Hot air oven: Principle, Working mechanism, Sterilization control, Advantage and Disadvantage
Introduction of Hot air oven
A hot air oven is the most common method of sterilization in the laboratory working on dry heat. Sterilization is the process of removing or destroying all microorganisms including viruses, bacteria, and their spores from the article or surface without destroying their quality and quantity. It is a physical method of sterilization due to dry heat. Factors influencing sterilization by heat are nature of heat i.e dry or moist, temperature and time, number of microorganisms, nature of microorganisms, type of microorganism, and presence of organic material. Mode of action: protein denaturation, oxidative destruction of essential cell constituents, and toxic effects of elevated levels of electrolytes. It works on the principle of conduction where heat is absorbed by the exterior surface of an item and then passed onward to the next layer. This method was introduced by Louis Pasture.
Principle of hot air oven
Electrical devices work on the principle of dry and hot air convection (that is circulation of heated air), conduction, and radiation. The hot air convection process is of two types. a. Gravity convection process: Heated air expands and possesses less density than cooled air which rises up and displaces the cooler air (the cooler air descends). It produces inconsistent temperature within the chamber thus has a slow turnover. b. Mechanical convection: Use of fitted blower or fan that actively forces heated air throughout all areas of the chamber. This dry heat destroys bacterial endotoxins (or pyrogens ) which are difficult to eliminate by other means. This property makes it applicable for sterilizing glass bottles that are to be filled aseptically. Dry heat kills by oxidation, protein denaturation, and toxic effects of elevated levels of electrolytes and it is more efficient.
Structure of Hot air oven and Functions
It consists of the following parts:
An insulated chamber surrounded by an outer case containing electric heaters
Door locking controls
Metallic cabinet with heating filament and fan fixed in the walls. Thermostat, temperature control, double-walled :(inner being a poor conductor and outer being metallic and air-filled space in between the layers) insulation keeps the heat in and conserves energy. Electrically heated, and provided with a fan or a blower to ensure rapid and uniform. Heating Mechanism:- Killing effect of dry heat on microorganisms is due to i) destructive oxidation of essential cell constituents, ii) protein denaturation and iii) toxic effect of elevated levels of electrolytes.
Uses of Hot air oven
Sterilization of articles that withstand high temperature and do not get burned e.g. Glass-wares, powders, forceps, scissors, scalpels, glass syringes, pharmaceutical products like liquid paraffin, fats, grease, and dusting powder, etc.
Handling procedure of Hot air oven
Wrap the articles or enclose them in a container of cardboard, aluminum, or paper. Mouths of flasks, test tube sand both ends of pipettes must be plugged with cotton wool. Articles to be sterilized such as Petri plates and pipettes may be arranged inside metal canisters and then placed. Place the articles at sufficient distances so as to allow free circulation of air in between them and to ensure uninterrupted airflow. Shut the door and switch on the hot air oven. When the thermometer shows that the oven air has reached sterilizing temperature, heating is continued for the required period of time (e.g. 160°C for an hour). Allow the temperature to fall up to 40°C (approximately 2 hours), prior to removal of sterilized materials; which prevents breakage of glassware.
Advantages of Autoclaving
Do not require water and there is not much pressure build-up within the oven making it safer to work.
Smaller than autoclave but can still be as effective.
Higher temperatures can be reached compared to other means.
This treatment kills the bacterial endotoxin, not all treatments can do this.
An effective method of sterilization of heat-stable articles only method of sterilizing oils and powders.
Protective of sharps or instruments with a cutting edge (fewer problems with dulling of cutting edges).
It does not leave any chemical residue.
It is non-toxic and does not harm the environment.
Disadvantage of Autoclaving
Some organisms like prion may not be killed or inactivated.
Plastic wares or heat-sensitive materials can’t be sterilized.
Glasses may become smoky due to high sterilization temperatures: The temperature holding period is at 160°C for 1 hour, 170°C for 30 minutes whereas at 180°C for 20 minutes.
Dry heat penetrates materials slowly and unevenly and is thus a time-consuming method because of the slow rate of heat penetration and microbial killing.
It requires a continuous source of electricity.
Sterilization control for hot air oven
A) Biological controls: 106 spores of Bacillus subtilis subsp. niger or spores of nontoxigenic strains of Clostridium tetani on paper strips are placed inside envelopes and then placed inside the hot air oven after complete sterilization inoculated in thioglycollate or cooked meat medium and incubated for sterility test under strictly anaerobic conditions for 3 to 5 days at 37°C. Growth in medium indicates the failure of sterilization.
B) Chemical control: Browne’s tube No. 3 shows a green color after sterilization at 160°C for 60 minutes ( color changes from red to green).
C) Physical control: Thermocouples and temperature chart recorder used.
Precautions during autoclaving
Sterilize dry substances.
It shouldn’t be overloaded.
Rubber goods, fabrics, any inflammable or volatile substances should not be put inside the oven.
The oven is allowed to cool gradually for about 2 hours or up to 40°C before the door is opened.