At Home Microbial Testing for Cosmetics


I recently started formulating cosmetic products this year. I had purchased the microbial test kit from Making Cosmetics and proceeded to test two of my formulations - a body lotion & a face lotion. I did not use an incubator for the testing, I left the vials at room temperature for a duration of time starting 6/29/21. Unfortunately, the natural temperature in my house fluctuates, and there is generally a high level of humidity each day, so the temperatures / environment were not as consistent as an incubator. 

So, after the first couple of weeks, there were no changes to either side of the test sticks. I left the vials for a couple more weeks, and then today is the first time I looked at them, and to my surprise, the face lotion vial has darkish gray mold? growing on the rose agar side. This is the only difference I see in both of the vial samples. 

My question is - does this mean my preservative fails the test? Or is this expected for how long I let them sit? Was it normal that there were no changes whatsoever in either vial for the first couple of weeks? Are these tests inaccurate due to the environment that I left them in?

I am currently using 2% phytocide aspen bark extract as my preservative system in both of those formulations. 

Thanks for all feedback!


  • PerryPerry Administrator, Professional Chemist
    edited July 22
    Yes, it means your preservative system fails. (phytocide aspen bark extract is not a reliable preservative)

    Just because you didn't notice any changes doesn't mean they weren't happening. Microbes are invisible until there is enough growth that you can see them.

    No, the tests are not inaccurate. However, they are also not adequate for preservative testing for a product you are going to sell. For that you will need a proper preservative efficacy test.
  • mmattiammattia Member
    Thank you very much for your response. 

    Do you have any recommendations for effective natural broad spectrum preservatives I can replace the phytocide with?

    In the future, if I were to test another product/preservative system, how long should I leave the vial before knowing if my preservative was effective or not? How can I be confident my preservative is definitely working?

    Also, I definitely agree that a professional PET will be my next step - I just figured that the at-home tests would be a good start so that I can send my products in after they pass the at-home test and save money on the ones that definitely won't pass. Do you have any recommendations on labs that do PET testing in the US?

    Thank you
  • PhilGeisPhilGeis Member, Professional formulator
    Please stop this.  Preservation is addressed by challenge testing - organisms are added  to determine kill.  tbhe test kit merely tries (and not adequately) to determine if your product is contaminated
    Tnhe at home test is useless for preservation assessment and pretty poor for anything but perhaps water.
  • mmattiammattia Member
    Thank you for your feedback. I am only using the test kit as a first line of testing. I will not be selling products that have not been professionally tested. The test kit is just my way of knowing which preservatives absolutely do not work before I spend hundreds at a lab. 
  • PhilGeisPhilGeis Member, Professional formulator
    The test kit tells you NOTHING regarding the preservative - or much else.  It might tell you if you screwed up making the product - nothing more.     
  • mikethairmikethair Member
    edited July 26
    In our microbial lab we use Mikrocount Combi tubes and a lab incubator for initial product development testing. Later we will send samples out to a certified lab for the preservative efficacy test.

    Over 10 years has worked perfectly.
    Dr. Mike Thair
    Cofounder & Chief Formulator
    Indochine Natural
  • PhilGeisPhilGeis Member, Professional formulator
    mmatia is confusing content with challenge.
  • MarkBroussardMarkBroussard Member, Professional Chemist
    edited July 24

    This might help you. 

    Preservation is a system approach.  One ingredient that is considered to be a preservative, will not protect your product across the board from bacteria, yeast and mold.  So, you will need to use a combination of preservative ingredients.  Some are more effective against bacteria, some are more effective against yeast and mold.  You need to combine a bacericide with a fungicide.  In addition, there are other components such as formulating your products at pH 4.8 or so (or lower), definitely below 6.0.  And, there are other ingedients that are preservative boosters that enhance the effectiveness of the preservatives by weaking the microbial cell wall, for instance.  And, chelating agents to bind metals.  So, a proper preservation system might consist of 5 or 6 individual ingredients that work in tandem.

    As it regards dip sticks, you can use these as probes or screens to determine if your preservation system is definitely not working as you have noted above.  When you first prepare a sample, it should be relatively clean microbially if you have sterilized your equipment before making the sample.

    (1)  Using a sterile swab, dip the swab into the sample and swirl it around to get a good distribution from different parts of your "product".  I usually dip a finger in the sample prior to sampling with the swab to try to introduce some microbes into the sample.  This might simulate the way an end user will actually use the product if your container is a wide-mouth jar, for instance.

    (2)  Swab a thin layer on each side of the dip stick just so you have an even coating of product on the dip stick surface.  Cover the entire dip stick surface and make sure you can clearly see the surface of the dip stick and that it is not obscured by product. If so, just wipe off any excess product.

    (3)  Best to use an incubator and place the dip sticks in the incubator and monitor them.  As long as I have room in the incubator, I just leave the dip sticks in the incubator for a couple of weeks or so.  If that is not possible and you live in a hot, humid environment, w/o air conditioning just leave them on a countertop.  This will simulate the way the actual product is stored in use.

    (4)  After 72 hours, examine the agar surface of the dip stick with a magnifying glass looking for any growth colonies.  And then monitor periodically over a 2 week period.  If you see any signs of growth whatsoever, then you know that you need to go back and revise your preservation system.

    If you do not see any signs of growth, you can send the sample in for Preservative Challenge Test.  Or, you can send it to a micro lab for a plate test and if that comes back clean, then submit for Preservative Challenge Test.
    Chemist/Microbiologist formulating in the Organic & Naturals arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program guidelines focused skincare & haircare products. 

    See website for details
  • PhilGeisPhilGeis Member, Professional formulator
    edited July 24
    All competent labs test for content before performing challenge testing. 

    Dip sticks are are poor - might tell you if the product is heavily contaminated - that your raw materials are foul, if you making hygiene is foul.  They say nothing of preservative efficacy.  
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