Before a skincare product goes on sale, you need to know two things: that it will stay the same on the shelf and in your customer's bathroom, and that it won't grow bacteria, yeast or mould once it's opened. Stability testing answers the first question. Preservative efficacy testing, or PET, answers the second.
Are these tests required in Australia?
Australian law doesn't name a single stability test or PET that every cosmetic must pass. But under the Australian Consumer Law, you are responsible for supplying products that are safe and of acceptable quality. Testing is how you show you've done that. Retailers, distributors and insurers often ask for test results too, so skipping testing can close doors as well as add risk.
What is stability testing?
Stability testing checks that a product keeps its quality over time. Samples are stored under different conditions and checked at set intervals for changes in:
- appearance and colour
- scent
- texture and thickness
- pH, for water-based products
- separation, graininess or crystals
- signs of rancidity in oils and butters
Typical conditions include room temperature, raised temperatures to speed up ageing, cold storage and repeated freeze and thaw cycles. Products are tested in the packaging they'll be sold in, because packaging can react with a formula or let in air and light.
Do oil-based products need stability testing?
Yes. Balms, oils and butters don't need the same preservation as water-based products, but they can still go rancid, grain or separate. That's why we stability test every custom product we make, oil-based ones included.
What is preservative efficacy testing (PET)?
PET, also called a challenge test, checks that a product's preservative system works. A lab adds known amounts of bacteria, yeast and mould to the product, then measures how quickly they are reduced over about 28 days. The method most labs use is ISO 11930.
PET matters most for products that contain water, such as lotions, creams, gels, mists and cleansers, because water lets microbes grow. It is also worth considering for oil-based products likely to get water in them, such as body scrubs used in the shower.
How are the results used?
Together, the two tests support:
- your shelf life: how long the product stays in good condition
- your preservative choice: evidence the system protects the product under real use
- your packaging choice: evidence the formula and pack are compatible
- conversations with stockists and insurers, who may ask how you know your product is safe
When should you test again?
A test result only applies to the exact product tested. Test again when you:
- change an ingredient or its amount
- change the preservative
- change ingredient supplier for a key ingredient
- change packaging, including a different pump, jar or material
- scale up to a new batch size or process
Common mistakes
- Testing in a glass jar, then selling in a plastic pump
- Choosing a preservative because it is "natural" without checking it covers bacteria, yeast and mould
- Promising a long shelf life before any testing is done
- Assuming a supplier's test of their base covers your modified version
How we can help
Every custom product made through our private label service is stability tested before production, and we offer packaging compatibility testing and extended six-month stability testing as add-ons. If you formulate your own products, our Stability and Safety Testing Masterclass walks you through setting up your own stability program. For more on choosing a preservative, read our guide to preservatives.
Frequently asked questions
How long does stability testing take?
Accelerated testing gives an early indication within weeks to a few months. Real-time testing at room temperature runs for as long as the shelf life you want to support.
Does every product need PET?
Every product containing water should have its preservative system tested. Oil-based products generally don't, unless they are likely to come into contact with water during use.
Can I use my supplier's test results?
Only for the exact product they tested, in the same packaging. If you change anything, test your version.