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Life at the interface – Preservative Tales

July 21, 2015

I don’t know why I feel like writing about preservatives tonight but I do so here’s another one.

One of the many reasons why a ‘one size fits all’ approach to preservative selection is foolish for a company is solubility.  Most cosmetic brands have products that range from all oil through to all water with most products fitting in the middle (creams, lotions, spritzers, serums etc).   The preservative is put into a product to keep microbes at bay thus keeping the product fresh and clean throughout its shelf life.  In order for it to do that it has to be where the action is likely to occur so that it can POUNCE into action like a cat waiting around the corner of the door to grab your leg………….

This ‘pounce readiness’ is linked to solubility.

The place in your formula where you are most vulnerable to micro attack is at the interface.

An interface is the meeting of two surfaces:

1) Air and product.

2) Product and container.

3) Product and finger.

4) Oil phase and water phase.

Point 1 is often addressed by choosing good quality packaging that minimises the time the product is exposed to the air (if the product is high risk)

Point 2 is addressed by having clean containers and a clean product.

Point 3 is actually linked to the volatility (chemistry) of the preservative.  This is where essential oils can help but more typically we use things like phenoxyethanol (or phenethylalcohol in organics) to do this job and protect our head space.

Leaving Point 4 to solubility.

The interface between the oil and water phases of a product are where microbes are served up everything they need to survive. Oxygen to breathe from the water,  water to drink and food to eat in the way of tasty organic (as in carbon rich) vegetable oils.  Many a formulation has failed a PET because the preservative was present but sleeping in the oil phase or too damn comfortable swimming in the water half of the formula.

Preservative scenarios 2

The ideal preservative for a water and oil containing formula sits nicely at the interface there like a security guard ready to fight and defend.

And that is just one reason why preservatives can mess with your formula so much, crash viscosity and break emulsions – they want to sit with your emulsifiers……

Testing for emulsifier/ preservative compatibility is just one of the things us cosmetic chemists do early on in our new product development work.  Testing for preservative efficacy is something that comes a little later, before a product launches but once we are relatively happy with the formula.

I drew you a picture so you didn’t have to just read my waffle.  Sorry that I put one too many ‘e’s’ in interface – I got a bit carried away.

Anyway, hopefully you can now see a little more clearly that preservative solubility is actually quite important.  I tend to go for preservatives that like oil and water a bit when I make a cream and go for preservatives that favour water when I make a spritz.  However, as with all things preservative wise the only way you can tell if things are working is to test and while that does require a slight investment of dollars it is well worth it!

Yay for cosmetic science.
Amanda x

PS: You can be taught this and more exciting stuff about Cosmetic Chemistry by me via New Directions where I run the Cosmetic Chemistry workshops.   Please enquire via the NDA website!

6 Comments leave one →
  1. Jodi permalink
    July 22, 2015 3:39 am


    Great drawing of preservative scenarios – it really helps to visualize what is going on. And, I love NDA’s EO’s and customer service but why don’t the recipes contain preservatives? E.g., this eczema cream:

    Not including preservatives is irresponsible and may harm someone. As you say, Amanda, unless the product is tested we really don’t know if it is contaminated. So how can we know that 2 weeks in the refrigerator is safe? (This is what Sylvia G. is recommending in the recipe posted on the NDA site.)


    • RealizeBeautyEd permalink*
      July 22, 2015 8:51 am

      NDA has many recipes on the website that are old and were put together by past staff members with varying backgrounds. The decision so far has been to keep them all and just add new ones so that is why there are some recipes that don’t fit with what I’m saying here. My advice is to always use your common sense with recipes like that and customise them based on best principals.

  2. July 22, 2015 5:38 pm

    HI Amanda – I appreciate what you are saying, and I am well aware that you personally have a lot of common sense and are familiar with best principals in cosmetics. However, I would argue that many NDA customers are nowhere near as aware of these principals as those who operate within the more professional/corporate side of the industry, and may not appreciate sufficiently the need to properly preserve most cosmetic products. They may take the absence of a preservative as an indication that they don’t need one.

    As Jodi pointed out, given the potential for raw materials to be contaminated with micro-organisms, keeping the finished product in a fridge does not remove the risk to the user. As I am sure you are aware, it doesn’t take many Pseudomonas aeruginosa cells to cause permanent blindness, and other potential contaminant organisms can cause skin problems, especially if applied to broken or damaged skin.

    Perhaps it would be better to add “preservative q.s.” to these formulations? I don’t like that phrase, but I think it’s better than nothing.

    • RealizeBeautyEd permalink*
      July 22, 2015 5:41 pm

      I get it, I do but I really didn’t want to be dragged into a conversation about a NDA formula on my blog as I am not responsible for the decisions to keep old formulations up on the website and am not being told to go through each one and change them. I would rather the old formulations come down but at the end of the day it is not my business but I totally support your point and will bring it up again!

      • July 22, 2015 6:00 pm

        Thanks Amanda – sorry, but I thought you had some say in that decision. I understand your position! I’ll go away now! 🙂

      • RealizeBeautyEd permalink*
        July 22, 2015 6:02 pm

        🙂 NO it helps me to know that it is causing problems as it makes my argument for taking them down a bit stronger. I just don’t want people to think that I am the one who makes these decisions at NDA, I am just one voice in a team. I appreciate the interest.

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