Question

During the constant rate period in the cross-circulation batch drying of solids by hot air, the rate of drying (kg moisture evaporated per unit time per unit area) is independent of -

• A. Moisture content of solid
• B. Temperature of solid
• C. Temperature of hot air
• D. Humidity of hot air

Hint:

During the constant rate drying period, the temperature of the wet solid surface remains constant and is equal to the wet bulb temperature of the drying air, protecting heat-sensitive food components from thermal damage.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Drying of solid foods occurs in distinct phases: the constant rate drying period and the falling rate drying period.

Step 2: Detailed Explanation:
- Constant Rate Period: During this initial phase of drying, the surface of the solid is completely covered with a continuous film of free water.
- The rate of water evaporation from the surface is equal to the rate of evaporation from a free water surface.
- Because the surface is kept continuously wet by rapid capillary water migration from the interior, the drying rate is entirely governed by external mass transfer conditions (such as air temperature, velocity, and humidity).
- It is completely independent of the moisture content of the solid during this phase, remaining constant until the critical moisture content is reached.
- Falling Rate Period: Once the critical moisture content is reached, the surface dries out, and the rate of drying becomes dependent on internal moisture diffusion (and thus dependent on the moisture content of the solid).

Step 3: Final Answer:
During the constant rate period, the rate of drying is independent of the moisture content of the solid, which corresponds to option (A).