Preservation is a tricky business. All beauty products that contain a large amount of water need to be preserved and all preservatives have some kind of drawback. Potassium sorbate has been used for a long time but has recently been very widely used in beauty products that have some kind of natural story about them. How natural is it?
Potassium sorbate is the potassium salt of sorbic acid. Sorbic acid gets its name from the latin name for the rowan tree, and sorbic acid was first isolated from rowan berries. S0 that is a nice story. Is the potassium sorbate in the product on your bathroom shelf produced by rosy cheeked peasants harvesting rowan berries and extracting it in their cottages? Not really.
Both potassium sorbate and sorbic acid are used in vast quantities in the food industry. They are related, distantly, to sugars. In nature most sugar is burnt for energy but some gets used to make other useful bits and pieces. For instance. the sorbic acid in rowan berries. I assume in the berries it has some kind of preservative activity.
Source of Potassium Sorbate
It wouldn’t be too difficult to make potassium sorbate from sugar if you put your mind to it. With the interest from the natural products sector I am a bit surprised nobody has done exactly that yet. Sorbic acid is not a million miles away from citric acid in basic structure, and citric acid is made from sugar cheaply and efficiently. But as far as a I know all the commercially available potassium sorbate comes from petrochemical sources.
A molecule of potassium sorbate from any source is identical once it is in beauty products, so it’s origin makes no difference to how useful it is in a formulation. It works pretty well as a preservative. In particular it is good at knocking out fungi which are the bane of the cosmetic formulator’s life. Bacteria are less susceptible so you usually need to use another preservative as well. You also have to watch out for interactions with other components of the formulation, so it isn’t an easy option. It is well tolerated by most people. There are a few people out there who get mild skin reactions to it. But is very much dependent on the nature of the formulation, so it might well be the case that some products with it will irritate, while others don’t.
Potassium Sorbate Safety
Sorbate is a part of the body’s metabolism. So if any gets through the skin it is dealt with quickly and easily – as you would expect given that it is widely used as a food additive. Not only your body, but micro-organisms are quite capable of dealing with potassium sorbate. Its potential for accumulation either in the body or in the environment is therefore pretty low.
The downside is that it isn’t all that stable in formulations either. Personally I am fairly relaxed about that. It breaks down slowly enough to give a pretty impressive shelf life. I have tested a five year old cream preserved with it and it has still been fine. But it has to be said that the parabens, which have rock solid stability, do last a lot longer. If you find a really old cosmetic product in your cupboard you should throw it away. If you can’t bring yourself to and are in two minds, if it has parabens listed on the ingredient list it will probably be fine. If it has potassium sorbate, maybe not.
So there it is. Its a safe, useful preservative for personal care and beauty products. Not as green as some people might suggest but it does the job. Most importantly, in a world where we we have more and more people living on the same sized planet, its easy biodegradability is a great plus point and one that should commend it to consumers and cosmetic chemists alike.
Notes on Potassium Sorbate for Chemists
It has a couple of CAS numbers: 24634-61-5 and 590-00-1. It is approved everywhere but there are some use restrictions. The maximum in the EU is 0.8%, but that is way more than you are ever likely to need. The EU doesn’t have much to say about potassium sorbate.
Potassium sorbate will stand you in good stead if you get to know it. In theory it is only effective at low pHs but can contribute to the overall microbiological quality even at higher pHs. Remember that that it is the salt of a weak acid and so tends to have a buffering effect. In particular watch out for it contributing to neutralisation of carbomers.
In my experience a reasonable oil level in a formulation will mean hardly any skin reactions reported. In straight aqueous formulations you stand a very good chance of somebody somewhere complaining that it has turned their skin red. This is so rare that you probably won’t pick it up in pre-market testing. So I wouldn’t recommend it for use in shampoos, gels and the like. Having said that it does depend a lot on the formulation’s particular characteristics so I don’t propose that as a hard and fast rule.
The Cosmetic Ingredient Review (CIR) for sorbic acid says that it should be safe at a level of 10% in the diet. So there is a pretty wide safety margin if you use it in a cosmetic product. The Environmental Working Group’s Skin Deep database gives it a hazard rating of 3 out of 10 for no obvious reason. As their credibility as experts on cosmetic ingredient safety is just below that of the Smurfs, I suggest we ignore their opinion.
Other than the Environmental Working Group who few take seriously, scaremongers have not picked up on this one yet. Indeed it ought to appeal to people concerned about the environment. In theory it should be possible to produce sustainable grades in the future as well. Lets hope they become available soon.