There is a lot written on the subject of ‘preserving’ anhydrous products and as a result there is much confusion. In the hope that we can dispel some myths, here are some basic guidelines.
Firstly, anhydrous covers many types of products as it simply means ‘without water’ and where they also contain no oil, such as a dry face mask where water is only introduced right before use, there is no need for any kind of preservative and as such this is a great way to make and pack certain products which can be preservative-free.
A preservative is generally only required where there is water present, so when a product is made with water and the product is not, for instance, a soap with a pH of 9 or higher (they are self-preserving) an effective preservative is required to prevent the growth of bacteria and other dangerous substances such as yeasts and moulds, and often the product would also require some form of challenge testing to ensure the preservative is effective as part of the CPSR process.
When considering the need to ‘preserve’ your product, one of the most common confusions is the difference between an antioxidant and a preservative, so to be clear, an antioxidant such as Vitamin E, Grapefruit Seed Extract or Rosemary Extract which are common additions to help add longevity to certain types of products are NOT preservatives and therefore are generally NOT suitable on their own to preserve any product that contains water.
Antioxidants are substances that prevent or reduce oxidisation and that is a problem common to oil-based products such as anhydrous scrubs, balms and body butters, as the oils and butters they are made from will ‘oxidise’ over time with this deterioration being known as rancidity.
It is common practice and effective to add an antioxidant to any of these types of product to help extend their useful life, but because the product does not contain water there is no need for a preservative. The confusion here is when a product might come into regular contact with water in use and as such the water might become a contaminant, becoming a source for bacterial growth within that product.
The most common product where this happens would be any kind of oil-based product which is designed to be used in the shower or bath and where its packaging allows water to enter in normal use with the most common ones being salt or sugar scrubs, although many of these scrubs may also contain other additions such as clays or botanicals, so to help you decide how to effectively preserve these products to ensure they remain safe in use here are some guidelines….
Preserving Salt or Sugar Scrubs
Where a scrub is ONLY made from oils and has a high percentage addition of either salt or sugar without any further additions they are considered to be self-preserving with no need to add a separate preservative. They will benefit from an antioxidant to extend the life of the oils but the addition of a further preservative is not generally considered necessary.
There is no easy way to tell if the amount of salt / sugar used will be effective at preserving though, so we would always advise to include a warning label saying ‘avoid introduction of water during use.’ Scrubs that have botanical exfoliants / additions or even something like pumice or clay have no "barrier" to microbial growth because these moisture-loving additions are within the oils and therefore these products should be preserved and even challenge tested in some cases as the clay / pumice / botanical may interfere with preservative function. “All very well” you might say, “and my scrub only contains oil and salt but I’d like my scrub to have a preservative in it because it is used in the shower and gets regular water contamination, so what should I do?” Well….
There are two main recommended approaches that are both valid in these cases, so basically the choice is yours, although there may be opinions on the better approach and much depends on the choice of preservative used. Firstly, add an oil-soluble preservative… Now there are opinions that suggest this is not fully effective because the preservative is trapped in the oils and therefore cannot be effective in the water contamination, but the oil itself provides a physical barrier to the microbes circulating / penetrating the product. The interface between oil and water is the zone where microbes are likely to "access" food - i.e. the oils.
If the oils contain a preservative, then this should effectively stop the microbes from feeding on the oil. Most oil based preservatives will actually dissolve slightly into water at the oil/water interface anyway, so this argument would suggest this is an effective approach. Secondly, add a water-soluble preservative… Adding a water-soluble preservative to oils requires an additional ‘emulsifier’ so although this approach makes sense in many ways, the presence of the emulsifier also makes the water more ‘available’ to microbes because it effectively removes the natural barrier that exists between oil and water, so this is the flaw in this approach and although the preservative may well be effective, the presence of the emulsifier changes the product into one which will more readily emulsify with the water contaminant and that may not be considered a desirable effect.
So… either approach is valid but might be questionable for different reasons, so what IS the best route to preserve anhydrous scrubs? The best thing to do is use a preservative that is 2 part - with both water and oil solubility characteristics. Phenoxyethanol / ethylhexylglycerin(Phenoxyethanol EHG) is an example of a widely used and effective choice, but any good preservative which can disperse in both oils AND water would be effective in this approach.
The obvious advantage of this approach is that there is no need to add an emulsifier which as described above, would effectively break down the natural physical barrier which exists between oil and water AND this type of preservative can more effectively dissolve out of the oils into the water contamination, providing more effective protection against the growth of bacteria and moulds in this contaminant.
So… we hope this brief article will help you choose the right approach to preserve your anhydrous oil-based product.
Should you choose the approach where a water-soluble preservative and emulsifier is added the most obvious choice for the emulsifier would be a polysorbate, although others exist.