This article covers desiccation of agar in environmental monitoring plates. We'll explain why it occurs, and how the issue can be managed effectively. Topics covered include:
Why agar desiccation is most pronounced during settle plate exposure
How airflow, temperature, and incubation conditions influence agar drying
The role of media formulation and neutralizers in agar stability
Why fill volume makes a difference in high-airflow environments
How deeper fills reduce the likelihood of cracking and shrinkage
How selecting the right prepared media supports reliable environmental monitoring
What causes agar desiccation during settle plate exposure?
Agar desiccation is one of the most common challenges in environmental monitoring, particularly when using settle plates in high-airflow environments such as laminar flow cabinets, isolators, and microbiological safety cabinets.
Desiccation occurs when moisture evaporates from the agar surface. This is especially problematic during passive air sampling, where settle plates may be exposed in environments with sustained airflow for up to four hours.
While some moisture loss is inevitable, excessive drying can cause the agar to shrink away from the edge of the Petri dish, or crack during exposure or incubation. Both outcomes reduce the effective sampling surface area and can negatively affect the recovery of test organisms after incubation, ultimately compromising the reliability of your environmental monitoring results.
When pronounced desiccation occurs, the operating environment is often the primary cause. High airflow velocity, elevated air temperature, or a combination of both, can accelerate evaporation, sometimes referred to as the “hair dryer effect.”
If environmental conditions can't be adjusted, laboratories may need to adapt their sampling strategy. In practice, this can mean replacing settle plates more frequently, for example, every two hours rather than using the full four-hour exposure period—to preserve agar integrity and support reliable recovery.
Adjusting fill volume to reduce the risk of agar desiccation
Changing the fill volume of your environmental monitoring plates is one of the most effective ways to reduce the risk of agar desiccation.
This is because deeper agar fill volumes contain more water. This slows evaporation and reduces the likelihood of shrinkage or cracking during exposure. For this reason, deep-fill plates are particularly well suited to use in high-airflow environments.
It’s also worth noting that extra-deep fill plates can be supplied with triple-vent lids, as the increased agar volume compensates for higher evaporation rates without compromising performance.
See the short animation below for a visual explanation of why desiccation of agar plates occurs, and how fill volumes can help address the issue.
Does media formulation affect agar stability?
As well as fill volume, the composition of biological prepared media plays a significant role in how agar behaves under high-airflow conditions. Irradiated media containing disinfectant neutralizers can be particularly prone to shrinkage and cracking in such environments.
While the inclusion of neutralizers is essential for applications such as glove dabs or surface swabs, their use in settle plates for passive air sampling isn't always necessary. If cracking of agar containing neutralizers is a persistent issue, it may be worth reviewing whether neutralizers are required for that specific monitoring application.
How can labs manage agar desiccation effectively?
Managing agar desiccation is rarely about making a single change. Instead, it involves aligning operating conditions, sampling strategy, and media selection.
By understanding how airflow, formulation, and fill volume interact, laboratories can select prepared media that remains stable throughout exposure and incubation—supporting reliable recovery and consistent environmental monitoring results. AnalytiChem offers a range of Redipor prepared media products in various fill volumes off-the-shelf, with tailor-made formulations also available to meet each customer's unique requirements.
The most effective approach to managing agar desiccation is to match plate design and fill volume to the realities of the working environment. Laboratories utilizing high-airflow conditions often benefit from using plates with deeper fills, while those using plates with neutralizers should consider whether they're essential for their passive air sampling methods.
If you’d like guidance on selecting the most appropriate Redipor prepared microbiology media for your application, or want to discuss fill volumes, formulations, or availability in your region, please contact our team. We’re here to help you choose media that performs reliably, even in the most demanding environments.