If you’ve ever looked at the ingredients on a bottle of shower gel, liquid handwash or body lotion, you’ll probably have noticed a few unfamiliar names towards the end of the list. Some of these may be preservatives, added to help prevent bacteria, mould and other microorganisms from growing in the product.
But handmade cold process soap doesn’t need an added preservative. And no, it’s not because I’ve discovered some miraculous preservative-free secret. It’s simply down to the chemistry of traditional soap.
First, why do cosmetics need preservatives?
Many cosmetic products contain water, and where there’s water, there is the potential for microorganisms such as bacteria, yeasts and moulds to grow. Products like lotions and creams can provide quite a welcoming environment for microbial growth, especially once you’ve opened them and started using them.
Preservatives help keep those products safe throughout their intended shelf life. They’re not something to be frightened of, and I would never suggest that a product is better or safer because it’s described as preservative-free. Actually, that usually gets alarm bells ringing for me.
But different products have different preservation needs, and a solid bar of traditional soap is rather different from a pot of moisturising cream.
Soap is naturally alkaline
In my recent post about the pH of handmade soap, I wrote about the fact that traditional cold process soap is alkaline, typically with a pH somewhere around 9 to 10.
Many microorganisms don’t grow particularly well in alkaline conditions, which makes traditional soap a less welcoming environment for them. That doesn’t mean bacteria can’t ever be found on a bar of soap, or that soap somehow sterilises itself. Some microorganisms are perfectly capable of surviving alkaline conditions.
But surviving and multiplying are two different things, and soap’s natural alkalinity helps make it difficult for many microorganisms to thrive.
There’s another reason: water
This might sound slightly odd, given that soap spends so much of its working life getting wet. A properly cured bar contains relatively little water that’s available for microorganisms to use. Scientists call this water activity. It’s not simply about how much water a product contains, but how much of that water is available to support microbial growth.
During curing, excess water gradually evaporates from my soap bars, helping them become harder and longer-lasting. The finished soap has a relatively low water activity, which makes it way less suitable for microbial growth. Combine that with its alkaline nature, and you’ve got a product that doesn’t need an additional preservative to remain microbiologically stable.
Clever stuff, chemistry.
Does that mean soap can’t go off?
Not quite!
If you’ve read my post about whether handmade soap expires, you’ll know that soap can deteriorate over time.
The oils and fatty components in soap can gradually oxidise, potentially leading to rancidity and those dreaded orange spots that soapmakers know all too well. But that’s a different process from bacteria or mould multiplying in a product. Preservatives intended to control microbial growth aren’t the same as antioxidants, which can help slow oxidation.
So while my soap doesn’t require an added antimicrobial preservative, that doesn’t mean it will stay perfect forever.
Does it matter how you store your soap?
Absolutely! Keeping your bar on a draining dish, away from standing water, allows it to dry between uses. This helps it stay firm and last longer, and avoids leaving it sitting in a persistently damp environment.
And just because a bar doesn’t need an added preservative doesn’t mean hygiene isn’t important. Proper manufacturing procedures, suitable ingredients and correct storage still matter.
Different soap formulations can also have different microbiological risks, particularly where unusual additives are involved, so preservation requirements need to be considered as part of the product’s cosmetic safety assessment.
So is preservative-free soap better?
Nooooo! And this is a point I come back to again and again. Preservatives have a REALLY important job to do in products that need them.
Preservatives have become something of a bogeyman in recent years, particularly in the beauty and skincare industry. They’ve been unfairly demonised, and the idea that ‘preservative-free’ automatically means ‘better’ has become a rather effective marketing tactic.
The irony is that most of us happily consume foods containing preservatives without giving them a second thought, yet there’s so much unnecessary fearmongering about their use in cosmetics. Personally, I’d much rather use a properly preserved cosmetic than one that was at risk of becoming contaminated simply because somebody wanted to advertise it as ‘preservative-free’.
Traditional cold process soap doesn’t need added antimicrobial preservatives simply because its own physical and chemical characteristics help prevent microbial growth. That’s not a marketing trick, and it’s not evidence that preservatives are bad.
It’s simply another interesting example of how different cosmetic products work. And another reason I find the chemistry of soapmaking so fascinating, even after all these years.
Vicki x
