Ratio of sodium benzoate to potassium sorbate for optimal synergy

If you wanted to be precise about it, how could one determine the optimal ratio of Sodium Benzoate to Potassium Sorbate in a solution? For example, I've seen 0.35% Na-benz to 0.20% K-sorb as a good starting point. Is Na-benz higher only to compensate for reduced disassociation?
Estimating dissociation at 50% and 75% respectively at my target pH of 4.2, that gives 219 ppm benzoate (438 ppm Na-benz) and 180ppm sorbate (240ppm K-sorb). Would having benzoate and sorbate at equal molarity be more antimicrobial than not?
If I start from 0.35% Na-benz and set K-sorb such that sorbate is equal to benzoate, then I end up with 0.24% K-sorb (by mass) instead of 0.20%. Is there any way to know if it's better or worse other than getting it challenge tested?
As an aside, the two research papers I've found that look into synergy with sodium benzoate and potassium sorbate turned up negative results. Is this effect even real? Do both of them together provide significantly more effect than using one by itself? (Which is to say, inadequate.)
⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️

Comments

  • AbdullahAbdullah Member
    edited December 2021
    They are often combined at 2:1 benzoate: sorbate so this may be a good ratio. 
  • PhilGeisPhilGeis Member, Professional formulator
    edited December 2021
    I'm not aware there is effective synergy.  The conclusion is often poorly supported in such papers - largely an opinion rather than meeting a predetermined justifiable performance level.  Can you share the ones you find compelling?
  • @PhilGeis I only briefly searched google scholar and read abstracts, skimmed tables and summaries. Here are the two I found FWIW...
    Isn't it popularly touted that together they provide "broad spectrum" protection? The Euxyl K712 brochure claims to be effective against E. coli at 0.5% total (after 168h).

    I found another paper looking at sodium nitrite, sodium benzoate, and potassium sorbate independently against E. coli and B. cereus, with the pH range of K-sorb being extended by high salinity and Na-benz showing the most significant effect from lower pH. Na-benz also being the most effective at elevated temperatures (25-35C).
    The impression I'm getting is that Na-benz starting from 0.1% can provide adequate protection alone at ph<4.0, even against E.coli. K-sorb does not, and needs to be in salt water to work against bacteria. Is there any benefit to combining them?

    (I am now leaning more towards Na-benz+Phenoxyethanol+Ethylhexylglycerin as a preservative starting point, but all I have at the moment is K-sorb, Na-benz, and Phenoxyethanol.)
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • MaydayMayday Member
    edited December 2021
    Abdullah said:
    They are often combined at 2:1 benzoate: sorbate so this may be a good ratio. 
    I have seen that recommended too, something like 0.15% K-sorb and 0.30% Na-benz. But I would really like to know why!
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • PhilGeisPhilGeis Member, Professional formulator
    Thanks - Think you should read these carefully - they do not support "synergism" of the combination.  
    By design, they're not that relevant to cosmetic preservation - metric was MIC in various broths without pH control.   "Synergism" defintion was  arbitrarty.

    The first two concluded no synergism and the last didn't address the phenomenon.

    1st. Tho difficult to follow - concluded twice no synergism e.g.  "The sodium benzoate + potassium sorbate combination exhibited no synergistic effect; .."

    2nd looked only at a Bacillus isolate - not relevant to cosmetic preservation - but concluded "The combined treatment of the two preservatives did not show any synergistic effect of the growth inhibition of B. subtilis."
  • PhilGeisPhilGeis Member, Professional formulator
    Please do not belei.ve supplier literature.  almost all claim broad spectrum through wide pH range.
    Think you're ok with Benozate/Phenoxy/EHG in a emulsion context - but add EDTA.
  • MaydayMayday Member
    edited December 2021
    PhilGeis said:
    Thanks - Think you should read these carefully - they do not support "synergism" of the combination.

    Whoops, I missed the mueller-hinton broth. I think that's also neutral pH, so the sorbate and benzoate would have no significant disassociation and the results are completely irrelevant to low-pH cosmetic formulas.

    My original question still stands though about the why and how for the use ratio for Na-benz and K-sorb.

    I checked out the 2nd ed of your book that you linked in another thread but didn't find specific discussion on concentrations for benzoate/sorbate. Is that sort of question addressed in the 3rd ed of Cosmetic Microbiology: A Practical Approach?

    I should add that I have not yet gotten any textbooks on cosmetic chemistry. If these questions are too elementary, feel free to just point me in a good direction and I'll really appreciate it! (I'm considering Steinberg's Preservatives for Cosmetics 2nd ed.) Thank you for the help so far

    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • MarkBroussardMarkBroussard Member, Professional Chemist
    @Mayday

    This does directly address your specific question, but:  Sodium Benzoate and Potassium Sorbate have the same, or very similar, mechanisms of action on the microbial cell wall.  Potassium Sorbate often causes a flushing reaction in people, so Sodium Benzoate is your better option of the two.  While they are often combined, the specific ratios have been developed by the manufacturers, but I am unaware of any published studies in that regard and perhaps this is company proprietary information.

    But, in developing a preservation system, you'll benefit more for using a mix of preservatives that have different mechanisms of action on the microbial cell.
    Chemist/Microbiologist formulating in the Organic & Naturals & Clean Beauty arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program/Clean At Sephora/Credo Clean guidelines focused skincare & haircare products. 

    See website for details www.desertinbloomcosmeticslab.com
  • Sodium benzoate+phenoxyethanol is a good combination. I would add 0.1% caprylyl Glycol it it instead of EHG. It is cheaper and more water soluble and almost same effective.
  • Abdullah said:
    Sodium benzoate+phenoxyethanol is a good combination. I would add 0.1% caprylyl Glycol it it instead of EHG. It is cheaper and more water soluble and almost same effective.

    @Abdullah I only mentioned the EHG because it's a component of Euxyl PE 9010, which I wanted to try out. What do you like to use Caprylyl glycol for? As part of preservative system or for some other affect?

    wrt EHG and Euxyl PE 9010, reading the CIR safety assessments for EHG, I think I'll just stay away from it unless I can make a good case for it. I'm not liking the higher potential for ocular irritation.

    Something interesting in the Phenoxyethanol safety assessment:

    Treatment with Phenoxyethanol also caused increased permeability of cellular membranes to potassium ions.

    So I wonder if Potassium Benzoate may be a better choice to combine with Phenoxyethanol since it'll increase the potassium ions floating around slightly. But since that's not widely available, Potassium Sorbate could be the better choice. Or just add a dash of KCl...

    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • 0.1-0.3% caprylyl Glycol or EHG both increase the penetration of other preservatives in microb cells so other preservatives function much better.
    Glyceryl Caprylate also does the same thing and i like it even more because it is cheaper and also increase the viscosity of surfactants.

    I prefer sodium benzoate over potassium sorbate because it is less expensive and more gentle. Also in surfactants, it functions better. 
  • PhilGeisPhilGeis Member, Professional formulator
    edited December 2021
    Dave Steinberg's books are excellent but I'm not aware he addresses the organic acid combination.  I'd worry less about synergism than, as Mark said, efficacy of comnbinations.  I know it takes a lot of resources but once you've established a good combination you can  prob reapply with confidence.  P&G did this with isothiazolinone/benzoate/EDTA in all its shampoos through all versions for over a decade.  Benzoate/Benzyl alcohol/EDTA was another combination that carried over well. - in shampoos atd up to neutral pH.
    Alternative systems can work but aren't so versatile.
  • PhilGeis said:
    Dave Steinberg's books are excellent but I'm not aware he addresses the organic acid combination.  I'd worry less about synergism than, as Mark said, efficacy of comnbinations.  I know it takes a lot of resources but once you've established a good combination you can  prob reapply with confidence.  P&G did this with isothiazolinone/benzoate/EDTA in all its shampoos through all versions for over a decade.  Benzoate/Benzyl alcohol/EDTA was another combination that carried over well. - in shampoos atd up to neutral pH.
    Alternative systems can work but aren't so versatile.
    Does Dave Steinberg have book about preservatives? 
  • Thanks Phil and Mark for the advice!

    Btw I just realized my usage of ppm at the top is nonstandard. I thought ppm was on a molar basis (ratio of molecules), but it is conventionally calculated by mass.

    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • Thanks for the book
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