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Drum drying defines a process used in the food industry to remove moisture from pastes, purees, and liquids with rotating, heated drums. The procedure converts wet food products into flakes or powder that can be rehydrated by the consumer. Drum drying uses a precise combination of steam pressure, temperature, rate of rotation, and thickness of product to retain flavor and color.
Food manufacturers typically use drums to make potato flakes, powdered milk, baby foods, dehydrated soup mixes, and some breakfast cereals. The process might also convert apples to apple sauce, but drum drying fruit presents a challenge because of its high sugar content. Drum drying preserves ample starch, along with additives and stabilizers, in the production of instant mashed potatoes.
Potatoes are washed and peeled as the first step in the dehydration process. They are then sliced and cooked to create a paste. The puree is applied to the outer surface of an internally heated drum to remove most of the water content. Usually, moisture evaporates within seconds, allowing removal of the end product from the drum via scrapers.
Equipment used for drying typically employs one or two cylinders made from stainless steel or cast iron that rest on a frame. Depending on the intended use, dryers can be purchased with single drums, double drums, or twin drums. Single and double drum dryers are most commonly used in fruit and vegetable dehydration.
Single drum drying, usually used for potatoes, feeds the puree on to the outside of the drum. Double drum drying employs two cylinders that rotate toward each other and is commonly used to make tomato paste. The distance between the drums determines the thickness of food layers. Twin drum dryers rotate away from each other, which controls dust during the food drying process.
A vacuum drum dryer operates in a similar fashion, but uses low temperatures for foods that might be damaged by extreme heat. Drums on this type of equipment are sealed inside a vacuum chamber and represent a form of dehydration considered expensive. Steam drum drying represents a more efficient method of dehydrating food products.
Drum drying came into use in the 1990s before spray drying techniques were developed. Spray drying exposes food with high moisture content to hot air inside a machine that rapidly removes water. These newer machines permit temperature adjustments for different types of foods and the size of particles desired.