Monday, October 18, 2010

Preservatives: Water activity and sugar/salt scrubs

p wrote this really great comment in this post (it's really long, so I'd encourage you to read the entire thing in the post as I'm editing it slightly for space): 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!

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


p said...

Glad my comment was helpful! And so glad to give a tiny bit back to a blog that has taught me so much! :)

Re your Disclaimer #2, I think the only way for the home formulator to figure out how much water is bound is to consult a table of aw values for different solutions. I tried to find a nice table online, but no luck - I just found isolated values (including those for saturated sugar and saturated salt solutions), plus a few lists like on wikipedia's page... did you happen to find one? I'd love to know the aw of a 20% glycerin solution, for example - wouldn't it be nice if 20% glycerin in a rinse-off product were sufficient to make a product self-preserving?

Also, does it matter that you're using 140% sugar to 100% oils? From what I understand, about water activity, the issue is the amount of sugar dissolved into the water introduced into the product - the oil doesn't really come into play, right?

Your Disclaimer #1 is a really good point. Do you know, does Phenonip kill the hardy fungi you mention? Do those fungi live in our homes (or on our bodies), or are they really exotic and unlikely to get into our products?


sarah said...

Our bodies contain commensal (normally found) organisms, mainly bacteria and yeast/fungi. Our bodies can thus be carriers for all manner of nasties.
Place a finger on a surface and you've transferred countless 100s or 1000s of bugs.

Not to mention the unseen things in the air.....

Susan Barclay-Nichols said...

Hi p! I realize didn't respond to your comment!

Although the product is composed of oils, we will introduce water into the product at some point - dipping wet hands into it, getting shower water in it, leaving the cap off - and that water is likely to sit on the top of the product and not integrate into the product. So the water isn't bound and won't be protected by the sugar or salt found in the scrub.

As far as I know, Phenonip is effective against these beasties - the data sheet lists its efficacy against one of the osmophilic yeasts (Saccharomyces cerevisiae) and one of the xerophilic fungi (Penicillin purpogenum). The data sheet doesn't list every single beastie, so I'm going to go with the idea that it does until I have time to do more research.

Here's what I found on xerophilic fungus:
Molds that do not require free water for growth are sometimes called xerophilic fungi. These fungi grow in air that has more than 60% relative humidity. The rate of growth increases as the relative humidity increases. The common xerophilic fungi belong to the genera aspergillus and penicillium. The spores of these fungi are very small (only a few microns) and when inhaled, they reach the tiniest alveoli of the lungs, often causing allergic reactions.

Here's what I found on osmophilic yeasts:
Osmophillic organisms are extremophiles that are able to grow in environments with a high sugar concentration. Osmophiles are similar to halophillic (salt-loving) organisms because a critical aspect of both types of environment is their low water activity, aW. High sugar concentrations represent a growth-limiting factor for many microorganisms, yet osmophiles protect themselves against this high osmotic pressure by the synthesis of osmoprotectants such as alcohols and amino acids. Nearly all osmophillic microorganisms fall under the yeast genus.

Osmophile yeasts are important because they cause spoilage in the sugar and sweet goods industry, with products such as fruit juices, fruit juice concentrates, liquid sugars (such as golden syrup), honey and in some cases marzipan.

So I think the answer is yes, they are found around the home!

Mahmood Ali said...


i would like to thank you very much for sharing your efforts.
That is the concept of open source, and the presentation, the website, and every thing is very helpful.
I am going to use your data in my seminar tomorrow god's welling, so maybe i am going to share that one as well :D

regards from Egypt, Mahmood Ali

Anonymous said...

Would you recommend Opitiphen Plus as a preservative in your sugar and salt scrubs?

Susan Barclay-Nichols said...

Hi Anonymous. I've decided I have no opinion on the topic of using Optiphen in anhydrous products, so I will refer you to this post on Optiphen and anhydrous products and let you make your own decisions!

Gabrielle said...

Wish I'd seen this brilliant post sooner - sorry for the very late comment.

I make a very basic, liquid-y, pourable, salt (or sugar, depending on preference) scrub and package them bottles with flip top/disc lids so there's far less chance of water being introduced.

They do require a very quick, light 'shake' just before use, but they work very nicely.

Customers like them because they don't have to juggle slippery lids and pots in the shower, much less dig around inside a pot while buckets of water are introduced into the product.

I do add just a dash of polysorbate 20 (not the 60 or 80 - they're just not necessary) which very lightly emulsifies the oil, enough so that it rinses cleanly away from the skin.

Hope this might help others looking for ways of eliminating water being introduced into their scrubs.

Gail Storment said...

From your June 27, 2013 blog, Chemistry Thursday: Why oil and water don't mix or more on solubility
I was reviewing your comments on this blog post and see a possible error. At least I think it is an error after doing some research. It is about ionic and covalent bonds and their breaking abilities.
Your statement: "Covalent bonding: Covalent bonds are those between non-metal atoms, like oxygen or nitrogen or carbon. They are about sharing electrons, and they are much harder to break than covalent bonds. They can be polar bonds or non-polar bonds. Molecules are about the covalent bonding."

This is saying covalent bonds are harder to break than covalent bonds and I think you meant ionic bonds as they are easier to break than covalent bonds.
You give us such invaluable information and I thank you for this.

Susan Barclay-Nichols said...

Eep! Thanks for picking that up, Gail! I'll correct it at once!

Anonymous said...

Hi Susan - Great post. I've been misled into believing that TCP is enough to act as preservative in scrubs and that bath salt don't need one. So I'd like to know which preservative will you personally recommend for both or either one. Thanks. Once again, great post!

Susan Barclay-Nichols said...

Sorry, what is TCP?

Anonymous said...

Vitamin E (Tocopherol). Thank you in advance.

Susan Barclay-Nichols said...

Hi Anonymous! Could you please leave your name with your posts. A cheery "Bye, (name)" is all I need, or I'll have to delete them because I don't allow anonymous messages on the blog.

Having said that, I have never heard Tocopherol or Vitamin E called TCP. Is this some new slang I missed out on? Vitamin E isn't a preservative. (Check out the post in the newbie section to learn more about this...) It's an anti-oxidant, and will not prevent contamination.

As for preservatives, I like Phenonip in mine, although there are others you can use. Check out this post on sugar scrubs, or go into the preservatives section and use the comparison chart to see what might work. Or do a search for sugar scrubs. I have many many recipes, and I mention in those what I like to use and why!