Drying
Name:
Drying
Detailed information:
(1) Polymerization of the glycerides of unsaturated vegetable oils induced by exposure to air or oxygen.See Drying Oil; Drier.(2) Removal of 90–95% of the water from a material, usually by exposure to heat. Industrial drying is performed by both continuous and batch methods. The type of equipment and the temperatures used depend on the physical state of the material, i.e., whether liquid (solution or slurry), semiliquid (paste), solid units, or sheet. Continuous drying: The rotating-drum dryer is used for flaked or powdered products (soap flakes); a heated metal drum revolves slowly in contact with a solution of the material, the dry product being removed with a doctor knife. In paper manufacturing, drying is performed by a battery of staggered, steam-heated, revolving drums located at the dry end of the fourdrinier machine, the paper passing around the drums at high speed; the moisture content is thus reduced from 60% to about 5%. In spray drying, milk, egg white, and other liquid food products are passed through an atomizing device into a stream of hot air. In tunnel drying, the product travels on a conveyor belt through a heated chamber of considerable length. Batch drying: Steam-jacketed pans are used if the material is in paste or slurry form, or in removable trays placed in an oven, if in solid units (fruits, vegetables, meats, etc.). The revolving-tube dryer, used for granular solids and coarse powders, is a long, horizontal cylinder in which a current of warm air runs counter to the movement of the material. Freeze-drying is a specialized technique utilizing high vacuum and low temperatures. See Dehydration; Evaporation; Freeze-drying.
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