Question
Hot air oven
Answer
The hot air oven is the standard laboratory instrument for dry heat sterilization, using electrically heated air, circulated by an internal fan for even distribution, to kill microorganisms by slow oxidative destruction of cellular constituents (rather than the protein coagulation mechanism of moist heat).
Standard operating conditions: 160°C held for 1 hour is the most commonly used cycle; alternative validated time-temperature combinations include 170°C for 40 minutes or 180°C for 20 minutes (higher temperature permits shorter holding time) — dry heat requires substantially higher temperature and longer exposure than moist heat/autoclaving to achieve equivalent sterilization, since dry heat kills more slowly by oxidation rather than the faster mechanism of moist-heat protein denaturation.
Construction and working: a well-insulated chamber with electrical heating elements, a fan to circulate hot air uniformly (avoiding cold spots that could leave organisms/spores unaffected), a thermostat to maintain the set temperature, and a timer to control the holding period; articles must be loaded loosely (not tightly packed) to allow adequate air circulation and even heat penetration throughout the load.
Materials suitable for hot air oven sterilization: glassware (pipettes, Petri dishes, flasks, test tubes), swabs, oils, greases and petroleum jelly, powders (e.g., talc), and metal instruments/sharp cutting-edge items that would corrode or blunt under moist heat (autoclaving).
Materials NOT suitable: rubber and most plastics (melt/degrade at these temperatures), and any heat-labile fluids/media (which would be damaged or degraded).
Sterilization controls: chemical indicator strips that change colour at the appropriate temperature, and biological indicators using spore strips of a heat-resistant organism (e.g., Bacillus atrophaeus spores are classically used for validating dry heat sterilization, as opposed to Geobacillus stearothermophilus used for autoclave/moist heat validation).
Advantage over autoclaving: does not cause rusting/corrosion of metal instruments and can sterilize materials (oils, powders, sealed glassware) that moist heat cannot adequately penetrate or that would be damaged by moisture.