Steam Sterilization Cycles, Liquids

Steam Sterilization Cycles, Part 2: Liquids

Following our first post about Steam Sterilization Cycles—where we addressed Gravity vs. Vacuum Cycles—this second article discusses the Liquids Cycle.

Just about every lab must sterilize some type of liquid solution, such as lysogeny broth (otherwise known as LB Broth), media, agar, buffer, saline, and water.  And as such, sterilization of a liquid in a vented container requires a special type and the proper application of a cycle known as the Liquids Cycle.

The Liquids Cycle

Liquids rely on the Liquids Cycle to avoid a phenomenon known as “boil-over.” Boil-over is simply a liquid boiling so violently that it spills over the top of its container.  Boil-over will occur if the pressure in your autoclave chamber is released too quickly during the exhaust phase of the cycle.  Significant liquid volume can be lost to boil-over, and this can result in unwanted spills on the bottom of your chamber and subsequent cleanup.

To help prevent boil-over during the exhaust phase, the chamber pressure must be released slowly.  This process is controlled by the sterilizer’s control system.  Controlling the exhaust rate allows the liquid load to cool off as the surrounding chamber pressure is decreased.

Be sure your autoclave is programmed and equipped with the right software and hardware to handle a slow exhaust Liquids Cycle.  Quite often, a Jacket Blowdown Option will be supplied to assist with minimizing boil-over.  During a Liquids Cycle, this option automatically exhausts the steam from the jacket (which surrounds the chamber) and thus allows heat to be drawn out of the chamber and the liquid load at a faster rate. The exhaust rate for a Liquids Cycle is different from a standard Gravity or Vacuum Cycle, where the chamber pressure is released quickly.

NOTE: It is important to recognize that larger liquid loads will take longer to both heat up and cool down. See the chart below:

Temperature Setting Liquid Quantity (ml) Time Setting (minutes)
250°F (121°C) 75 25
250°F (121°C) 250 30
250°F (121°C) 500 40
250°F (121°C) 1000 45
250°F (121°C) 1500 50
250°F (121°C) 2000 55


Steam Sterilization Cycles: Boil-Over

Steam Sterilization Cycles: Boil-Over

The autoclave operator should keep this in mind, especially for large beakers or carboys.  For instance, a 30-minute Liquids Cycle for a 500mL flask won’t necessarily achieve the same results if sterilizing a 5 L flask.  It is a best practice to A) validate your liquid loads with hermetically sealed biological indicators and B) minimize container volumes so that cycle times remain manageable.

Now you know what causes boil-over and how to avoid it when running a Liquids Cycle in your autoclave. However, while the Liquids Cycle prevents boil-over, it may also allow a small percentage of the liquid load to evaporate. So, when sterilizing very small volumes, it becomes imperative to minimize or eliminate evaporation.

To mitigate evaporation, more advanced sterilization cycle applications may be necessary. Please stay tuned for our next article in our Steam Sterilization Cycles series, where we will discuss the Air-Overpressure Cycle.

While you wait for our next article in this series, we encourage you to ask us any questions you may have. Contact us today.

17 Questions to Ask Before Buying Your Next Autoclave

17 Questions to Ask Before Buying Your Next Autoclave

With so many models, sizes, options and components to choose from, how can you ever really know exactly what you need to make the most out of your investment?

These questions will help you to make informed decisions by outlining what is most important to consider and know about owning an autoclave.