Hot Air Oven: Parts, Types, and Uses 

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.

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 uniform heating of the chamber assists in the arrangement of the heating element. 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.

Calibration knob

The calibration knob is present, which sets the desired temperature of the thermometer for reading the oven’s temperature.

Fan

 A fan in the oven maintains adequate air circulation in all chamber parts.

Shelves 

Depending on the model, the shelves can range from 2 to 3 frames, usually stainless steel. For the proper circulation of the equipment, the selves inside this equipment are perforated.

Temperature sensor

The temperature sensor measures the temperature within the hot air oven. Then displays it on the controller screen.

Load indicator

The load indicator indicates overloading in the hot air oven.

On/off switch

The switch turns on and off the oven.

Safety thermostat

A safety thermostat protects the oven and specimen from over-temperature if the controller does not function.

Door

Solid doors are present with a silicone rubber gasket and lock.

Principle of Hot Air Oven

The sterilization in a hot air oven occurs by convection, conduction, and radiation. Electric current provides dry heat in the heating elements inside the double-walled chamber of the instrument. The outer surface of the glassware and other materials kept inside the equipment for sterilization absorbs the heat, which slowly passes to the center of the material one layer at a time by the phenomenon termed as conduction. In this way, the whole material reaches the required temperature for sterilization.
The dry heat oxidizes the water molecules in the microorganisms damaging the cells, which ultimately kills the organism. The materials are placed in this sterilization equipment 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 the instrument on. 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 instrument. The standard time is 160°C for 60 minutes or an hour.

Temperature °C	Time (minutes)
120	480
140	180
150	150
160	60
170	30
180	20

Then turn it off and allow it to cool to 40-60°C, preventing the breakage of glassware.

Types of Hot Air Oven

Forced air 

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. 

Static air 

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. 

Uses

Hot air oven is used to sterilize glassware, metallic instruments, and some chemicals in powder form, oils, and fats.

1. Materials suitable for sterilizing in the instrument 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 
2. It can also be useful at a lower temperature (80-100°C) to dry routine glassware.

Do not sterilize plastic and rubber instruments in this equipment because these instruments can melt due to high heat

Sterilization Control

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.

Biological control

Spores of the nontoxigenic strain of Clostridium tetani are useful 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. 

Precautions

1. Ensure the materials kept for sterilization in the hot air oven are dry.
2. Do not use rubber goods, fabrics, and inflammable or volatile substances inside the oven.
3. Avoid overloading the oven. Keep space for proper air circulation.
4. Wrap the glassware like Petri dishes and pipettes well from the outside.
5. Cool it to 60°C before opening its door of the. It prevents the cracking of glassware.

Advantages of Hot Air Oven

1. As the equipment uses dry heat, sterilization does not require water. However, water is compulsory in moist heat sterilization.
2. While using this method of sterilization, inactivation of bacterial endotoxins occurs.
3. Sterilization of oils and powder can only occur in the hot air oven. They cannot be sterilized in autoclave as clumps may be formed due to moisture.
4. Non-corrosive for metals and sharp objects.
5. Easy to install and has a low operating cost.

Disadvantages

1. Sometimes prions may not be killed using the dry heat method of sterilization.
2. Due to the high sterilization temperature, glassware may become smoky.
3. Plastic wares and rubbers can not be sterilized in a hot air oven because these items will melt at a higher temperature.

References

• 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.