Unique Functions and Use of a Dry Heat Sterilizer
by dewi susanti
Basically, the method and equipment used in sterilization largely depends on what kind of material requires sterilization. This is because different materials have varying capacities to withstand the processes involved. Therefore, before using a dry heat sterilizer, you should be well versed in its unique functions in order to gauge its appropriateness for the material in consideration.
Types of Dry Heat Sterilizers
A dry heat sterilizer is generally designed as an oven that produces hot-air convection (convection is the heated air circulation within an oven chamber). Hence, there are two types of hot-air convection ovens:
- Gravity convection: This type takes advantage of the fact that hot air expands and bears a lower density as compared to cool air. Therefore, heated air circulation occurs within an oven chamber as hot air rises while cool air descends. Unfortunately, this type of convection gives inconsistent temperatures and a comparably slower turn over as compared to mechanical convection.
- Mechanical convection: This is an efficient process in which heated air is blown through all the areas of an oven chamber. Blowing of heated air circulates heat equally throughout the entire oven, which ensures a uniform temperature. Many dry heat sterilizers use this convection method.
How a Dry Heat Sterilizer Works
The key aspect of dry heat sterilization (the process of sterilization using a dry heat sterilizer) is destruction of microorganisms by coagulation (denaturing) of proteins. This process utilizes a material’s capacity for conduction in which heat absorbed through the outer surface is transferred to the interior. After a period of time, the item undergoing sterilization achieves sufficient temperatures for sterilization to occur.
Suitable Materials for Dry Heat Sterilization
Since dry heat sterilization needs high temperatures and long exposure time, it is most suited for sterilizing materials that are heat-stable and non-aqueous. Such materials would not be effectively sterilized using steam in moist heat sterilization.
Various types of materials that may be used in a dry heat sterilizer include: powders that are impermeable to moisture, metal, glassware, anhydrous fats and oils, as well as various oil-based injectables. Such items include syringes and metal instruments since they are heat resistant.
The Process of Dry Heat Sterilization
Items that require sterilization are normally placed in separate units, which are sealed or closed during the process of sterilization. Each specific item requires heating at a particular temperature and for a certain period of time. Moreover, different temperatures of heating require varying durations of time:
(i) A temperature of 160oC requires 180 minutes sterilization time
(ii) A temperature of 170oC requires 60 minutes sterilization time
(iii) A temperature of 180oC requires 30 minutes sterilization time
There are also other special conditions necessary for particular preparations in ensuring elimination of microorganisms. The temperature within an oven must be constantly regulated and monitored throughout the sterilization process. Following the sterilization process, containers which were temporarily closed are thereafter sealed through aseptic techniques. This is necessary in preventing microbial recontamination.
The capacity of a dry heat sterilizer to effectively sterilize may be validated using a bioindicator strain (Bacillus subtilis spores including CIP 77.18 and var niger ATCC9372). The D-value (decimal reduction time or time required at a specified temperature to destroy 90% of bioindicator organisms) for this bioindicator ranges from 5-10 minutes at a temperature of 160oC.