Freeze-drying (lyophilization) setup: initial freezing temperature range During lyophilization of a dense bacterial suspension in sealed vials, to what approximate temperature range is the material first frozen prior to primary drying (sublimation)?

Introduction / Context:
Freeze-drying (lyophilization) preserves microbial cultures by freezing followed by vacuum-driven sublimation of ice (primary drying) and desorption (secondary drying). Correct initial freezing is crucial to maintain viability and structure of the culture matrix.

Given Data / Assumptions:

• Dense cell suspensions are aliquoted into small vials with a suitable cryoprotectant (for example, skim milk, sucrose).
• Samples are rapidly frozen before applying vacuum.
• Lower freezing temperatures produce fine ice crystals and better microstructure preservation.

Concept / Approach:

Laboratory lyophilizers commonly use prefreezing in dry ice–ethanol baths or on shelves capable of reaching approximately -60 to -80°C. This ensures rapid solidification and minimizes ice crystal damage. Warmer ranges like -20 to -30°C are insufficient for optimal vitrification and may compromise viability for sensitive strains.

Step-by-Step Solution:

Verification / Alternative check:

Method guides recommend prefreezing near -70°C; many shelf lyophilizers cool to around -50 to -80°C before evacuation. Dry ice baths approximate -78.5°C, matching the selected range.

Why Other Options Are Wrong:

• -20 to -30°C and -30 to -48°C: Too warm for optimal prefreezing; larger ice crystals form.
• -48 to -58°C: Borderline for some systems; colder is preferred for rapid freeze.
• -5 to -15°C: Inadequate; not fully frozen and unsuitable for lyophilization.

Common Pitfalls:

• Confusing storage temperatures with prefreezing targets; true lyophilization favors colder prefreeze.
• Skipping cryoprotectants, leading to postreconstitution loss of viability.

Final Answer:

-60 to -78°C