Minerals in the water may deposit on the bottom of the kettle causing residue, which can affect product performance after prolonged use. Add appropriate amount of white vinegar or citric acid to the portable soup kettle, then add water until the mixture completely covers the sediment in the kettle and soak overnight. Pour out the mixture in the kettle, then pour pure water into the kettle, repeat several times until there is no trace of vinegar or sourness left.