The agar used can be a broad spectrum agar such as Soybean Casein Digest Agar (SCDA), commonly referred to as Tryptic Soy Agar (TSA) which will allow the growth of most non-fastidious organisms. A selective agar may be used if you are trying to isolate a certain type of microorganisms, such as Sabouraud Dextrose Agar, which is used for the cultivation of fungi.
Following incubation, the agar plate(s) are read on a colony counter and recorded. A detailed description of sampling techniques and incubation periods can be found in ISO 8573-7. It is recommended that the viable organisms be identified in order to assess the impact on the environment and product.
It may be beneficial to conduct particle counts of the compressed air lines before conducting microbiological counts to more accurately determine the microbiological sampling time. This can be performed under the general rule that the higher the particle count the higher the microbiological count. Correlation between these two variables should be determined so that particulate testing can be used to predict future microbiological results.
Industry offers various other instruments which also serve as microbial detection systems. These instruments may or may not comply with the ISO 8573 standards and are offered here for informational purposes only. See Table 1.
The process of conducting a routine environmental monitoring program of your compressed air lines will assist you in identifying the microorganisms, if any, that may be present in your system. Once you know the types and quantities of these potential environmental contaminants the sooner you can start to identify their impact on your finished product, and you can begin to develop systems to protect your product.