Skip to content

Preservative Adventures with Sodium Levulinate and Sodium Anisate

October 2, 2016

Sometimes things don’t always work out as you expect. This is a story about that.

One of my favourite natural preservatives at the moment is a blend of glycerine with Sodium Levuninate and Sodium Anisate.  I love it because it is clear, colourless, odourless and high performing.  I’ve put a few formulations through Preservative Efficacy Testing with this preservative and had them pass – creams, spritz, cleansers (gel and emulsion based) and masks – so my faith in this little beauty is grounded in science which re-assures me.

So what’s odd then?

Well it turns out that I forgot to adjust the pH down on one of the products I just got results back for so the resting pH was 6.15 instead of the pH 5 that I wanted it to be.  I usually adjust the pH of my formulations containing this ingredient because the manufacturer says that the product works best in acidic conditions.  It’s not that pH 6.15 isn’t acidic – anything less than pH 7 is acidic – more that the pKa value of Levulinic Acid is 4.61 so at pH 6.15 it would be almost 100% salt, not acid and I’ve been told it is the acid that has the preservative action rather than the salt.

Aside disk of pKa. 

All of the acids we use in cosmetic science are ‘weak’ acids and as such exist in a mixture of their salt and their acid form. The point at which both acid and salt forms are in balance is acids pKa value. Above the pKa (more alkali) the acid will exist mainly as the salt and below, mainly as the acid. At the pKa it’s 50:50.  A change in pH of 1 jumps the ratio up very quickly, so that around 90% of the ingredient is in its acid or salt form depending on if it jumps up or down.  So for Levulinic Acid a pH of 5.61 means 90% Salt whereas a pH of 3.61 is 90% Acid.

The PKa value is a constant and each acid has one, some have more (such as citric acid).

And back to the oddness of what’s going on in my formula

But then again, the manufacturer does say that the Sodium Levulinate/ Levulinic Acid can work between pH 4-6 while the p-Anisic Acid/ Sodium Anisate work between pH 4-6.5.

So maybe pushing a pH of around 6 isn’t so bad after all.

Especially given that my  pH 6.15 product passed PET. 

On another note some stability testing I’ve been doing on a formula using this preservative have also showed the pH to rise from 6.15 – 6.45 over 16 weeks in the oven (equivalent to around 15 months real-time) and that has been playing on my mind – are these products now un-preserved? I need to get them micro-tested to check….

So I had a look into the science and found that Sodium Levulinate is commonly used as a meat preservative.  In this patent here you see the pH of the samples tested are between 5.8 and 6.23 and the preservative worked well in all cases in spite of the fact that even at pH 5.8 we have only around 55% as Levulinic Acid.  This paper talks about Sodium Levulinate being the preservative rather than levulinic acid and this paper found that Levulinic Acid produced disappointing results when compared with Lactic or Acetic acid for meat washing – a bit of a web search on google scholar confirmed my suspicions that it is the Sodium Levulinate that is used more than the Levulinic acid.

There is another reason why I’m insanely interested in this is solubility. 

At pH below 5.5 the above blend does indeed switch from its salts to its acids, you can see that visibly as while the salts have good water solubility the acids do not. As the pH reduces the preservative starts to form little white crystals if it is in a water-dominant formula, this wouldn’t be such a problem in an emulsion as the acids would usually find something else to dissolve or disperse into but if your product is predominantly water there will be trouble!

From what I’ve looked at, got results on (PET) and seen with my own eyes a pH of 6 in a water-dominant product is probably fine and will allow for the use of this preservative without fear of the preservative coming out of solution.  Lower the pH and you will have a problem, not necessarily with preservation but with product clarity.

But what about the rise in pH over time?

I suspect that what is happening, and especially in the presence of a catalyst such as heat, is that there is a shift towards the salt formation over the acid form. I suspect this because the starting pH of the preservative is between 7-8.5 (mine was pH 8 @ 18C) and the ability of the reaction to go both ways.  Whether this impacts the long-term micro stability of the product is yet to be seen (or measured). The only thing I can say is there must be a reason the manufacturer gives a cut-off pH of 6.5. But what is I wonder?

Micro contamination typically reduces the pH of the product as the microbes produce CO2 as a bi-product of respiration.  This CO2 acidifies the environment which could, at least in theory, push the equilibrium back towards the acid form (for the preservative) and therefore more within the target range of action. That said, one wouldn’t want to wait for micro contamination to bring the pH down, a buffer solution would be a better idea!

Overall  I think  there is enough evidence to say that the salt form of both actives are effective preservatives and that the pKa values of these ingredients is, for whatever reason of minor importance for this application.  Ignoring the pKa values is something I’m fairly happy to do but even though it makes no logical sense given what I’ve said above to stick to the manufacturers guidelines of max pH 6.5 I am lacking the real-time evidence at this point to back that up.   What I would say though is that in a cream formulation a very water-soluble preservative would be un-desirable as it would leave the interface between the oil and water vulnerable.  So, it would make logical sense for the pH to be lower to reduce the water solubility (by increasing the percentage of the preservative that is in the acid form) and thus keep the preservative at the interface.  This is irrelevant in a water-only or water-dominant formulation where the contamination risk is predominantly in the water phase.

Picture below:

At low pH (below 5.5) the salts in this blend change to their acidic form. The acids have low water solubility as we can see here with the precipitate that has formed.  This is a problem in a water-dominant formula but is less so in an emulsion as the acids can better disperse in a multi-phase system. 

levulinic-acid-and-anisic-acid

So yet again something that I took for granted as being simple has potentially turned out to be quite different indeed.

I might well try the preservative at pH 8.5 in a water-dominant formula to test this theory (I know that one sample doesn’t constitute a good scientific trial but it’s a start).  I could also try a cream formula at pH 7 to see how that fairs (my other creams have been at pH 4.5 and pH 5.2 and passed).

But for now that’s that.

Thinking about stuff is always exciting and very interesting wouldn’t you agree?

 

12 Comments leave one →
  1. Christopher permalink
    November 9, 2016 3:49 am

    Very interesting! Thanks for sharing.

  2. Carmella permalink
    March 3, 2017 3:11 pm

    Hi! I loved reading this! Thanks for putting this out there. I found one supplier of this system saying it is good up to PH 5.5. I see you use it higher.. if you don’t mind me asking, what is the commercial system you buy? I found it as dermasoft. Thanks!!

    • RealizeBeautyEd permalink*
      March 3, 2017 3:22 pm

      Hi there, I buy dermosoft also. While I’m not doubting the information given by the supplier I used to create supplier date as part of my job and realise that they don’t always have the time or opportunity or reason to test the boundaries of their ingredients further. As a consultant that is my job to both find the rationale and test for differences. I have no reason to suspect that this preservative can’t work at slightly higher pH in some formulations and have pushed that boundary with good results (micro pass) but I can’t guarantee that this stands for all formulations. A supplier will give the information in a conservative way so as to better guarantee satisfaction so don’t be surprised if ingredients don’t work above and beyond what the supplier suggests. My only caveat on that is to be prepared for failures and to be careful to test everything thoroughly if you are like me and like to take on these projects 🙂

  3. Carmella permalink
    March 7, 2017 10:47 am

    Thanks for writing this! So for a water based formula like a serum,would you recommend a formula with a 5.5 PH? Or 5.0 to account for fje upward drift? I see the manufacturer recommends max PH of 5.5. If I formulate at 5.5 it may drift over that and be above their recommended range??

  4. RealizeBeautyEd permalink*
    March 7, 2017 1:08 pm

    Hi Carmella, why are you expecting the pH of your product to drift upwards? That isn’t normal. Otherwise in answer to your question I would suggest that it will depend on your formula. What I’m talking about here is the issue specific to creating a low oil phase, crystal clear formula where a low pH pushes the preservative to become slightly insoluble. If you have a larger oil phase and are not so fussed on a crystal clear product you can push the pH down a bit and that will be fine. The supplier data does hold true for an emulsion as in an emulsion the best place for the preservative is at the interface which is achieved better when the product is in a more acidic form. It just isn’t so relevant for a low oil phase spritz.

  5. Carmella permalink
    March 7, 2017 2:01 pm

    Thanks so much for the reply. I am thinking of using this in a mask or toner with NO oil phase and was worried about the insolubility issue you observed at low ph. Do you remember the exact ph where that occurred? I am going to do some similar tests over here soon! Thanks again!

    • RealizeBeautyEd permalink*
      March 7, 2017 2:39 pm

      It says in the article – below pH 5.5 the preservative favours its acidic form and above that the salt. Both have anti-microbial properties but as the salt form has a higher water solubility it can ignore the oil phase if the pH is too high and that would be a disaster in an emulsion. With regard to pH shifting this can happen as I’ve experienced but it is always a problem and should be addressed. I should have stressed that pH drift over time is not normal or acceptable 🙂

      • Carmella permalink
        March 7, 2017 3:00 pm

        Thanks once again. Last thing, I can only find this preservative system as dermasoft 1338 and see it has a max PH of 5.5, but notice you mention the manufacturer states a max at 6.5… is this another supplier? Did you find another spec sheet? Just curious. Much thanks again and have a great night.

      • RealizeBeautyEd permalink*
        March 7, 2017 5:02 pm

        No that is the correct thing. I looked at the chemistry, its chemical reactions and worked my conclusions on there while pairing those with my PET results. I used to create specifications for a manufacturer so I kind of got where they were coming from and felt comfortable in stretching it. I wouldn’t expect other people to feel as confident in challenging supplier data though 🙂

  6. Carmella permalink
    March 7, 2017 6:30 pm

    Wow! Is there a place online where we may view some of your formulations? With this preservative or not?

    • RealizeBeautyEd permalink*
      March 7, 2017 6:47 pm

      No, that’s my private consulting work. One can’t give away everything 😉

      • Carmella permalink
        March 7, 2017 9:09 pm

        👍

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: