by - Pauline W. Fallis,
Dry heat sterilization is probably the least used and most misunderstood method of sterilization.
It is usually used for heat safe items that can not be disassembled or may rust. 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. Death of microorganisms occurs in dry heat by the process called oxidation. In other words, the slow process of coagulating the protein of the cell. The sterilization process is accomplished in HOT AIR OVENS, and it is longer than in steam sterilizers, due to the lack of moisture.
There are two types of hot air ovens used in dry heat sterilization. Those that use gravity convection and those that use mechanical convection. Convection is the term used for the circulation of the heated air within the chamber of the oven.
In Gravity Convention Ovens, air is heated and rises. As you know, heated air expands and possesses less density, thus less weight than cooler air. Therefore heated air rises. The cooler air falls as it is displaced by the rising heated air. Due to this rising and falling of heated and cool air the temperature tends to be uneven in the chamber. Therefore, sterilizing in a gravity convection oven is sometimes difficult, because you cannot be sure that you will uniformly achieve the required minimum temperature for the required minimum time. Due to this temperature variation, monitoring a gravity convection oven is often difficult. Examples of gravity convection ovens are a regular kitchen oven, or a toaster oven.
Mechanical Convection Ovens, are the most effective type. The oven contains a fan or blower which continually circulates the heated air to maintain a uniform temperature throughout the chamber. Most commercially available dry heat sterilizers are of this type. An example is the home convection oven.
Like other methods of sterilization there are both advantages and disadvantages for using dry heat as a method of sterilization. Advantages:
- Dry heat can sterilize items that can not be sterilized in steam or chemical sterilizers, such as powders and oils, or those that are prone to rust.
- Dry heat can be used for glassware, as it will not score or erode the surface as, steam might do.
- Dry heat will not corrode or rust instruments or needles.
- Dry heat will sterilize instruments containing many parts that can not be disassembled. Disadvantages:
- Dry heat penetrates slowly and unevenly.
- Dry heat requires long exposure times to effectively achieve sterility.
- Dry heat requires higher temperatures that many items cannot be safely exposed to.
- Dry heat requires specialized packaging materials that can sustain integrity under high heat conditions.
- Dry heat may require different temperature and exposure times, depending on the type of item being sterilized.