Microbial testing for cosmetics is the quiet discipline that stops a good formula from becoming a recall. Five tests cover almost every situation a brand will meet: total aerobic count at release, the challenge test, water system monitoring, preservative efficacy over the shelf life, and in-use testing on the finished pack. Each answers a different question, and substituting one for another is where projects go wrong.
1. Total aerobic count at release
The release test counts bacteria, yeast and mould per gram or millilitre and compares them against the limits in the relevant standard. For most leave-on products the limit is stringent, and for eye-area and infant products it is stricter still. A pass at release is the minimum, not a guarantee: the test proves the batch left the plant clean, not that it will stay clean.
2. The challenge test
The challenge test – described in ISO 11930 in the EU – inoculates the product with defined organisms at a defined level and measures how quickly the preservative system reduces them over 7, 14 and 28 days. It proves whether the preservation system works in the actual formula, which is why a preservative that performs in water can fail in a formula with a high surfactant or a high oil load.
3. Water system monitoring
Water is the highest-risk ingredient in a cosmetic plant. Monitor at the point of use, not only at the outlet, with a defined sampling schedule and a trend chart. A system that passes at the outlet and fails at a dead leg is a system with a biofilm problem that will eventually reach a batch. Ask the filler for twelve months of trend data – a single passing result proves nothing.
4. Preservative efficacy over shelf life
A challenge test at release is a snapshot. The preservative has to remain effective at month 12 and month 24, which means testing at stability pulls, particularly after the product has been opened once. A system that passes at release and fails at month 12 is common in formulas with a high level of a volatile solvent, because the solvent evaporates and the water activity changes.
5. In-use testing
In-use testing asks whether the product survives the customer. Recruit 20 to 30 subjects, give them the product in the real pack, and ask them to use it normally for four weeks; then test the contents. A jar of cream used with fingers will show a higher count than a pump bottle of the same formula. A product intended for a jar needs a stronger system than one intended for a pump.
6. When a test can be skipped
An anhydrous product with no water and no water-containing raw material does not need a challenge test in most jurisdictions, though it still needs a release count. A single-use sealed sachet filled and sealed under validated conditions may avoid a challenge test if the safety assessor justifies it, but the justification has to be written and supported by process validation rather than assumed.
7. Commercial profile
Standard microbial programme, three to five weeks of laboratory time, cost proportionate to the sample count. See the cream range and the stability guide for the tests that run alongside.
