To which I answered: We preserve sugar and salt scrubs because you never know how the end user will contaminate the product. I give my friends and family a little spatula to use with my manicure scrub to keep their wet and icky hands out of it, but you know they're dipping their fingers in there every once in a while and leaving behind beasties! And I know that I dip my wet hands into my sugar scrub in the shower all the time - I do dry them off, but there's water everywhere and some of it is bound to get into the jar! I know some people think salt or sugar should preserve the product, but I don't trust it. I'd rather use 0.5% to 1% of a nice oil soluble preservative and ensure we won't be seeing mould growing on the top of the container any time soon!
To which Sarah answered: Thanks, Susan, my thoughts exactly but how do we explain to people that sugar and salt won't necessarily prevent contamination? 'Experts' have stated that the amount of water entering the product is negligible and will be contained within the salt and sugar content, and won't be a problem, but how do we know for sure? I always use a small amount of preservative, as I don't want to take any risks.
What we're creating with a salt or sugar scrub is a hypertonic solution where the salt level is higher on the outside of the cell than the inside. Water comes out of the bacteria and they die.
But wait! All of this is talk about water and we're making anhydrous scrubs! Ah, yes, but we're introducing water into the mix and that's where the problem arises. We're adding a lot more than we think when we dip wet hands into the product or when hygroscopic materials - like sugar or Dead Sea Salts or olive oil - draw water to our product. We're adding water into the product with things like Epsom salts, which have water molecules bound to the magnesium sulphate, or proteins or cationic polymers or some emulsifiers! And we're adding water by merely having them in our shower area - open it for a second or two in a warm, foggy bathroom and you'll find a little H2O in your product. Even though we're calling them anhydrous scrubs, there's little bits of water scattered throughout the product.
A note on hygroscopic ingredients: Once a sufficient amount of water is introduced into our product, the bacteria that has managed to live in our products will get enough moisture to start replicating. Or the yeast will come out of their dormant state and start fermenting the sugars into alcohol and carbon dioxide. In both cases, they'll multiply as long as the water holds out.
How much salt or sugar do we need to preserve our specific scrub? I saw one statistic saying 20% sucrose is enough, but I can't confirm that without a ton of testing, and I haven't been able to find a recommended salt amount. I'm sure I could do more searching and find some kind of information, but considering it takes about 3 seconds to add 1% Phenonip to my scrubs, I think I'll go make some breakfast instead.