Total Aerobic Microbial Count (
TAMC) and Total Combined Yeast and Mold Count (
TYMC) tests are the industry standards for measuring bioburden, but they do face specific "accuracy" hurdles when applied to oils due to the physical and chemical properties of the sample.
Here is a breakdown of why these tests can be tricky for oils and how labs typically correct for it:
1. The Immiscibility Problem
Oils do not mix with the water-based buffers and agar media used in standard TAMC/TYMC methods.
- The Issue: Microorganisms are often trapped within the oil phase, making it difficult for them to come into contact with the growth media on a plate. This results in falsely low counts.
- The Fix: Laboratories must use solubilizing agents or surfactants (like Tween 80/Polysorbate 80) to emulsify the oil into the diluent, ensuring any microbes are released and can grow.
2. Natural Antimicrobial Properties
Many oils (especially essential oils like tea tree, oregano, or peppermint) are naturally antimicrobial.
- The Issue: The oil itself may inhibit or kill the very microbes you are trying to count during the testing process, even if they were present in the original batch.
- The Fix: Neutralization is required. According to USP <61> / Ph. Eur. 2.6.12, labs must perform "Suitability Testing" to prove that the oil's antimicrobial activity has been neutralized (often via dilution or chemical neutralizers) so that a known "spike" of bacteria can still be recovered.
3. Masking and Detection Limits
- Low Bioburden: Oils generally have very low water activity, meaning they don't support much microbial growth. If the contamination is extremely low, standard "plate count" methods might miss it.
- Alternative Methods: For oily products that can be filtered, the Membrane Filtration method is preferred. It involves passing the oil through a filter to "catch" all microbes, then rinsing the filter to remove inhibitory oil residues before placing the filter on agar.
Summary of Accuracy
While the tests are accurate
if validated properly, a "straight" test without emulsifiers or neutralizers will almost certainly provide an inaccurate (underestimated) result.