The drying characteristics and properties (color and shrinkage) of carrots (as a representative agricultural product) were experimentally examined in a fluidized bed under reduced pressure. Dry hot air and superheated steam were used as the drying gases. Rice and carrot powders (0.125-0.355 mm in diameter) were used as the fluidizing particles, in addition to glass beads (0.12 mm in diameter).It was confirmed that the drying rate using a fluidized bed was much higher than without a fluidized bed (hot-air drying), regardless of the type of fluidizing particles used. Under reduced pressure, both with and without a fluidized bed, the drying rate was higher than that at atmospheric pressure using hot air. The drying rate was sufficiently high for fluidized-bed drying with superheated steam, though the drying rate was higher with hot air than with superheated steam. As the drying temperature increased, the volume ratio (befor/after drying) of the sample increased. At high drying temperatures (373 and 423 K in the present study), the color of the sample changed; in other words, a heat-induced change in the properties of the carrot was observed. At a low drying temperature (333 K in the present study), the drying method did not affect the color of the carrot; i.e., the color of the dried material was maintained even in a fluidized bed under reduced pressure when the drying rate was higher.
INTRODUCTIONThe drying process affects the quality of a dried product. In particular, heat-sensitive materials such as food, agricultural products, and pharmaceuticals need to be kept at a low temperature while being dried. Vacuum and freeze dryers have been used for the low-temperature drying of such materials. However, these dryers have certain problems; a long drying time and a large amount of energy are required for drying. It is necessary to manufacture a dryer that has a low drying temperature, high drying rate, and high energy efficiency for drying heat-sensitive materials.