Drying Characteristics of Carrots Immersed in a Fluidized Bed of Fluidizing Particles Under Reduced Pressure

Abstract

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.

Keywords:

• Carrot
• Fluidized-bed drying
• Fluidizing particles
• Superheated steam
• Vacuum drying

Notes

H.A. = hot-air drying (without fluidized bed), H.A.F.B. = hot-air fluidized-bed drying, S.H.S.F.B. = superheated-steam fluidized-bed drying.

a: the value of V/V ₀ is almost the same at 333 K regardless of the other conditions, and the values are small.

b, c, and d: the value of V/V ₀ is larger than that of group “a.” As the temperature increases, the value of V/V ₀ increases. For “d” (H.A.F.B. (GB) at 423 K), in particular, the value of V/V ₀ is extremely large.

The color change (RGB change) can be divided into four groups (“a,” “b,” “c,” and “d”) for the drying temperature and pressure. For hot-air drying (H.A.), the color change is smaller than in the cases with the fluidized bed in all groups. As the temperature increases, the color change becomes larger (“a” and “d” < “b” < “c”). Under reduced pressure at 333 K (“d”), the color change is larger than in the cases at atmospheric pressure at 333 K (“a”), but the difference is small.

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldrt.