Sunday, October 17, 2010

Question: Why are we using preservatives in salt or sugar scrubs?

In this post, Sarah asks: One thing I'd like to know is the specifics as to why we preserve anhydrous sugar and salt scrubs - I've had so many discussions with regards this (I say we should, to be safe) and would like it clarified if poss.! I know you've touched on it before but could you do it again please?!!

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.

Let's first take a look at osmosis, which is all about the movement of water molecules from an area with a high concentration of something to an area with a low concentration of something. If we put bacteria into a glass of water with some salt, the bacteria will move the water with the salt into it, which will cause it to shrivel up and die. (Fluid leaves the bacteria causing the cell wall to contract, which causes the cell membrane to separate from the cell wall in a process called plasmolysis.) This is how preserving with sugar or salt works for our food products. It moves the water out and salt into the bacteria and they die.

Here's a nice animation about how this works! And to learn more about osmosis, click here

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. 


sarah said...

Beautiful, thank you! It may not convince the die-hards but at least I know my scrubs will be safer than theirs!!

lynda said...

Susan, Have you ever used LiquaPar Oil for a preservative for anhydrous mixtures?

My favorite supplier doesn't carry Phenonip.

p said...

Great post, as usual! I'm in the minority here, leaving my sugar scrubs unpreserved....

My reasoning is that any water introduced to the product will dissolve sugar until it reaches its saturation point. So the question is, is room temperature water saturated with sugar a hospitable environment for beasties?

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!

Would love to know what you think of my reasoning, Susan! It's entirely possible I've missed something important....

Susan Barclay-Nichols said...

Hi Lynda. No, I've never used LiquPar oil for anhydrous mixtures, but it should work well in scrubs. I don't use it as I can't get it locally, and it's fairly similar to Phenonip.

Susan Barclay-Nichols said...

Hi p. I think your reasoning is sound, and this is an interesting topic, so I've written a post about it here.

Sarah said...

Food for thought on all sides, thanks p and Susan.
The only real problem then is with the end user!! How do we know if, like me, they've accidentally dropped the scrub in the bath then shaken it off and put it back on the shelf?! I know I've preserved mine (and it's only for me) so I'm not too worried but what about a scrub that one of your customers have bought? Will continue to preserve mine but also respect others wishes not to.

Mesha said...

So I am confused- Should you preserve the entire batch of scrub or just the portion that isn't sugar?
Thank you!

Brittanie said...

I am about to make my first batch of scrubs and have been debating using my Optiphen preservative. I use it for my lotions and body butters with water.

Here is something interesting - my salt scrub I buy from Trader Joes has NO preservatives. Just sea salt, oils, and lavender essential oil.

If the big guys are selling their scrubs with no preservatives, I think we will be fine! I've had my scrub for a couple months and have definitely introduced water into it via my hands and shower.

Cher said...

How is the anyhydrous preservative going to preserve the water that gets into the jar? Am I missing something? Thanks!

Susan Barclay-Nichols said...

Hi Cher! Can I pause for a moment and ask about what you mean by an anhydrous preservative? I've never seen anything called an anhydrous preservative. I've seen ones suggested for anhydrous products because they are more or all oil soluble, but they wouldn't be called an anhydrous preservative. Is this what you mean?

There's quite a controversy about this topic, so I encourage you to take a look at this post - Debate: Water soluble or oil soluble preservatives in an emulsified scrub. I think you'll find loads of interesting things to think about there!

As an aside, though, Phenonip isn't just oil soluble, so it's a good one to use here. (It was recommended to me by a cosmetic chemist...) But take a look at that post because there's so much more than oil or water solubility to consider!