Define the Roller Drying (Drum drying) Method?

One of the important techniques for drying liquid food products is the roller drier. Like spray drying, roller drying can only be used for liquid products, which can either have a low viscosity or be highly viscous to paste like. Roller dryer is also called drum drying. The metal rollers are heated from inside with condensing steam and the product dries as a thin film by contact with the cylinder surface. While spray drying is done purely by vaporisation, evaporation is the main process in roller drying since the saturated vapour pressure is equal to the atmospheric pressure. The product to be dried is spread as a thin film on to the surface of the hot drum and after one revolution is scraped off by a knife, in the form of flakes, scales or powder. A large part of the roller dried whole milk powder is destined for chocolate manufacture. Roller drying is extensively used for the manufacture of 'instant' potato products, potato flakes, and also for all liquid food products including infant foods, fruit products, eggs, milk and beverages.