Testing Cosmetics for Bacteria, Yeast, and Mold

Preservative Efficacy Testing

Testing Cosmetics for Bacteria, Yeast, and Mold

Any cosmetic formulation containing water must to be tested for bacteria, yeast, and mold. This is called Preservative Efficacy Testing (PET). Pre-market testing is performed on formulations containing water. There is Challenge Testing and Plate Counts Testing.

Challenge Testing is performed in the lab for one month or two months. Millions of bacteria, yeast, and mold will be injected into the product, and it will determine if your preservation system is effective.

Plate Counts Testing is performed to determine if the formulation is not contaminated with bacteria, yeast, and mold. This test does not tell you how long your preservation system will be effective.

Microbial Testing

  1. New Formulation – Challenge Testing
    Challenge testing is a one-time test on the formulation to make sure the preservative system is effective. There is 1 month and 2 month challenge testing. After the formulation is exactly how you want it, a sample batch is made and a portion of it is sent to a cosmetic testing lab for challenge testing.
  2. Batch Testing
    Batch testing is performed on every single batch made to assure there is no bacteria, yeast, or mold in the formulation at that moment. The test only tells you for that moment if there is any bacteria, yeast, or mold in it.

Challenge Testing Method 1 Month

One month challenge testing is now standard in the cosmetic industry. In the past, I did two month testing but now I do the one month challenge test. One month testing is less costly and you can find a lab to do one month challenge testing for approximately $600 per sample. The cost range is $500-$900, generally $600

Batch Testing for Bacteria, Yeast, and Mold

Batch testing is performed on a sample from the batch of a formulation containing water. Every time you make a batch of product, that batch needs to have a bacteria, yeast, and mold test. This test will ensure there is no bacteria, yeast, and mold in the batch you just made, that GMP was followed, and no contaminants were introduced into the batch. Batch testing must be performed on formulations containing water before it is sold on the market. Plate Counts Testing is the method for batch testing. The DIY kits are also used to test a batch.

Aerobic Plate Count (APC) Fungal/Yeast Count (F/YC) aka Plate Counts

Plate counts is a test performed in a cosmetic lab on the sample from the batch. It tests for bacteria, yeast, and mold. The test will tell you the counts of the bacteria, yeast, and mold. It proves GMP was used and if the sample is free of pathogens at that point in time. No pathogens are allowed in any cosmetic formulations.

It does not tell you how long the formulation’s preservation system will be effective. Stability testing does that. Cost: $30-$40.

Do-It-Yourself Test Kits (for Batch Testing and to test the sample before sending it to a lab)

The DIY kit allows you to perform your own testing for bacteria, yeast, and mold, like the plate counts test listed above except that it does not give the plate counts. It will show if bacteria, yeast, or mold is present in the formulation. Cosmetic lab testing is more reliable and should be used in conjunction with the DIY kit. We use the DIY test kit before sending samples to the lab. If we see any growth on the slide, we change the formula preservative system as it won’t pass testing in the cosmetic lab. We also use it for real-time testing as shown in the video.

The kit is simple to use. Apply the formulation such as lotion or cream to each slide with a sterilized swab, place the slide in warm area, preferable an incubator and read it in 24-48 hours (longer if not using an incubator, about one week). For a water solution, dip the slide into the solution.
Pink Side of Slide: Rose-Bengal-Agar (RB) yeast and mold it appears on this side.
Yellow Side of Slide: Nutrient-TTC-Agar (NUT) bacteria appears on this side.

Refer to the directions that come with the kit. Cost: $5 -$8 per test

Real-Time Testing

The product is stored at conditions that product will likely go through during shipping, storage., and usage. The product is monitored until it degrades and fails. At this point the shelf life is determined. Real-time stability testing is the most accurate to determine shelf life of the product. But, it is not feasible since it can take 2-3 years. Therefore, Accelerated Stability Testing is done. A sample should be retained and tested over time for real-time testing. We retain samples of our formulations and perform real-time testing on our formulations.


Learn more in-depth about testing and formulating cosmetics by taking a Formulation Course here.

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