Preserving Mouthwash Using Salt

Thank you in advance for your help!

We have currently formulated a mouthwash with 20% ethanol. However, the burn is pretty bad. 

Is it possible to preserve a mouthwash using a combination of 10% salt and 20% glycerin/xylitol? 

There has also been a patent filed on using 0.25% menthol as a preservative. 

Please provide your thoughts. It would be of great help!

Thanks again!


  • PharmaPharma Member, Pharmacist
    Most microbes will be able to just barely grow in 10% brine. Fortunately, the faster growing ones are usually not spoilage germs but lactobacilli... however, there are a few exceptions such as Staphylococcus aureus. This being said, you have to add at least 20% glycerol or similar to further reduce water activity (which will then be, at a rough estimate, so low that only 'extremophiles' will thrive). Whether or not you're fine with the risk of accidentally acquiring such microbes is up to you (these are likely to be yeast which otherwise spoil sugary foods). Given that it's a mouthwash, additionally lowering pH to further reduce growth as would commonly be done is not a viable option.
    Also keep in mind: Low water activity will only stop most microbes from growing but they will not die. It is therefore possible that different microbes will still grow, most of all yeasts and mounds in headspace especially once the bottle is not so full anymore.
    Did you try to gargle with 10% salt water? Sea water has half as much... sure, diluting it 11 times will result in isotonic salt water and that's barely noticeable but since you noticed 20% ethanol, you're likely not diluting. Have fun with such a hypertonic salt solution *shudder*.
    Menthol at 0.25%? Doesn't sound safe. Besides, patent claims are often similar to cosmetics claims: they are unsubstantiated and sometimes more imaginative than actually working.
    Just add some decent preservative ;) .
  • @Pharma

    Thank you so much for your detailed response! 

    Do you think we should just stick with the 20% ethanol and call it a day? We also use 20% glycerin.

    Do you think that we can drop ethanol to 15% considering we use glycerin at 20%? 

    Thank you again! Really appreciate your help :)
  • PharmaPharma Member, Pharmacist
    15% ethanol plus 20% glycerol will definitively inhibit any microbial growth. Again, <20% ethanol won't kill anything either (adding detergents might however render it *cidal). I'd go with the alcohol and would most certainly prefer such a blend over the salt version.
  • @Pharma

    Thank you so much for your help, Pharma! It is greatly appreciated! 

    I'm sorry for bothering you, but may I please ask you this last question:

    There is a product that preserves their mouthwash using only essential oils. They are a reputable company and have been around for over 2 decades. 

    Here is their ingredient list: 

    Deionized water, vegetable glycerine, extracts of echinacea angustofolia, echinacea purpurea, gotu kola, pure essential oils of peppermint, red thyme, cinnamon bark, eucalyptus globulus and lavender, plant saponins.

    How are they able to preserve using essential oils? All the chemists on here say that essential oils do not and cannot work.

    Just want to hear your thoughts on this. 

    Thank you again so much for your help!
  • Cafe33Cafe33 Member
    edited July 2020
    Well, while doing much research on cleaning products which are recommended for use with COVID, I came across a few using Thymol as an active ingredient at around 0.23%. Not sure if it is permitted in mouthwash but perhaps this is the "red thyme" they are using.  It might be worth looking into it. Look at the Thymox brand. 

  • @Cafe33 Thank you! Appreciate your help :)
  • PharmaPharma Member, Pharmacist
    Thyme, cinnamon, and lavender is a blend known for its fairly broad spectrum bactericidal activity ;) . Also, saponins show some activity and are likely to boost antimicrobial activity.
  • Yes pharma I am thinking the plant saponins being natural surfactants will boost preservation?! Also eucalyptus oil?!
    Dr. Catherine Pratt
    (B.Sc with HONS I , Ph.D Analytical/Organic Chem and Microbiology), Cosmetic Chemistry IPCS)
  • @Pharma @Dr Catherine Pratt

    Thank you both so much!

    If you don’t mind me asking just one more question:

    would adding sodium bicarbonate and aloe Vera make it impossible to preserve using the essential oils listed above? 

    I apologize for all the questions. Thank you again!
  • To be honest I don’t just use essential oils to preserve products. If you want an organic preservative there is one called Leucidal which is basically fermented radishes nothing scary!! Check it out!!
    good luck!!
    Dr. Catherine Pratt
    (B.Sc with HONS I , Ph.D Analytical/Organic Chem and Microbiology), Cosmetic Chemistry IPCS)
  • PharmaPharma Member, Pharmacist
    With the alcohol and glycerol inside, no worries. Without... you'd have to run some tests to see.
    There's a huge difference between chemovars. Example: lemon thyme won't really work and lavender EO from wild L. angustifolia is more active than lavendine EO from hybrid species.
  • Cafe33Cafe33 Member
    What about using pure thymol and then adding a few EO's as claim ingredients?
  • PharmaPharma Member, Pharmacist
    That sounds as bad as it may taste... dunno... salt or thymol... meh...
    Besides, thymol and other essential oils are oil soluble and need something which brings them into solution and alcohol fits the bill.
  • @Pharma ;
    Thank you for your help! Really appreciate it!

    I wonder how low we can go with alcohol if there is already a 20% glycerin concentration. 
  • PharmaPharma Member, Pharmacist
    As an educated guess, 10% ethanol is likely the lower limit.
  • @Pharma
    thank you for your help!
  • One last thing, sorry! I know I said that multiple times now :(

    When calculating glycerin in the formula, you must count it by volume and not by mass correct? 

    By mass we are using 25% glycerin, but by volume it’s only 20%.

    Just want to confirm. 

    Thank you!
  • No you can weigh it as well, just because it is a liquid doesn’t mean it cannot be weighed.
    Dr. Catherine Pratt
    (B.Sc with HONS I , Ph.D Analytical/Organic Chem and Microbiology), Cosmetic Chemistry IPCS)
  • @Dr Catherine Pratt

    Thank you. However, it was my understanding that when formulating everything must be measured by volume because every liquid/solid has a different density. 

    I just wanted to confirm if this is true. 
  • PharmaPharma Member, Pharmacist
    We're talking rules of thumb and educated guesses here ;) . Hence, it doesn't really matter, the more you add the better.
    If you want it precise, I refer to mass-% since this is the measure usually used. Density changes with temperature and changes with dilution whereas a kg weighs always 1'000 g.
    Vol-% is something traditionally used for alcohol. This, however, is an exception and for most applications completely antiquated. Obsolete and outdated things used for centuries have the tendency to stick around for longer than anyhow useful, just think about imperial units. Even NASA uses these though rounding errors and unit mix-ups caused more than one issue in the past... at least they now also use the metric system and SI units. Guess learning it the hard way means loss of years of work and hundreds of millions of $$ (sometimes literally) going up in flames (CLICK).
  • @Pharma

    Your responses are so detailed and helpful! Thank you!

    I am concerned about mass vs volume measurements for alcohol because they say 20% is usually enough to be used a preservative and want to make sure they are referring to the volume weight because the difference is significant between the mass and volume of alcohol. 

    I guess we will try the 25% glycerin by mass and about 12-15% ethanol by volume. 

    Hopefully this is enough to pass the stability testing.

    Thanks again!!!
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