Subtopic: Preservation of food by cooling and freezing
Question Statement:
Which of the following statements regarding the freezing of food products and Plank's freezing time equation is/are CORRECT?
Options:
(A) Plank's equation assumes that food freezes completely at a single constant initial freezing temperature
(B) The thermal conductivity of frozen ice is approximately four times greater than that of liquid water at the same temperature
(C) Rapid freezing produces smaller and uniformly distributed ice crystals compared to slow freezing, preserving cellular membrane integrity
(D) During freezing, the concentration of solutes in the unfrozen liquid phase progressively increases, causing depression of the freezing point
Answer Key:
A, B, C, D
Step-by-Step Solution & Explanation
All four statements are foundational principles of food freezing engineering:
- Plank's formula assumes all latent heat is removed at constant initial freezing point $T_f$ without accounting for sensible cooling (Option A is correct).
- Thermal conductivity of ice ($k_{\text{ice}} \approx 2.2\text{ W/m K}$) is ~4 times that of liquid water ($k_{\text{water}} \approx 0.6\text{ W/m K}$) (Option B is correct).
- High freezing rate promotes rapid intracellular nucleation of small ice crystals, minimizing puncture damage to cell walls (Option C is correct).
- As pure ice crystallizes out, the remaining solution becomes more concentrated, progressively lowering the cryoscopic temperature (freezing point depression) (Option D is correct).