Moist heat kills microorganisms by denaturation and coagulation of proteins, which occurs at a lower temperature and in less time than the oxidative damage caused by dry heat — making moist heat the more efficient sterilizing method.
Methods of moist heat sterilization
Below 100°C:
Pasteurization — heating milk/liquids to 63°C for 30 min (holder method) or 72°C for 15–20 sec (flash/HTST method) to kill pathogens (e.g., Mycobacterium, Salmonella, Brucella) without destroying nutritive value; not a true sterilization method.
Vaccine bath — 60°C for 1 hour, used for preparing bacterial vaccines.
At 100°C:
Boiling — 100°C for 10–30 minutes; kills vegetative bacteria but not all spores; used for disinfection of instruments when autoclaving is unavailable.
Tyndallisation (intermittent sterilization) — steaming at 100°C for 20 minutes on three successive days, with intervening incubation periods that allow spores to germinate into vegetative forms (which are then killed on the next cycle); used for heat-labile fluids that cannot withstand autoclaving.
Steam at atmospheric pressure (Koch’s/Arnold’s steamer) — 100°C, used similarly to Tyndallisation.
Above 100°C (under pressure):
Autoclaving — the standard, most reliable method; steam under pressure (15 lbs/sq inch → 121°C for 15–20 minutes) achieves sterilization by killing even bacterial spores; used for surgical instruments, culture media, dressings, and most laboratory glassware.
