Autoclaves are most commonly used to sterilize equipment and materials such as surgical instruments and test tubes with high-temperature steam and pressure. These pieces of equipment can also sterilize liquids, among other uses.
Autoclaving liquids presents some challenges compared to autoclaving solid objects, which are easily overcome with proper preparation. Learn more about how to autoclave water, buffers, agar and other liquid media.
Key Reasons for Autoclaving Liquids
The following are primary reasons why a professional in industries like medicine, biotechnology, chemical manufacturing and food processing may autoclave liquid materials:
- Sterilization
- Waste disposal
- Material Processing
- Component extraction
- Dissolved gas removal
Overcoming Challenges With Autoclaving Liquids
The main challenge with using an autoclave for liquid sterilization is choosing an appropriate cycle time. You want to ensure the liquid gets to a high enough temperature to be sterilized without boiling over or becoming overcooked. Cooling the chamber too slowly can also cause boil-over, overcooking or broken containers. Boil-over results in a loss of liquid and a mess in the autoclave, and overcooking can ruin your liquid. Note that water cannot be overcooked.
Choosing a Cycle Time
You’ll want to consider a few different factors when choosing your cycle time — the amount of liquid, its viscosity and the container type. The more liquid you have, the longer it will take to reach the proper temperature for sterilization. Thicker, more viscous liquids also take more time, while thinner liquids are often quicker.
Your container types can also affect cycle time since metal heats faster and conducts heat better than glass or plastic. If you have various liquids in multiple container types, calculate cycle time based on the one that will take the longest.
Some autoclaves have an F-zero (F0) feature that decreases the time heat-sensitive liquid spends at high temperatures, cycle times and energy use. Users determine how much to shorten the cycle time by consulting F0 empirical tables.
Reaching Sterilization Temperatures
Most liquids have a higher heat capacity than solids, so they take longer to heat and cool, increasing the cycle time. Additionally, the chamber can reach sterilization temperature before the liquid does, making it difficult to tell if the liquid got hot enough. It’s a good idea to use flexible sensors submerged in the media to be sure you’re sterilizing liquid and not just the containers. Using the temperature of the liquid to control the cycle is called load-sensed timing.
If possible, it’s best to use at least two temperature sensors in different containers since the chamber may heat unevenly. Also, because the sensors will read different temperatures if one of the containers breaks, this setup allows you to stop the cycle and prepare to start over if necessary.
Balancing Temperature and Pressure During Cooling
A rapid cooling system can cause an abrupt change in pressure and result in the liquid boiling over. However, decreasing cooling times is necessary to avoid overcooking the load.
You can control the pressure using air ballast. This method increases the pressure by injecting compressed air into the chamber. Higher pressure prevents cracks, spills and boil-overs that can occur at lower pressures and reduces the temperature safely.
Combine air ballast with circulating cool water through a chamber jacket or coils. A super-fast method adds an external fan to increase air circulation, transferring heat from the load to the chamber more efficiently.
How to Sterilize Liquids in an Autoclave
Though choosing a cycle time for liquid sterilization can be complicated, using an autoclave to sterilize it is relatively straightforward.
First, prepare the containers properly. The correct preparation varies depending on the containers and liquid used.
- For buffers, liquid media or water in containers with lids, fill to the fill line. Cap the containers loosely, and secure the caps with autoclave tape.
- For agar in containers with lids, fill only half full, then cap loosely and secure with tape.
- For any liquid in test tubes with caps, fill ⅔ full and cap firmly. Place the test tubes in a labeled rack and stick autoclave tape to the rack.
- For any liquid in containers without lids, cover the containers with foil and autoclave tape.
Once the load is ready, place it in the autoclave. Even if the autoclave is empty, it may still be hot and have steam inside, so use caution when you open it. After putting the containers in the autoclave, add conductive water to the indicator line. While distilled water can sometimes be used, deionized water generally won’t work properly. Check your autoclave manual to determine what kind of water the manufacturer recommends for your model and application.
Add any load sensors you want to use. Ensure the sensors are fully submerged in the middle of the liquid, where it will take the longest to heat, and not touching the container. If the liquid is especially viscous, you may want to tap or vibrate it after inserting the sensor to release any air bubbles since bubbles near the sensor could give you inaccurate readings of the liquid’s temperature.
Close the autoclave and secure the lid to create an airtight seal. Set the desired cycle. It’s best to watch the autoclave for a minute or two to ensure it’s sealed and running correctly. If it isn’t, you’ll want to cancel the cycle.
Once the cycle is complete, you can remove the load from the autoclave. There may still be steam inside, so be sure to step out of the way when you open it. Additionally, the containers will still be hot, so remove them using tools or heat-resistant gloves.
Why Autoclaving Liquids Is Best
An autoclave is a highly effective piece of equipment, especially for sterilization applications. It can reach high temperatures and pressures to kill microorganisms and deliver consistent, reliable results.
This equipment is also highly versatile, enabling professionals to use them for a wide range of liquids. Autoclaving can also promote safer environments, particularly in facilities that handle hazardous or infectious materials. They also reduce the need for chemicals, providing a cost-effective and more environmentally sound solution.
