In freeze drying (lyophilization) of biological food materials, direct sublimation of frozen ice crystals into water vapor without passing through an intermediate liquid phase requires the drying chamber absolute pressure to be strictly maintained below:
Options:
(A) $611.65\text{ Pa}$ ($4.58\text{ mm Hg}$)
(B) $101.325\text{ kPa}$ ($760\text{ mm Hg}$)
(C) $2.34\text{ kPa}$ ($17.5\text{ mm Hg}$)
(D) $12.50\text{ kPa}$ ($93.8\text{ mm Hg}$)
Answer Key:
A
Step-by-Step Solution & Explanation
At the triple point of pure water ($T_{tp} = 0.01^\circ\text{C}$, $P_{tp} = 611.65\text{ Pa} \approx 4.58\text{ mm Hg}$), solid, liquid, and vapor phases coexist in equilibrium. Sublimation (direct phase transition from solid ice to vapor) is thermodynamically possible only when the system pressure is below the triple-point pressure ($P < 611.65\text{ Pa}$). In practice, industrial freeze dryers operate at $10\text{ to }50\text{ Pa}$.