Home Cosmetic Science Talk Formulating Ethanol and Glycerin For Preservation

  • Ethanol and Glycerin For Preservation

    Posted by Alex777 on January 27, 2020 at 5:58 pm

    Hi,

    Does anyone know how much ethanol is needed to preserve a liquid formulation if the liquid formula uses 20% glycerin?

    20% ethanol seems to be the standard. 

    I’ve also read an article that says for liquid formulas, 5-10% ethanol is sufficient. 

    The same article also states that the percentage of ethanol used in a formulation applies to the free water content. So if 100mL of glycerin is used in 500mL solution, the 20% ethanol would apply to the remaining 400mL of solution. 

    The link to the article I am referring to is listed below:
    https://www.researchgate.net/publication/318380276_Antimicrobial_Preservatives

    Any feedback would be greatly appreciated!

    Also, if you have had experience challenge/stability testing with ethanol, please share!

    Thank you!

    Alex777 replied 4 years, 2 months ago 2 Members · 3 Replies
  • 3 Replies
  • pharma

    Member
    January 27, 2020 at 7:46 pm
    Ethanol and glycerine preserve mainly by reducing water activity. Hence, % of water counts and ethanol and glycerine show an additive effect.
    Let’s assume that 40% glycerol results in full preservation (no microbial growth), 20% would therefore be half the preservation needed. Ethanol, as you say, requires about 20% to stop microbes from growing though you only require the remaining 50% = 10% ethanol.
    It’s important to note that this will only result in a no-grow formula, not an ‘everything dies’ product. In order to kill most microbes, you’d have to add at least 40% ethanol and that’s why low % ethanol or glycerine is commonly combined with other preservatives which do kill the little fellows.
  • Alex777

    Member
    January 27, 2020 at 10:48 pm

    @Pharma 

    Thank you for your input. 

    Just to confirm, if using 20% glycerin, only 10% of ethanol is needed to preserve?

    I assumed the following:

    If I have a solution that is 454mL. 20% of that solution is glycerin (91mL). 

    454mL - 91mL = 363mL 

    I assumed I would use 

    363mL x 20% Ethanol = 73g Ethanol. 

    In your case, I would be using 45.4g ethanol.

    (454 x 10% Ethanol = 45.4g)

    Can you confirm which volume of ethanol would be appropriate here?

    Thanks!

  • Alex777

    Member
    January 27, 2020 at 10:56 pm

    Also, does sodium bicarbonate lower water activity?

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