What pH is self-preserving?

I keep seeing different answers on this one, so I'm hoping to get a more definitive answer. I had assumed 10+ had the ability to self-preserve, but I see some places saying 9. That seemed low to me, but I could be wrong. One product I noticed that didn't seem to be preserved (unless I missed something!) is Dr. Bronner's unscented hemp castile soap (the baby-mild). When I tested the pH of the soap I got around 9.5. Wouldn't that be too low?

Thank you!

Comments

  • We can postulate on what pH is self-preserving. It is very much Formula dependent. How can you know? By sending it off to be tested. Preservation is an educated guess proven by testing.
    markfuller@microformulation.com Microformulation.com 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.
  • edited August 2018
    The reason you keep on seeing different answers is that more parameters are involved than pH, like e.g. water activity. Moreover there is no definite limit. Although unlikely to end up in a cosmetic product some microbes are growing optimally around a pH of 10.
    https://en.wikipedia.org/wiki/Alkaliphile
    ISO 29621 defines >=10 as low risk products.
  • Thank you both!
  • edited August 2018
    And some microbes thrive at a pH of around 2 (Helicobacter pylori in stomach).
  • The ISO 29621 Second edition 2017-03 provides some definitive answers on this issue. Quoting from this document:

    "Alkaline pH may also create a hostile environment and may in some products be used as part of their preservative system. Liquid soaps with alkaline pH (pH 9,0 to pH 10,0) present an environment unfavourable for the growth of some microorganisms (see Reference [11]). Hair curl relaxers, due to their extreme pH (around 12), prevent the growth of virtually all microorganisms that would be likely to contaminate cosmetic products (see Reference [12]).
    The reason for this is that the extreme pH, either acidic or alkaline, makes it necessary for microorganisms to expend energy on maintenance of intracellular pH rather than growth. When pH is used in combination with chelating agents, glycols, antioxidants, water activity and high surfactant levels, an environment can be created which will not support microbial growth."

    Works for us. Of course every new product we produce is Challenge Tested, and each batch manufactured is tested in our microbial lab before release.




    Dr. Mike Thair
    Cofounder & Chief Formulator
    Indochine Natural
  • Is it necessary to have Preservative Challenge Tests done on products with physiochemical factors that would make them low risk according to ISO 29621 i.e. a pH > 10 or Aw < 0.75 ? 
  • Is it necessary to have Preservative Challenge Tests done on products with physiochemical factors that would make them low risk according to ISO 29621 i.e. a pH > 10 or Aw < 0.75 ? 
    In my opinion just good practice to do Challenge test for the formulation, then all bases covered.

    Dr. Mike Thair
    Cofounder & Chief Formulator
    Indochine Natural
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