Question
Enumerate various methods of moist heat sterilisation. Add a note on laboratory Autoclave.
Answer
Methods of moist heat sterilization:
- Below 100°C: pasteurization (holder 63°C/30 min; flash/HTST 72°C/15–20 sec) — disinfection only; vaccine bath (60°C/1 hour).
- At 100°C: boiling (10–30 min) — kills vegetative organisms, not all spores; Tyndallisation (intermittent sterilization, three successive days of 100°C steaming with intervening incubation); free-flowing steam (Koch’s/Arnold’s steamer).
- Above 100°C (under pressure): autoclaving — the standard, most reliable method.
Note on the laboratory autoclave 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 kills all microorganisms, including bacterial spores, by protein denaturation/coagulation.
Construction and working: a double-walled chamber; steam is generated (internally or supplied externally) and admitted, displacing air (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.
Applications: sterilization of surgical instruments, dressings, culture media, glassware, and rubber goods in laboratories/hospitals.
Sterility monitoring: chemical indicators (autoclave tape, Bowie-Dick test) and biological indicators (Geobacillus stearothermophilus spore strips), confirming true sterilizing conditions were achieved throughout the load.