Q.68 Autoclaving is used as a sterilisation technique. The conditions used for autoclaving are:
- 99° C and 20 Psi
- 105° C and 10 Psi
- 121° C and 15 Psi
- 100° C and 15 Psi
121° C and 15 Psi is the standard autoclaving condition for sterilization.
This combination achieves 15-minute spore kill (6-log reduction of Geobacillus stearothermophilus) via saturated steam at 2 atm absolute pressure, universally adopted for heat/moist-tolerant materials in microbiology labs.
Option Analysis
99° C and 20 Psi
99°C below steam sterilization range; insufficient for bacterial endospores even at high pressure. Used for pasteurization, not sterilization. Incorrect.105° C and 10 Psi
Approaches boiling but inadequate spore lethality; 10 psi yields ~115-118°C, requires 60+ minutes. Non-standard, inefficient. Incorrect.121° C and 15 Psi
Gold standard: 121°C saturated steam at 15 psi gauge (~103 kPa, 2 atm absolute) for 15-20 minutes. Achieves D121 value matching spore thermal death time. Correct.100° C and 15 Psi
100°C = boiling water temperature; 15 psi should yield ~121°C but option specifies 100°C (implying dry heat or measurement error). Ineffective for sterilization. Incorrect.Conditions used for autoclaving leverage moist heat thermodynamics where 15 psi gauge pressure elevates steam temperature to 121°C, enabling rapid protein denaturation/coagulation in microorganisms including Clostridium, Bacillus spores.
Physics of Saturated Steam
Atmospheric boiling = 100°C. Each 1 psi gauge raises boiling point ~2.7°C. 15 psi → ~121°C. Latent heat of vaporization (2260 kJ/kg) provides superior heat transfer vs. conduction, penetrating fabrics/porous loads effectively. F0 = 15 minutes at 121°C.
Sterilization Validation
Biological indicators (Geobacillus stearothermophilus ATCC 7953) confirm 12-log reduction. Chemical indicators (Browne/Bowling tape) verify 121°C attainment. Time-temperature equivalents: 134°C/3 min = 121°C/15 min via z=10°C decimal reduction kinetics.
Cycle Parameters Comparison
Temperature (°C) Pressure (psi) Exposure Time Application 121 15 15-20 min Liquids, glassware, unwrapped instruments 134 30 3-10 min Wrapped surgical tools, porous loads 115 10 30+ min Heat-sensitive media (rare) Load Considerations: Liquids require slower ramp-up to prevent boil-over; wrapped instruments need pre-vacuum cycles for air removal ensuring steam penetration.
