Preservative for Low Water Levels.

Hi All, 
I have a foaming sugar scrub with, according to my calculations, 3.5%-4% water (mostly coming from a surfactant). Now when it comes to preservatives, what would be maximum water level in % to be eligible for anhydrous product and not have preservative at all. According to USP <1112> if the water activity is under 0.91 I'm ok for bacteria and under 0.6 ok for yest and mold. 
I understand that it is a high sugar content product (23%) and bacteria just loves sugar, so I'm realizing that not having a preservative at all might not fly by, but do I really need the full dose? How much preservative (say phenoxyethanol) would be substantial to take care of 3.5-4% water?

Thank you all for your help!!!


  • MicroformulationMicroformulation Member, Professional Chemist
    Well, firstly keep in mind that water activity (Aw) must be calculated, not inferred. David Steinberg does a great explanation of this concept in his book, "Preservatives for Cosmetics, Third Edition."

    Secondly, preservation must be done conservatively and must err to the most cognizant protection of your end user. Supposing a "partial" dose of a preservative skirts this caveat. What real benefit are you achieving by limiting the "dose?" Preservatives are generally safe when used properly so I argue that you are doing more of a disservice with this approach.

    Next, you must keep in mind not only your production preservation scheme but in products such as this the introduction of water and microorganisms by the end user. This particular product is a textbook example of when this needs to be a factor.

    Lastly, I would be sure to adequately preserve but "when is it enough or alternatively what is the right usage rate?"  This comes about through experience. Ultimately, the only real and subjective answer would come about through PET testing. Microformulation Cosmetic Consulting provides Custom Formulations for both large Commercial accounts as well as smaller entrepreneurs. We can provide Naturally compliant Formulations under the NSF, NPA, Whole Foods and USDA Organic Certifications. BS.Pharm Albany College of Pharmacy, Union University.
  •  Thank you so much for your reply!
    I do agree with you on all of it. Usually I'm very cautious and do preserve everything on full scale. However this case, I felt was a bit different.
    The product will come out of the tube unlike so many scrubs that come in a jar. So this will reduce contamination by a consumer. (but then you never know what might happen:).
    Secondly, since this is a foaming product I found that the phenoxy being a solubilizer by nature, reduces my viscosity.  Of course I might go to another preservative but then I'm adding to the cost. And parabens (As much as I love them) are out of the question with our "parable free" claim:) MIT and other are in the same boat. 
    PS thanks for the book reference, I'll try to find it:) 
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