A hot air oven is an essential laboratory equipment used to sterilize heat-stable media, chemicals, laboratory glassware, and other equipment (such as scalpels, scissors, and blades). It uses dry heat to kill microorganisms and operates from 50 to 200 °C. It is found in hospitals and microbiology laboratories where medical professionals and laboratory technicians use it for sterilization and disinfection.
The dry heat sterilization achieves a killing effect by denaturation of protein, oxidative damage of the essential cell constituents, and the toxic effect of the elevated levels of electrolytes.
Table of Contents
Parts of Hot Air Oven
The different parts of the hot air oven are:
Double-walled insulated chamber
Generally, an oven consists of a double-walled, insulated chamber. It helps to prevent the escaping of the heat outside the chamber. The inner layer is a poorer conductor of heat, while the outer layer is metallic. Heating takes place by the electric current. The heating element is arranged according to the uniform heating of the chamber. For the regulation of the chamber, there is a built-in thermostat.
Tubular air heaters
Tubular air heaters generate heat within the inner chamber. Two tubular air heaters are present on both sides of the inner chamber.
The calibration knob is present, which sets the desired temperature of the thermometer for reading the oven’s temperature.
A fan in the oven maintains adequate air circulation in all chamber parts.
Depending on the model, the shelves can range from 2 to 3 frames, usually stainless steel. For the proper circulation of the hot air oven, the selves inside the oven are perforated.
The temperature sensor measures the temperature within the hot air oven. Then displays it on the controller screen.
The load indicator indicates overloading in the hot air oven.
The switch turns on and off the oven.
A safety thermostat protects the oven and specimen from over-temperature if the controller does not function.
Solid doors are present with a silicone rubber gasket and lock.
Working Principle of Hot Air Oven
The sterilization in a hot air oven occurs by conduction. Dry heat is provided in the heating elements through the electric current inside the double-walled chamber of the instrument. The outer surface of the glassware and materials kept inside for sterilization absorbs the heat, which slowly passes to the center of the material one layer at a time. In this way, the whole material reaches the required temperature for sterilization.
The dry heat oxidizes the molecules damaging the cells, which kills the organism. The materials are kept inside the hot air oven for at least an hour to destroy resistant spores. However, holding time can change depending on the temperature.
Handling of Hot Air Oven
Consideration of the following things is necessary for the handling of a hot air oven:
- Use materials suitable for dry heat sterilization only.
- Wrap the materials or equipment in paper or newspaper or enclose them inside a container of cardboard or aluminum. Metal canisters are also suitable for wrapping. Cotton wool can plug in the open ends of test tubes, flasks, and pipettes.
- Arrange the articles appropriately in the chamber to allow the free circulation of the air.
- After placing it correctly, shut the door, and switch on the hot air oven. After that, the temperature will start to rise.
- When it reaches the appropriate temperature, check the time for holding the material to the required temperature. The holding time for sterilization depends on the temperature of the hot air oven. The standard time is 160°C for 120 minutes.
|Temperature °C||Time (minute)|
Then turn it off and allow it to cool to 60°C, preventing the glassware’s breakage.
Types of Hot Air Oven
A forced-air hot air oven is better than a static air oven because its fan keeps the hot air moving throughout the oven. It maintains a consistent temperature throughout the oven by preventing the hot air from rising to the top of the oven. It keeps the cooler air at the bottom.
It is an open type of sterilizer. The heating coil is present at the bottom of the unit. By gravity convection, the hot air rises inside the chamber. Temperature is less uniform than the forced air type hot air oven.
Hot air oven is used to sterilize glassware, metallic instruments, and some chemicals in powder form, oils, and fats.
- Materials suitable for sterilizing in the hot air oven are:
- Glassware: Petri plates, test tubes, flasks, pipettes, syringes,
- Glass or aluminum Petri dishes (not plastic dishes) Glass tubes (rimless) and bottles fitted with aluminum caps or non-absorbent cotton wool plugs. Autoclaving is also suitable for bottles. Glass flasks and cylinders (cover the open end with aluminum foil or paper, tied with string). Glass pipettes (graduated and Pasteur) with ends plugged to a depth of about 20 mm with non-absorbent cotton wool.
- Metallic instruments: forceps, scalpels, scissors
- Chemicals: in the powder form, oils, fats, petroleum jelly
- Pharmaceutical products: liquid paraffin, fats, grease
- Glassware: Petri plates, test tubes, flasks, pipettes, syringes,
- Hot air oven can also be used at a lower temperature (80-100°C) to dry routine glassware.
Quality control testing is essential to test the efficacy of any instrument. Biological indicators are used to test the functioning of the hot air oven.
Spores of the nontoxigenic strain of Clostridium tetani are used as the microbiological test for quality control. Paper strips impregnated with 106 spores are placed in the envelopes and inserted into suitable packs. After sterilization, the strips are removed and inoculated into thioglycollate or cooked meat media and incubated for sterility test under strictly anaerobic conditions for five days at 37°C. If there is proper sterilization, it will kill the spores, and there won’t be any growth.
- Ensure the materials kept for sterilization in the hot air oven are dry.
- Do not use rubber goods, fabrics, and inflammable or volatile substances inside the oven.
- Avoid overloading the oven. Keep space for proper air circulation.
- Wrap the glassware like Petri dishes and pipettes well from the outside.
- Cool the hot air oven to 60°C before opening the door of the hot air oven. It prevents the cracking of glassware.
Advantages of Hot Air Oven
- Sterilization can be done without water, but water is compulsory to use in moist heat sterilization.
- Bacterial endotoxins are inactivated.
- Oils and powder can be sterilized only in the hot air oven. They cannot be sterilized in autoclave as clumps may be formed due to moisture.
- Non-corrosive for metals and sharp objects.
- Easy to install and has a low operating cost.
Disadvantages of Hot Air Oven
- Sometimes prions may not be killed using the dry heat method of sterilization.
- Due to the high sterilization temperature, glassware may become smoky.
- Plastic wares and rubbers can not be sterilized in a hot air oven because these items will melt at a higher temperature.
- Ananthanarayan, R., & Paniker, C. (1980). Textbook of microbiology (1st ed.). Orient Longman.
- Manandhar, S., & Sharma, S. (2017). Practical Approach to Microbiology (3rd ed., pp. 8-9). National Book Centre.