I've done a bit of homework on this, and the answer seems to be no... From what I understand, the key measurement here is the "water activity" of a solution, the amount of water available to bacteria and mold. The water activity (aw) of pure water is 1. Every beastie is different, but most bacteria need an aw > 0.9 to grow, and most yeasts need an aw > 0.8. From what I've been able to find, the aw of a saturated room temp sugar solution is 0.83, enough to prohibit the growth of bacteria! Mold is borderline, but from what I understand, the molds that have an aw of around 0.8 don't thrive in an a solution with an aw of 0.83 - they can survive for a time, but their numbers dwindle over time. The aw of a saturated salt solution is even better - 0.75, so there shouldn't be any issue with beasties there!
So as far as I can tell, if the water introduced to the product isn't so abundant that it dissolves all the sugar or salt, then the water should be saturated with sugar and salt! So no beastie proliferation! Incidentally, this water activity stuff seems to be a lot of the reason that honey is awesomely self-preserving - it's aw is down around 0.6!
Wow, great research, p! And thanks for sharing it with us!
Water activity is a really important part of preserving our products, and it's one of the reasons we don't need to preserve our anhydrous products that don't make their way into the bath or shower. So let's take a look at it a little further (although p has done some great research there!)
Water activity is defined as the water requirements for survival or growth of microorganisms. But water activity is not the same as the amount of water in a product. In some cases, the water is bound to other molecules (say, Epsom salts) and isn't free for usage by the microbes. In other cases, the water is bound by humectants like sorbitol or glycerin (anywhere from 10% to 20% will bind water). So water activity is actually a measure of the amount of free (unbound or active) water molecules present in our products. Water activity increases or decreases with with increases or decreases in pressure and temperature. pH also plays a role.
When we dissolve a solute like salt or sugar into water, the amount of water available to our beasties decreases so we say the water activity is reduced. Reduce it enough, and you've got an environment inhospitable to microbes. If the microbes don't have enough water, they die or go into a dormant state. (Remember the post the other day on osmosis? This is how salt or sugar kills them!)
So, it looks like that not using a preservative in your sugar or salt scrub will work with two disclaimers...
Disclaimer one: There are some microbes that can live in really hostile environments - like the xerophilic fungus (0.61 to 0.99) or the osmophilic yeasts (0.65 to 0.81) - and there are some, mostly yeasts, that will go into a dormant state waiting for water to come their way and bring them back to life. Add a little unbound water to the mix (let's say you have wet hands and leave a puddle in the top of the product), and you've got yourself a fungal party!
Disclaimer two: How do we figure out how much water is bound in our products and how much isn't? There's a lengthy formula that takes into account the water in the product, the partial pressure of the water vapour above the surface of the product, the relative humidity, and temperature, and by figuring all of that out, we can figure out the water activity of the product at that moment.
If I take a look my sugar scrubs, I use about 140% sugar to 100% oils, which means I'm well above the numbers for killing bacteria and yeast. So should I use a preservative in my salt or sugar scrub? I'm still firmly on the side of "yes", because I'm always worried about what the end user will do with the product, but there is some evidence here that you could use lower levels of preservatives or possibly none. (Please do not take this that I am endorsing not using preservatives in scrubs!)
Thanks for the question, research, and work you put into your comment, p! It's definitely food for thought!
If you'd like to learn more about water activity, here are a few links:
Water activity (really interesting site).
Water activity theory (from wateractivity.org)