In many labs, preparing growth media is the autoclave’s primary job. There’s no doubt that steam sterilization is the best approach for this task. Unfortunately, the combination of high temperature and rapid pressure shifts that you see in an autoclave chamber create challenges when preparing or processing liquids—even when the autoclave includes special “liquid cycles.”

Under the best of circumstances, it’s not unusual to lose 10% to 20% of the liquid from an open container during autoclaving processing. Under less ideal circumstances the chamber pressure drops more quickly than the load can cool. This causes the liquid to violently boil, and can result in a “boil-over”: growth media overflows its container, fouling your autoclave chamber and potentially entering the sterilizers’ plumbing. When it cools, the media can gum up valves and block pipes. Sealing the container that’s holding the liquid media to “prevent spills” is not a good alternative: if the autoclave hasn’t been set up for processing sealed containers, it’s highly likely some may rupture or burst, either during the sterilization process, or when the unit is being unloaded.

In either case, the sample is wasted, the cycle is ruined, lab techs and researchers are potentially injured, and at the very last everyone is stuck cleaning up a big annoying mess.

Tips to Prevent Sample Boil-Over

Fortunately, there are ways to prevent the frustration and waste of boil-over. Priorclave North America offers these four tips to protect the integrity of your samples and prevent messes during media prep:

Tip #1: Only fill sample containers halfway

You should assume liquids will boil up at least 20 percent. Some froth even more. This is especially with LB agar, but other agars, broths, and growth media are not immune. If you do not fill your containers more than 50 percent, you leave plenty of room to boil without boiling over.

Tip #2: Place your samples in a deep tray

Even if you follow all the tips to prevent the likelihood of boil-over, no process is infallible. Sometimes a cycle doesn’t run as planned, and unexpected results happen. Cleaning a tray is relatively easy and far preferable to cleaning the autoclave chamber and plumbing.

Tip #3: Add at least ¼” water to the tray

Adding water to the tray buffers the effects of temperature and pressure changes as the chamber heats and cools. Additionally, if the sample does boil over or a bottle cracks, a tray with water will be easier to clean.

Tip #4: Hope for the best, but plan for the worst

Select a tray that is deep enough to contain your water buffer, all liquid samples, and the remnants from burst containers. This will mitigate the effects of a bad cycle, and make your life easier when it’s time to clean up the mess. Again: dumping out a messy tray is much easier and faster than cleaning out an autoclave chamber

Consider an Air Ballast Option

If your lab runs a lot of liquid loads, you might consider choosing an autoclave with an air ballast. The air ballast serves two functions: first and foremost, it mitigates rapid chamber depressurization by flooding the chamber with cool, pressurized air as the autoclave vents hot steam. Additionally, the influx of fresh air helps cool the load. Together, these effects help the pressure remain more stable and allows the sample to gradually cool to a safer temperature. That minimizes boiling and loss to evaporation.

Protect the Integrity of Your Research With Priorclave

Priorclave autoclaves are designed for research. All Priorclave autoclaves are built with special attention to liquid loads, nutrient broth, and media preparation. Our chamber and valve design discourage evaporation, bottle ruptures, and excessive boiling. Priorclave’s optional air ballast system provides the benefits of equalized chamber pressure and eases the cooling process, reducing the likelihood of burst bottles and boil-over.

Have questions or want to discuss your lab’s autoclave needs? Please feel free to reach out to us today.