Banana powder is formed by using banana pulp, which is mechanically chopped and then processed with hydraulic shear using a colloid mill, turning it into a paste. Sodium metabisulfite is then used to brighten the yellow color of the paste. The paste is then dried by either spray- or drum-drying, although the latter is more common because none of the paste is lost while drying. The drum-drying also produces about 2% more powder and also dries it more thoroughly. Regardless of the drying process banana powder can generally only stay fresh on the shelf for about a year before passing its expiration date