A Pragmatic Guide
I have many customers who are just starting out and have no idea what shelf-life to quote for their products. The shelf-life is an indication of how long a product will last, how long it will continue to perform as intended. It is up to the Responsible Person (RP) to decide what shelf-life to assign, since they know the most about their product, but how do they decide what shelf-life to use? This article aims to provide a pragmatic guide to stability testing, enabling you to make informed decisions that ensure the quality and longevity of your cosmetic products.
A product can ‘go off’ in a number of ways. Here are some typical changes, and what to look out for:
- Colour – your product can change from one colour to another – when does it become a problem? Some colour change is normal, and it is acceptable to include a colour range in the product specification e.g. White to off-white. But if there is a drastic change, this may indicate a stability issue.
- Aroma – this can be caused by a wide range of causes and again some change is normal, but check for off-notes and products that smell unpleasant. This is unlikely to be a safety issue, but will cause an increase in customer complaints.
- Volume – is it increasing or decreasing? This can indicate a chemical reaction is taking place, and further investigation will be needed.
- pH – a small numerical change can indicate a significant change in pH because this is measured on a log scale. Most cosmetic products should be in the skin-safe zone for pH (4-8), and some preservatives only work in a narrow pH range, so the pH can be critical for product safety.
- Viscosity / feel – does the product clog the dispenser? Is it pouring differently than intended?
- Separation – the product is no longer fully mixed – this issue raises questions about the formulation.
Types of Stability Testing
Stability testing allows a brand owner to find out what changes are happening and when. The results from the stability tests can also help the RP to decide what shelf-life can be justified.
Product stability is affected mostly by heat and/or light and /or oxygen, therefore it is sensible to test for all these parameters. Keep samples from each batch to monitor and support shelf-life claims.
Room Temperature Testing
This method involves keeping samples from each batch and monitoring their performance over time. It provides insights into how your product might behave in real-world conditions. All products should be tested at room temperature as a minimum. The length of time should eventually cover the whole of the shelf-life of the product.
Elevated Temperature Testing
Particularly important for water-containing products, this involves storing samples at higher temperatures (greater than 35°C) for three or six months. This method predicts how your product might fare over extended periods, helping predict a shelf-life so you can use it before the real-time room temperature testing has been completed.
Freeze/Thaw Testing
Storing samples at room temperature or a higher temperature for a period of time (e.g. 24 hours), followed by storage in a freezer for the same period of time, repeated 3 to 5 times. This gives a good indication of longer-term stability to justify a longer shelf-life and also how the product will cope with transportation or storage in unheated warehouses.
Testing Under Light
Samples can be stored on a windowsill, or in a special light cabinet. If changes in the product are observed this could be avoided by choosing opaque packaging.
Final Packaging Testing / Packaging Compatibility Testing
Conduct stability testing using the final packaging. This not only assesses product stability but also examines the compatibility of your chosen packaging with the formulation. If the product had shown changes when stored under light, choose an opaque pack, and repeat the test again, to make sure that the pack provides sufficient protection. Also consider leaving some packs unopened to check the effect of exposure to oxygen. It is also worth considering pack fill volume – if you have a small volume in a large pack the top of the pack will contain oxygen that could degrade your product, even when it is unopened.
Determining Shelf-Life
In combination with the stability testing done on samples, also carry out continuous observations. Track changes in appearance, colour, odour, and pH (if relevant) over time. This data becomes evidence supporting your product’s shelf-life claims. Don’t forget to record everything. Document your observations in a report stored in your Product Information File (PIF).
What stability tests are needed before getting a CPSR?
For anhydrous products 1 month storage at room temperature on a windowsill is a good indication that the product is stable enough to be sent for testing and to get a CPSR. If there are going to be any significant changes that affect safety, they are likely to occur quite soon. I will start a safety assessment with this amount of data; however this amount of data would not necessarily justify a shelf-life of longer than 6 months. So further testing is needed if you want a longer shelf-life.
For water-containing products a challenge test will be required, and before this test is initiated, it is a good idea to ensure that the product is sufficiently stable. I would always recommend testing a product for at least 3 months at an elevated temperature (>35°C). If there are no changes at this temperature, an initial shelf-life of 12 months would be reasonable. If the tested at the elevated temperature was continued until 6 months, a shelf-life of 24 months would be reasonable. However, some products are not stable at these high temperatures. This doesn’t mean you have to scrap the product, but longer-term testing at room temperature would be required to justify the longer shelf-life.
For bar soaps, and powder products, no specific stability testing is required since these types of products are very stable.
What testing should I do?
There are many guidelines and descriptions online to explain and describe a stability testing protocol. Choose storage options of: higher temperature, room temperature, fridge, freezer, under light and freeze-thaw for a full assessment of the product. Add in testing of the product in its final packaging at fridge, room temperature and elevated temperature for a fuller picture. This testing can be done concurrently. With 6 months of testing under this kind a protocol, you will learn loads about your product, and be able to justify the shelf-life you have chosen.
A lot of this testing can be done in your own setting; however, the ability to store samples at higher temperatures is very difficult to achieve at home and therefore a specialist laboratory can be useful.
The following UK-based companies carry out stability tests (and may also do other analytical testing and services such as claims substantiation / user trials etc):
Emsworth emsworthstabilityplus.co.uk/
Sensory Dimensions sensorydimensions.com
Source Bioscience sourcebioscience.com
Marfleet Analytical marfleetanalytical.com
In conclusion, there is no one-size-fits-all stability test. Tailor your approach to your unique formulations and business needs. Remember, results from stability testing can save you headaches down the road and bolster the confidence of your customers.