Question
Enumerate moist heat methods of sterilization. Write in detail about any one method
Answer
Moist heat methods of sterilization:
- Below 100°C: pasteurization (holder method 63°C/30 min; flash/HTST 72°C/15–20 sec) — disinfection only, not sterilization; vaccine bath (60°C/1 hour).
- At 100°C: boiling (100°C/10–30 min) — kills vegetative organisms, not all spores; Tyndallisation (intermittent sterilization, 100°C steaming on three successive days with intervening incubation); free-flowing steam (Koch’s/Arnold’s steamer).
- Above 100°C (under pressure): autoclaving — the standard, most reliable method.
Detailed description — Autoclaving Principle: raising pressure above atmospheric raises the boiling point of water correspondingly — steam under pressure (15 lbs/sq inch) reaches 121°C; held for 15–20 minutes, this combination of high temperature, moisture, and pressure kills all microorganisms, including the most heat-resistant bacterial spores, by denaturing and coagulating their proteins.
Construction and working: a double-walled chamber in which steam is generated (internally or supplied externally) and admitted into the chamber, displacing air (a poor heat conductor, which must be fully evacuated via a discharge valve or vacuum pump for the set temperature to be reliably achieved throughout the load); a pressure gauge, safety valve, and thermometer monitor conditions; a timer controls the holding period, after which steam is released and the chamber allowed to cool before opening.
Standard cycle: 121°C at 15 lbs/sq inch for 15–20 minutes (longer for larger loads or rubber goods).
Applications: sterilization of surgical instruments, dressings, culture media, glassware, and rubber goods in hospitals and laboratories.
Sterility monitoring: chemical indicators (autoclave tape, Bowie-Dick test for effective air removal/steam penetration) and biological indicators (spore strips of Geobacillus stearothermophilus), which must show no growth after processing — the most definitive control, confirming true sterilizing conditions were achieved throughout the load.