Dry heat sterilization
Dry heat sterilization of an article is one of the earliest forms of sterilization practiced. Dry heat, as the name indicates, utilizes hot air that is either free from water vapour, or has very little of it, and where this moisture plays a minimal or no role in the process of sterilization.
The Dry-Heat sterilization process is accomplished by conduction; that is where heat is absorbed by the exterior surface of an item and then passed inward to the next layer. Eventually, the entire item reaches the proper temperature needed to achieve sterilization. The proper time and temperature for Dry-Heat sterilization is 160 °C (320 °F) for 2 hours or 170 °C (340 °F) for 1 hour. Instruments should be dry before sterilization since water will interfere with the process. Dry-heat destroys microorganisms by causing coagulation of proteins.
The presence of moisture, such as in steam sterilization, significantly speeds up heat penetration.
There are two types of Hot-Air convection (Convection refers to the circulation of heated air within the chamber of the oven) sterilizers:
- Gravity convection
- Mechanical convection
Gravity convection process
As air is heated, it expands and possesses less density (weight per unit volume) than cooler air. Therefore, the heated air rises and displaces the cooler air (the cooler air descends). The method of Dry-heat gravity convection produces inconsistent temperatures within the chamber and has a very slow turn over.
Mechanical convection process
A mechanical convection oven contains a blower that actively forces heated air throughout all areas of the chamber. The flow created by the blower ensures uniform temperatures and the equal transfer of heat throughout the load. For this reason, the mechanical convection oven is the more efficient of the two processes.
Instruments used for dry heat sterilization
Effect on microorganisms
Dry heat coagulates the proteins in any organism, causes oxidative free radical damage, causes drying of cells and can even burn them to ashes, as in incineration. Mechanisms. (1) Protein denaturation, (2) Oxidative damage, (3) Toxic effect of elevated electrolyte (in absence of water).
Dry heat at 160 °C (holding temperature for one hour is required to kill the most resistant spores). The articles remain dry. It is unsuitable for clothing which may be spoiled.
1. Red Heat. Wire loops used in microbiology laboratory are sterilized by heating to 'red' in bunsen burner or spirit lamp flame. Temperature is above 100 °C. It leads to sterilization.
2. Flaming. The article is passed through flame without allowing it to become red hot, e.g. scalpel. Temperature is not high to cause sterilization.
3. Sterilization by Hot Air
Hot Air Oven (Sterilizer). It is one of the most common methods used for sterilization. Glass wares, swab sticks, all-glass syringes, powder and oily substances are sterilized in a hot air oven. For sterilization, a temperature of 160 °C is maintained (holding) for one hour. Spores are killed at this temperature. It leads to sterilization.
A Hot Air Oven is an apparatus with double metallic walls and a door. There is an air space between these walls. The apparatus is heated by electricity or gas at the bottom. On heating, the air at the bottom becomes hot and passes between the two walls from below upwards, and then passes into the inner chamber through the holes on the top of the apparatus. A thermostat is fitted to maintain a constant temperature of 160 °C.
- Ninemeier J. Central Service Technical Manual (6th ed.). International Association of Healthcare Central Service Materiel Management. p. 159.