How Do I Know Which Preservative to Choose?

One of the most frequent questions we get via the technical help desk is with regard to product preservation and more specifically which, out of our range is the best preservative to use. While the question seems reasonable, simple and relatively easy to answer the reality is somewhat different as we will now discover.

Product preservation is complex and not something that you can find a 'one size fits all' answer to. Take a look at a range of cosmetics by a multinational and you might be surprised to find that they rarely use just one preservative (or even one common blend). In my time working with multinational brands I've seen many a new product development cycle stall while the optimal preservative system is worked out with the help of microbiological testing, stability and irritation studies. Done properly, the micro testing alone for new product development can run into the thousands of dollars, something that the small brand owner can rarely afford. However, there are things that the small brand owner and hobbyist can do to increase the chance of their cosmetic formulation remaining safely preserved and for a start, it is all about the base...

The Base Formulation

Physical Space:

Tempting as it is to think of preservation and the base formula as separate entity's this mindset should be discarded and instead we should think of the preservative as an integral piece of the formulation puzzle. Preservatives work within the formula, often sitting at the interface of the oil and water phase (where the emulsifier sits) which is great as this is where the formula is most vulnerable. Vulnerability at the oil/ water interface is due to a number of factors not least that the oils, being exposed to water can start to oxidise. In addition to that the water oil combination provides microbes with the ideal environment in which to thrive. With this in mind it isn't hard to see why your choice in preservative (in an oil and water containing formula) plays a crucial role in formulation stability - get the wrong preservative and it could compete with your emulsifier or solubiliser, pushing the phases apart and leading to a complete melt down! To avoid disappointment we advise trying out a number of different preservatives in your formulation base and carrying out basic stability (Freeze/ thaw as a minimum) to rule out any preservative/ emulsifier/ solubiliser incompatibility issues - don't leave choosing your preservative to the last minute (an afterthought) and don't assume that when you change preservatives your formula will remain the same. It won't.

Acid/ Alkali Balance:

Another formula base issue to consider is to do with pH both during and after manufacturing. You might be aware of active ingredients that are pH sensitive - AHA's, Peptides, Amino Acid, Milk Extracts, Silk etc. - preservatives are no different. Preservatives such as Sodium Phytate, P Anisic Acid, Sodium Benzoate, Potassium Sorbate and our Plantaserv D are all pH sensitive, working best at acidic pH of below 5.5. During manufacture (especially of surfactant containing products such as shampoo and cleansers) pH of the formula can change quite dramatically from very alkali (pH 9-10) to more acidic once the pH has been adjusted. Depending on the manufacturing time and preservative choice this could either be un-remarkable or have devastating consequences on the products micro stability. How to solve this? Know your preservative - is it pH sensitive? What pH range are you operating at? What pH are you adjusting to? How long is the preservative exposed to sub-optimal conditions? Understanding this ahead of time can help you prevent (or at least understand) unexpected micro failures.

Temperature:

Temperature stability is another thing that can really creep up and bite you when you are not looking if you don't consider it. Some preservatives such as phenoxyethanol and phenethyl alcohol are pretty heat stable and can be added into a formula hot or cold while others such as Plantaserv D, Plantaserv M and the p Anisic Acid are less heat-robust and could go from hero to zero after a bit of boiling and are therefore best added cold (or at least below 50C). To avoid disappointment make sure you consider your method of manufacture and temperature requirements before selecting a preservative. In some situations it is prudent to use two preservatives - one heat stable for the hot phase (which may stay hot for a good few hours) and one for the cool down phase to give it an in-use micro boost.

Any Other Additives:

To add an extra layer of complexity to choosing a preservative it is also important to know if your formula contains any additives that can de-activate a preservative. It is not uncommon for preservatives to be de-activated (or at least have their activity reduced) by surfactants, especially anionic or cationic ones - benzalkonium chloride, cetrimonium chloride and benzyl alcohol are examples. In addition to that additives such as colours, metal ions (zinc oxide or titanium dioxide) and biological actives such as peptides can also cause issues and even de-activate the preservation system. A quick look at the preservatives chemistry can give clues to how compatible it will be with a particular base and that is something we can help with although as with all preservative questions the only sure fire way to know what's going on is to run micro testing.

Formulation Philosophy:

Once we have considered all of the above it is possible to be left with a reduced set of preservatives to formulate with. The next step is to consider the formulation philosophy - will you be working towards an organic certification, want the formula to be all natural, cost effective, innovative or even 'preservative free'? New Directions stock a range of preservatives to fit a variety of philosophies including palm free and organically certified.

Price Point:

Last but not least is price. While nobody wants to be dictated to by money, especially when being creative the reality is that for the vast majority of us cost prices matter, formula price per Kg matters a lot and distribution chain price structure often leave little room to play with. The difference in price per Kg between our natural and synthetic preservatives is quite dramatic and can make a big difference in your final product price. It is really important to factor the price of your preservative into your costings as soon as possible to ensure you can afford to tick all of the boxes you desire!

So how do you work out which is the best formulation for your product?

We have tried to lay all things on the table here, not to scare you or to make it even more impossible to choose but to help you move forward with the best possible solution, so that you avoid making manufacturing errors (either in temperature, pH or ingredient incompatibilities) that might render your carefully chosen preservative disabled. Not every formulation will require such stringent consideration; some contain no remarkable ingredients, have a low free water content, can accommodate powerful, synthetic preservatives and naturally sit at an agreeable pH but this isn't true for everything and if you are wanting to make safe, effective preservatives then having an appreciation for all of the above is an essential first step.

Our Thursday Ingredient Help Desk is the place to come and talk through your preservative concerns and failing that we have our email support, Facebook page and Thursday phone hotline. However, we hope that you now have some appreciation for the complexity of the preservative dilemma and won't be too disappointed if we tell you that the only clear cut way to know if your formula is adequately preserved is to test it.

And to think that we haven't even discussed packaging, GMP or Leave-on/ Rinse off considerations yet. Oh My Goodness!

Amanda Foxon-Hill

27 October 2014