Question Bank • QB_APE_DRY_006 MSQ • 2 Marks

Section 6: Agricultural Process Engineering

Evaporation and Drying

Subtopic: Osmotic dehydration and freeze drying

Question Statement:

Which of the following statements regarding the stages and physical mechanisms of freeze drying (lyophilization) are CORRECT?

Options:

  • (A) Primary drying involves the sublimation of free frozen ice crystals under vacuum, with the product temperature maintained strictly below the collapse temperature ($T_c$).
  • (B) Secondary drying involves the desorption of bound, unfrozen water from the solid matrix at elevated shelf temperatures ($20^\circ\text{C to }40^\circ\text{C}$).
  • (C) The condenser coils in a freeze dryer are kept at very low temperatures (typically $-50^\circ\text{C to }-80^\circ\text{C}$) to trap sublimed water vapor as ice.
  • (D) Primary drying is complete when the product shelf temperature falls below $-40^\circ\text{C}$.

Answer Key:

A, B, C

Step-by-Step Solution & Explanation

Analysis of freeze drying stages: - Option A is CORRECT: During primary drying, ice sublimes under high vacuum. Product temperature must remain below the collapse temperature ($T_c$) or glass transition temperature ($T_g'$) to avoid structural collapse. - Option B is CORRECT: Secondary drying removes remaining un-frozen bound water ($5\text{--}10\%$) through thermal desorption, reducing final moisture to
lt; 2\%$. - Option C is CORRECT: The refrigeration condenser acts as a cryogenic vapor pump by condensing and desublimating water vapor onto chilled plates at $-50^\circ\text{C to }-80^\circ\text{C}$. - Option D is INCORRECT: Primary drying is complete when product temperature rises to match the shelf temperature, signaling no more latent heat of sublimation is being consumed.
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