While microbial testing could be classified as safety testing it is so important that I think it
deserves its own category. Whenever you create a new formula that will be put in the hands of some consumer you need to ensure that the preservative system you’re using is adequate for preventing dangerous microbial growth. Some people and brands make it a point to advertise that their products are made “without preservatives.” In my opinion this is dangerous and bad practice. Products that are not properly preserved can spread disease and make people sick. Just don’t do it.
There are two primary types of microbial testing you need to do.
Contamination test — This is a test you need to do on every batch of product you sell! It is a
simple matter of taking a sample and testing to see whether it is contaminated or not. If it’s not, then proceed with packaging & distribution. If it is, don’t sell it! To determine whether you have initial contamination, bacteria and fungi counts, sometimes alled Standard Plate Counts or Aerobic Plate Counts, are done. The test is simple, a sample product is mixed with a neutralizing agent to knock out preservative activity, if present, and then added to bacterial or fungal growth agar in a petri dish. After that, it is incubated at an appropriate temperature for 2-5 days and then bacterial or fungal colonies are counted. The amount of microbes are evaluated to determine whether they pose a consumer risk or not. Typically, a bacterial count of fewer than 10 per gram is ok since cosmetics are not required to be sterile to be safe. Your company will decide on what level of growth is acceptable but 10 per
gram is standard in the industry.
Preservative Efficacy Test – This is a test in which you purposely introduce microbes into your
batches, then watch the samples over time to see whether your preservative system is good
enough to kill off the microbes. If it’s not, you need to improve your preservation system. The
PET or micro challenge test should be done on your initial samples and on your 8 week room
temperature and 45C stability test samples. This can help you determine the length of time that your products will remain microbe free.
Microbial challenge tests consist of five basic steps:
Step 1 is evaluating the product for initial microbial contamination.
Step 2 is the initial inoculation into 5 different containers each of which
have a different microbial colony. The initial microbe concentration is determined.
Step 3 is to incubate the samples at room temperature for 7 days.
Step 4 involves counting the microbes to see if there was a change.
Ideally, they’ll all be killed off.
Step 5 is a further extension of the test with evaluations being done at 14, 21, and 28 days.
If the preservative is working, there will be minimal to no microbial growth over the course of the test.
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