Dry heat sterilization (killing or removal of all microorganisms, including bacterial spores) technique requires longer exposure time (1.5 to 3 hours) and higher temperatures than moist heat sterilization. Various available methods of dry heat sterilization are; hot air oven, incineration, flaming (wire loop) etc.
Dry heat ovens are used to sterilize items that might be damaged by moist heat or that are impenetrable to moist heat (e.g., powders, petroleum products, sharp instruments).
Principle of Dry heat sterilization using HOT AIR OVEN
Sterilizing by dry heat is accomplished by conduction. The heat is absorbed by the outside surface of the item, then passes towards the centre of the item, layer by layer. The entire item will eventually reach the temperature required for sterilization to take place.
Dry heat does most of the damage by oxidizing molecules. The essential cell constituents are destroyed and the organism dies. The temperature is maintained for almost an hour to kill the most difficult of the resistant spores.
The most common time-temperature relationships for sterilization with hot air sterilizers are
- 170°C (340°F) for 30 minutes,
- 160°C (320°F) for 60 minutes, and
- 150°C (300°F) for 150 minutes or longer depending up the volume.
Bacillus atrophaeus spores should be used to monitor the sterilization process for dry heat because they are more resistant to dry heat than the spores of Geobacillus stearothermophilus. The primary lethal process is considered to be oxidation of cell constituents.
There are two types of dry-heat sterilizers:
- the static-air type and
- the forced-air type.
The static-air type is referred to as the oven-type sterilizer as heating coils in the bottom of the unit cause the hot air to rise inside the chamber via gravity convection. This type of dry-heat sterilizer is much slower in heating, requires longer time to reach sterilizing temperature, and is less uniform in temperature control throughout the chamber than is the forced-air type.
The forced-air or mechanical convection sterilizer is equipped with a motor-driven blower that circulates heated air throughout the chamber at a high velocity, permitting a more rapid transfer of energy from the air to the instruments.
Advantages of dry heat sterilization
- A dry heat cabinet is easy to install and has relatively low operating costs;
- It penetrates materials
- It is nontoxic and does not harm the environment;
- And it is noncorrosive for metal and sharp instruments.
Disadvantages for dry heat sterilization
- Time consuming method because of slow rate of heat penetration and microbial killing.
- High temperatures are not suitable for most materials e.g. plastic and rubber items cannot be dry-heat sterilized because temperatures used (160–170°C) are too high for these materials.
- The time and temperature required will vary for different substances and overexposure may ruin some substances.