Question

Method of drying which works on the principle of sublimation -

• A. Drum drying
• B. Spray drying
• C. Thin layer drying
• D. Freeze drying

Hint:

Sublimation requires operating below the triple point of water.
Freeze drying (lyophilization) is the only common industrial drying method that operates under these low-temperature, low-pressure conditions.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Sublimation is a thermodynamic phase transition where a substance changes directly from a solid state to a gaseous state without passing through an intermediate liquid phase.
Drying methods that utilize sublimation preserve the structural integrity and nutritional quality of heat-sensitive materials.

Step 2: Detailed Explanation:
Let us analyze the principles behind each drying method:
- Freeze drying (also known as lyophilization) operates by freezing the material first to convert its water content into ice.
The frozen product is then placed in a vacuum chamber, and the pressure is lowered below the triple point of water (\(0.01^{\circ}\text{C}\) and \(0.006\text{ atm}\)).
By applying a controlled amount of heat under these conditions, the ice sublimates directly into water vapor, leaving behind a highly porous, dehydrated structure.
This method prevents shrinkage, preserves heat-sensitive vitamins, and maintains flavor, making it ideal for high-value foods and pharmaceuticals.
- Drum drying operates by spreading a thin layer of liquid feed over the outer surface of a rotating, steam-heated metal drum to evaporate water under atmospheric or vacuum conditions.
- Spray drying atomizes a liquid feed into a stream of hot gas to quickly evaporate water, producing a dry powder.
- Thin layer drying exposes agricultural crops to a flow of hot air to evaporate moisture from their surfaces.
Thus, freeze drying is the only method that works on the principle of sublimation.

Step 3: Final Answer:
The method of drying which works on the principle of sublimation is freeze drying.