While it is not possible to have someone else just tell you what will definitely work and be best in your case (we always have to test these things), it is possible for us to help by explaining the chemistry and action of the preservatives we stock. It's also useful at times to see how one goes about running an experiment to test how different preservatives perform in your base. To that end, we have produced a preservative master data sheet, a spreadsheet that puts all the important data about our preservatives in one place thus making evaluating them (at least on paper) easier. Secondly, we've taken that one step further and completed an experiment exploring how a range of the preservatives we stock (all of which are natural, nature-identical or naturally derived) performed in our simple, natural cream base formula. We hope this help you build the confidence to run some experiments of your own so that you can break free from preservative phobia!
We chose our simple, tried and tested natural moisturiser cream. This is the one we use in our Cosmetic Chemistry workshop and our go-to for this type of experiment as it contains all-natural chemistry, is easy to create, cost effective and has a medium to rich texture suitable for most application types.
|A – Water Phase||Demineralised Water||To 100%||Carrier/ Solvent|
|Acacia & Xanthan Gum||0.7||Thickener/ Emulsion Stabiliser|
|B – Oil Phase||Jojoba Oil||8||Emollient, liquid wax, barrier support|
|Shea Butter||4||Emollient, barrier protection|
|Oleyl Oleate||2.5||Emollient, skin slip and active penetration|
|Emulsifying Wax Vegetable||3.5||Emulsifier, non-ionic|
|C – Cool Down||Vitamin E – Mixed Tocopherols||0.5||Antioxidant for product and skin protection|
Heat phase A to 70-80°C taking care to hydrate the gum fully. The gum can be pre-mixed into your glycerine if desired to prevent clumping.
In a separate vessel weigh and heat phase B to 70-80°C.
Combine hot phase A and B once removed from the heat. Use a propeller mixer or equivalent to mix while cooling to roll temp.
When cool add phase C to the combined phases A + B. Continue with the propeller mixer until well combined.
Finish with a homogeniser mix to ensure particle size distribution is even. Measure pH, adjust if necessary using a propeller mixer to blend in pH fluid, re-test then pack.
The table below shows which preservatives we chose for this experiment. A large batch of un-preserved cream was made and split to give equal portions of base. Each portion was finished by adding one of the preservative options as described below. Before and after adding the preservative, the pH was measured and the impact on the pH of the cream noted. The pH was then re-adjusted to around 5.5 to make comparing the impact on viscosity fair and equal.
Preservatives are notorious for destroying the viscosity and stability of cream formulations and this can be quite distressing for those starting off on their cosmetic chemistry journey. This phenomenon happens for one of two main reasons. First, the preservative may reduce the surface tension that exists between the oil and water phases, relaxing the cream too much and crashing its viscosity. In some cases, the addition of a preservative will lead to an immediate drop in viscosity followed by a fast recovery as the preservative takes up its spot at the oil:water interface but in other cases the changes are irreversible. Second, adding the preservative changes the formula pH too much and that affects the emulsion stability. We call this ‘shocking the emulsion’ and it can also happen when we add AHA's or high pH actives to a pre-emulsified base. If this is the root cause, the preservative can be added to some of the water from the water phase prior to adding it to the emulsion. This small preservative: water combo can then be pH adjusted to better match the base pH you want prior to adding it.
In this experiment, none of the preservatives crashed our viscosity and in fact, many increased it, sometimes significantly indicating that none of the chemistry tested, reduced the formula surface tension or shocked the formula too much. This may be because the emulsifier we chose is very robust or just because the chemistry of these preservative actives is very easy going.
Another thing that can go wrong in this type of testing takes a bit longer to notice. You may choose a preservative or preservative strategy that doesn't work and mould, bacteria or yeast may grow. Mould and yeasts are almost always clearly visible and can appear quite quickly. Bacterial contamination may be a bit harder to spot. In any case, the best way to get a measure on what's going on microbially is to send your samples off for microbial testing.
The simplest and cheapest way to get your products tested is via a cosmetic micro count and characterisation. This is a one-time test that takes a sample from your product and looks to see what's growing. Cosmetic products don't have to be sterile but they should be free from pathogenic microbes and this test will confirm that either way, costing less than $200 per sample.
Any third-party testing is going to come with a price tag and for clients who are really not ready to make those investments for whatever reason can sit tight and watch and measure their samples over a one to two-month time frame. As I mentioned above, mould and yeast are easy to spot, you won't miss them but to spot bacteria you will need to re-measure pH, check on the viscosity of the product (did it reduce or does it look lumpier?) and finally have your samples become bubbly / taken on an aerated look? Finally did your product change colour in a weird way (not as you would expect i.e. with patches of colour or a colour that's not typical from oxidation). While the absence of these features can't guarantee your product is clean, these are all useful things to be aware of and look out for. Just make sure you don't apply your creams to broken skin (or even at all) while you are conducting this testing, just in case!
A final common thing that can happen with preservatives in emulsions is that they slowly destabilise the cream. Preservative chemistry is, by its nature surface active and may influence emulsion stability on a subtle level that's still important and worth looking into. This may show up as a cream separating a little or becoming thinner, thicker or more acidic (due to preservative chemistry changes rather than microbes) over time. The best way to investigate this is long-term accelerated stability testing but that also costs time and money. The next best way to get at least some idea is to run freeze/ thaw tests at home in your freezer. Three to Five rounds of freezing a small sample and then letting it thaw over 24 hours per condition, evaluating visually after each round will give you at least some idea of what is going on in there. If your product starts to look worse for wear, that could be a sign it will separate over its shelf life.
All of these tests and observations are often best when you do them across a range of versions of the same product where you have changed only one component (such as the preservative as described here). That way, while you can't guarantee that the best performing option performs well enough to pass official testing, you can pick out the best performer from your trials and take that forward with some confidence, thus reducing your costing burden and chances of disappointment.
All of the samples tolerated the preservative variants well, remaining stable throughout our testing period of three months at room temperature with additional freeze/ thaw testing.
Some preservatives reduced the pH of the cream upon adding them and some increased it but none destabilised the cream in the process and were able to have the pH re-adjusted to our benchmark level of 6.07.
All of the preservatives actually increased the viscosity of the cream base with some increasing it dramatically which was a bit of a surprise. The viscosity of each sample remained stable over time and there was very little change in pH. The Plantaserv U version also contained 0.5% Sodium Citrate to help maintain the optimal pH.
Visually, none of the preservatives changed the colour of the cream either initially or over time. All samples remained off-white to cream in colour.
The final aesthetic assessment was around odour. Naticide has a distinctive odour and at 1% it is noticeable although not unpleasant in this formula base. Plantaserv M has a slightly chemical odour (almond-like, similar to Naticide but less strong) from the benzyl alcohol/ benzaldehyde chemistry and Plantaserv N has a very slight chemical note to it too although this is less noticeable in the base.
In terms of price impact, the preservative strategies explored here range from as little as $0.15 per Kg of bulk for Potassium Sorbate/ Sodium Benzoate to nearly $15 for the probiotic ingredients Leucidal and AMTCide Coconut. In terms of price, some preservatives are literally just that, there to preserve the product and stay quiet and as such, perhaps don't warrant the too much financial investment. However, some preservatives offer other benefits in your formula, potentially increasing skin hydration, modifying how the product spreads or feels, improving barrier functioning or giving far superior performance when formally tested. Further, as your preservative is likely to be one of the more irritating ingredients in your formula you may, after carrying out some further research, prefer some chemistry over others. In addition, there is your ingredient philosophy. While all of these options come from our natural range, not all meet the requirements of organic certification. In addition, some chemistry is palm derived and some is not suited to claims of 100% naturally derived.
No matter how much we say about preservatives there are always brand owners who wish to avoid their use and ask if keeping their product in a fridge or airless container will suffice.
The best way to answer that question is to show you what happened to this cream when it was a) un-preserved (left hand side) and b) under preserved with only potassium sorbate added at 0.5% (right). Both had their pH adjusted to pH 5.5 so as to give the potassium sorbate a chance as pH 6 is at the borderline of its activity.
After around 1 month at room temperature these creams had both been spoilt but in different ways with different microorganisms growing. Not being a microbiologist it's hard to tell what exactly is growing but the colour differences alone point to these two samples containing different dominant moulds or yeasts.
The sample on the right with potassium sorbate has suppressed the growth of the more orange / brown coloured organism but has since been overwhelmed by this black fluffy mould – a situation that wasn't entirely un-predictable. Potassium Sorbate is a gentle and slow acting preservative with limited range, focusing on yeast and mould control rather than bacteria. That said, it looks to my untrained eye that mould has grown on here so maybe without the back up from an antibacterial specialist, the mould easily overwhelms this chemical. It's possible.
In the complete absence of anti-microbials, the base cream let whatever was in the cream from the start grow, again a state that was to be expected given the nutrient nature of the base cream. Microbes need water, fats and a pleasant environment to thrive and a cosmetic emulsion gives them all of that and more. Microbes enter the cream base on the ingredients we use, during manufacturing, filling and when we test and use the product. There's no doubt the microbes growing in the un-preserved cream would be those that thrive on its pH, water and fat content so change those and you may well get a different result. What's unlikely is that we could change any of those things enough for this cream to not need a preservative strategy.
While this experiment has focused on preservatives (chemicals with a distinct anti-microbial action), there are different strategies that can be employed to keep a formula microbe free without using this type of chemistry. For this reason, it's often best to think of a formulation's preservative strategy rather than just its preservatives.
Preservative strategies can include modifying the products pH, choosing more protective packaging, adding chelating agents such as EDTA, Sodium Phytate, Hydroxyacetophenone or other things, decreasing the formulations free water content by adding glycols such as Pentylene Glycol (Hydrolite 5) to the mix or formulating with self-preserving materials such as salt, sugar and alcohol at high levels. As a chemist's formulating knowledge and confidence increases its possible to explore many of these strategies to create preservative-free yet well preserved products but just as we have done here, all of these strategies impact the whole formula and require careful testing and evaluating.
We hope you've found this experimental report helpful and that it helps you to design your own experiments and tests. The take-home message is that there is a lot you can do at home to trail and test different preservatives and preservative strategies with very little financial investment but you do need to have a plan and you do need to carry out your experiments carefully and properly. Take your time, change one thing about your formula at a time, note down your observations and finish by taking your best performing versions forward for full professional testing. That's a sure-fire recipe for success!