about an hour, wash it with dishwashing detergent for effective results. Once the discoloration is gone, boil it with rice washing water for about 10 minutes to form a film on the surface, making it less prone to blackening. You can also rub it with sliced lemon or boil apple peels.