A scrub - emulsified or not - is an anhydrous or non-water containing product. We don't normally preserve anhydrous products as beasties need water to grow. A scrub, however, will come into contact with water as we dip our hands into it when in the shower or at the sink - no matter how much you think you've dried your hands, you'll still have some beading down your arm when you're in the tub! - so it needs a preservative to prevent anything in the water from contaminating the product. Which kind of preservative? It's an anhydrous product, so we would need an oil soluble preservative so it'll mix in, but the water that is introduced into the product is water soluble, so we would need something water soluble.
Here's the argument from the Making Skincare Facebook page (which I copied from a public forum):
If water may be introduced to the product or the product used in a humid bathroom then a preservative is advisable. An expert microbiologist advises that if trying to preserve an anhydrous product (including all oil+sugar/salt scrub) the oil soluble preservative will get locked in the oils so will not reach any water, if water was introduced into the product. So if you added an oil soluble preservative then that preservative will stay in the oils and not move over to where the water is located to protect that water against bacteria and mould so would be useless. So contrary to what you may have read, we should really use a water soluble preservative in an anhydrous product which means we’d need to add an emulsifier to get that preservative mixed in properly with the oils.What does this argument mean to us? The argument is that we should be using a water soluble preservative with an emulsifier in an anhydrous scrub to ensure the preservative is not locked in the oils. I've read that the water soluble preservative should be mixed with an emulsifier to make it more available for the water that might enter the scrub.
Having said this, we are generally talking about emulsified scrubs on this blog, which means there's already an emulsifier in the mix. You wouldn't need to add another emulsifier to the mix.
There are many arguments for and against this position. Liz on the Dish forum wrote this...
Seems possible, if concerned use a preservative blend that is water soluble and oil soluble. Or use a water soluble one and add an emulsifier. However if you have an emulsifier in the mix anyway, I don't see why an oil soluble preservative wouldn't work and even better. I mean you already have it fully soluble in the oils ready to go with the emulsifier if water is introduced, unlike the water soluble preservative as it's not soluble in the formula until water is introduced. This seems to me we'd get highly concentrated spurts of preservative instead of a homogenous mixture. Anyone else?
Phenonip is one of the standards in the industry as the go to preservative for anhydrous products, even though it's not highly water soluble it does have some water solubility. But again the question of, "Is the preservative available?"...is a good one, but we wouldn't know for sure unless we test for either type.My guru from the Dish forum, LabRat, suggested that for an emulsified scrub, "I would use 0.3% to 0.4% Phenonip. Phenonip contains Phenoxyethanol (and) Methylparaben (and) Ethylparaben (and) Butylparaben (and) Propylparaben (and) Isobutylparaben). All of these ingredients are soluble in oil." He repeats this suggestion again and again when asked what preservative should be used in an anhydrous or emulsified scrub. He also suggested, "I would use 0.3% to 0.4% Phenonip, LiquaPar PE or LiquaPar Optima."
So to sum it all up, when it comes to preserving scrubs, we have the following positions...
1. We don't need to preserve a salt or sugar scrub because the salt or sugar bind the water so there's no chance of contamination.
2. We need to preserve an anhydrous scrub with a water soluble preservative combined with an emulsifier so it is more available to the water when it enters the scrub when we use it.
3. We need to preserve an anhydrous scrub with an oil soluble preservative.
4. We can preserve an emulsified scrub with an oil soluble preservative because it's emulsified by the emulsifier, like Polawax or Incoquat BTMS-50.
5. We need to preserve an emulsified scrub with a water soluble preservative.
Phenonip, a preservative that is both oil and water soluble, for eight years, and it appears* to be working. I've read that because of these varying levels of solubility, a paraben mix would be a good preserver of oil based scrubs, so I feel pretty happy with my choice.
*I say "appears" because I've never had any growth or visible contamination in any of my emulsified scrubs. I have not had my products challenged tested as I can't afford it, although it would be very interesting to see the results!
I've said in the past that I wouldn't use Germaben II in a sugar scrub. Have I changed my mind? Kinda...sorta...maybe? I don't think I have, although I'm always open to learning more. The parabens we find in this preservative are less oil soluble than the combination we find in Phenonip, and I haven't seen information that says that Germaben II is a better choice than Phenonip in an emulsified scrub yet.
I've written this post to open up the debate and encourage a discussion. If you have some thoughts, please share them. If you have some links, please include them. I think this is a great topic and all opinions are welcome. (As usual, be nice!)