Question

The drying process in which a solid form of water is directly converted to vapour form?

• A. Spray drying
• B. Drum drying
• C. Freeze drying
• D. Tray drying

Hint:

Freeze drying = Sublimation (Ice $\rightarrow$ Vapour under vacuum below triple point). Produces porous, high-quality rehydratable food products.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Sublimation is the direct phase transition of a substance from the solid state to the gaseous state without passing through an intermediate liquid phase.
Key Formula or Approach:
\[ \text{Ice (Solid)} \xrightarrow{\text{Sublimation at } P < 611.65\text{ Pa}} \text{Water Vapour (Gas)} \]

Step 2: Detailed Explanation:
In Freeze Drying (Lyophilization):
1. The food product is first completely frozen to solidify all free water into ice crystals.
2. The frozen product is placed in a high-vacuum chamber where pressure is reduced below the triple point of water (\(< 611.65\text{ Pa}\) or \(4.58\text{ mm Hg}\)).
3. Controlled heat is applied, causing the solid ice to sublimate directly into water vapour without melting into liquid water.
This preserves heat-sensitive vitamins, volatile aromas, and biological structure.

Step 3: Final Answer:
Therefore, direct conversion of solid ice to vapour is Freeze drying, matching option (C).