Question

The cut of moisture content between constant rate and falling rate of drying is called as ..........

• A. Equilibrium moisture content
• B. Critical moisture content
• C. Entropic moisture content
• D. Cut‐off moisture content

Hint:

Drying curve landmarks:
- Critical: The "breaking point" where the process slows down.
- Equilibrium: The "finish line" where drying stops.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the technical term used in drying kinetics to denote the specific moisture level where the drying mechanism changes from surface evaporation to internal diffusion.

Step 2: Detailed Explanation:

• When a moist food item is dried, the process follows a distinct "drying rate curve" (Plot of drying rate vs. moisture content).

• Constant Rate Period: Initially, the surface is fully wet, and drying occurs as if it were a free water surface. The rate is governed by external conditions (air velocity, humidity).

• As drying continues, a point is reached where the rate of internal water migration to the surface can no longer keep up with the evaporation rate from the surface. The surface begins to dry out.

• Critical Moisture Content (\(X_c\)): This is the specific moisture content at which the drying rate begins to decline. It marks the end of the constant rate period and the start of the falling rate period.

• Falling Rate Period: After this point, the drying rate is governed by internal mass transfer resistance (diffusion).

• Equilibrium Moisture Content (EMC): This is the final moisture level the product reaches when it is in balance with the surrounding air; drying stops here.

Step 3: Final Answer: The transition moisture content is the Critical moisture content.