Why To Autoclave Liquids With a Load Probe

In our previous Sterilization Cycles post on the F0 Cycle, we briefly reviewed the process for autoclaving liquids with a load probe. This post further discusses the benefits of using a load probe for sterilization validation.

A load probe is a temperature sensing probe located inside the laboratory’s autoclave chamber. It is configured such that the user can place the tip of the probe within the load being sterilized. Typically, it is placed in the coldest or most difficult to sterilize location. The coldest point in a liquids load is the center of the largest volume (i.e. in the center of a 2L flask). Similarly, the coldest point in a solid load is at the center of the densest bag or pack.

Laboratory autoclaves without a load probe measure temperature at the chamber drain (the coldest point in a chamber) and account for sterilization time when the chamber reaches 250°F. The ambient chamber temperature, however, can often be higher than the temperature at the center of the load—a phenomenon referred to as “load lag”¹. This occurs because as steam condenses on a load, the heat must be conducted from the surface to the center of the load. While this is occurring, the load temperature “lags” behind the chamber temperature. If unaccounted for, a discrepancy between the chamber temperature and internal load temperature heightens the risk of non-sterilization.

When running a sterilization cycle with a load probe, the autoclave uses the load probe temperature reading as the primary control for the cycle. Therefore, if the cycle parameters are set for 30 minutes at 250°F, the sterilizer doesn’t start the 30 minute timer until the load probe senses a temperature of 250°F. The reason behind delaying the timer is a phenomenon referred to as load lag.

In order to guarantee sterilization is achieved, the load must reach at least 250°F (121°C) for a specified amount of time. For example, most LB broth must be sterilized for 15 minutes at 250°F. For loads that exhibit a large load lag (i.e., dense, solid loads or containers with greater than 500 ml of liquid), a load probe should be used to measure the temperature within the load itself.

Using a load probe ensures the load has been subjected to a temperature adequate for sterilization for the required amount of time.

[1] The size of any given load has a considerable impact on the amount of load lag. For example, a 500ml flask with 300ml of liquid will have a lower lag (i.e., the load will match the chamber temperature quicker) than a 1,000ml flask with 600ml of liquid.