20 Jul 2021

All About Hot Air Ovens

Anjali Singh, MS

As a content and community manager, I leverage my expertise in plant biotechnology, passion for tissue culture, and writing skills to create compelling articles, simplifying intricate scientific concepts, and address your inquiries. As a dedicated science communicator, I strive to spark curiosity and foster a love for science in my audience.

Table of Contents

Why You Should Use a Hot Air Oven

Maintaining an aseptic environment is the primary requirement of tissue culture labs. Multiple strategies are put in place by tissue culturists to avoid any harmful contaminants. They include regular cleaning of the culture rooms, wiping the culture area with alcohol, sanitizing hands before working, wearing lab coats, autoclaving all equipment and culture media, and using effective chemicals like PPM (Plant Preservative Mixture) to prevent culture contaminations.

But did you know a hot air oven can also be used to sterilize some of the lab equipment? Yes, they aren’t common to hear but they also have some contribution to maintain the aseptic condition for tissue culture processes.

This article introduces hot air ovens, their principles, workings, and applications. So, let’s begin!

What is a Hot Air Oven?

By definition, a hot air oven is a sterilizing machine or device that is used to sterilize the equipment and some other materials using dry heat. It is also known as a forced-air circulating oven. The process of dry heat sterilization using a hot air oven was originally developed by Louis Pasteur.

Image: An illustrative labeled diagram of a hot air oven.

Source: Biology Reader

Principle

The hot air oven works on the principle of heat conduction. The body of the object absorbs heat, which then moves at the core of the object. This leads to the sterilization of the whole object layer by layer.

The dry heat is circulated inside the chamber at a temperature ranging from 50-300 ºC to sterilize thermally stable objects. The system involves an oxidative mechanism to kill the microorganisms. The cellular materials of microorganisms and spores are oxidized by the dry heat of the oven to kill them.

What Can be Sterilized Using a Hot Air Oven?

The hot air oven can be used to sterilize materials that don’t catch fire, melt, or change forms when exposed to high temperatures. It can be used to sterilize glassware like Petri dishes, flasks, pipettes, and test tubes; powder, like starch, zinc oxide, and sulfadiazine; materials that contain oils; and metal equipment, like scalpels, scissors, and blades.

Do not sterilize substances like surgical dressings, rubber items, or plastic material.

Parts of Hot Air Ovens and Their Functions

Given below is a list of major parts of a hot air oven and their functions:

1. External cabinet: It’s made of stainless steel and covers the inner chamber.
2. Glass wool insulation: Glass wool is filled in between the inner chamber and external cabinet to provide insulation to the oven.
3. Inner chamber: It’s made of stainless steel to support long-lasting operations. The material has corrosion or oxidation-resistant properties. It also contains slots to place the trays or racks.
4. Tubular air heaters: Two tubular heaters are present in the oven that generate heat inside the chamber for the sterilization of materials.
5. Motor-driven blower: It facilitates the uniform circulation of the air.
6. Temperature sensor: It measures the temperature of the oven.
7. Tray slots: They are present in the inner chamber to place the trays on which materials are kept. They are made of aluminum that is corrosion resistant.
8. PID temperature controller: It accurately controls and maintains the temperature inside the oven chamber for the entire cycle and displays temperature values.
9. Load indicator: It indicates how much load the oven contains.
10. Safety thermostat: It’s a safety part that protects the machine and equipment when the temperature controllers don’t work. It’s also known as an over-temperature protection device.

Types of Hot Air Ovens

1. Gravity Convection Ovens

This oven doesn’t use a fan to move air uniformly inside the oven chamber. Rather, the hot air inside the chamber will naturally rise when it expands and become less dense than its surrounding air. As the air goes up, it will lose its heat energy and fall down, repeating and cycling the process. They are ideal to sterilize powdery materials.

2. FORCED CONVECTION OVENS

These ovens contain fans that circulate hot air inside the chamber. The forced air circulation by the oven provides unique temperature uniformity and rapid heat recovery. They are used in labs for cultures or sample procedures.

3. MECHANICAL CONVECTION OVENS

They provide stability, uniform airflow for superior temperature control, and even heat distribution. They heat faster and uniformly distribute the heat inside the chamber. Mechanical convection ovens are utilized in testing and manufacturing.

How to Operate a Hot Air Oven?

Operating a hot air oven is very simple. You only need to follow a few steps given below:

• Plugin the oven and switch it on.
• Preheat it for about 30 minutes before loading the materials.
• Set the desired temperature and time relative to the volume of materials you are loading in the oven for sterilization.
• Load the materials in the oven without overcrowding them. Maintain proper spacing between each material for its proper sterilization.

You can regulate the temperature inside the oven between 50-300 ℃. But, the most commonly used temperature-time relation to killing microorganisms is 170 ℃ for 30 minutes, 160 ℃ for 60 minutes, and 150 ℃ for 150 minutes.

Advantages of Hot Air Ovens

• It doesn’t need water to sterilize
• It can work at a higher temperature than autoclave and is faster.
• It kills the bacterial endotoxins.
• It’s the only method to sterilize oils and powders.
• It’s economical and safe.
• It’s more fitted in the laboratory environment.
• It takes lesser time to complete the operation than an autoclave.
• It has a simple operating procedure.

Disadvantages of Hot Air Ovens

• It can’t kill the heat-resistant endospore and microbial agents like a prion.
• In comparison to the autoclave or other moist heat sterilization methods, a hot air oven has a lower heat capacity and it’s slower in killing microorganisms.
• It’s not suitable to sterilize all equipment, like plastic wares and heat-sensitive materials.

Understanding the working of the instruments and devices helps you to operate them better and keep them in the best condition to enhance their shelf life. At Plant Cell Technology we are trying our best to give you all the information you should have regarding the tissue culture processes. And, if you think we are missing something out or you’ve got an interesting idea, do reach us and share your views. We’ll be happy to incorporate them in our coming blogs and videos.

Happy Culturing!!

Source: Giphy

Written by: Anjali Singh

Anjali is a scientific content writer at PlantCellTecnology. She joined the company in 2020 with her technical knowledge of tissue culture, a background in Plant Biotechnology, and research skills. Apart from writing educational articles for our tissue culture enthusiasts, she also helps them with their queries on the tissue culture processes.

Before joining PCT, she has worked with various other biotech industries as a Scientific content writer and holds good experience in laboratory work and research.