During the process of cosmetic design, product development and production, one of the most imperative steps is stability testing. Stability testing of cosmetic products aims to guarantee that new or modified products hold their intended physical, chemical and microbiological quality standards as well as their functionality and aesthetics during the intended shelf life and consumer use. Stability testing can be performed in real-time or, most commonly, under accelerated conditions and assess the following parameters:
- Stability and physical integrity of cosmetics under suitable conditions of storage, transport and use;
- Chemical stability;
- Microbiological stability;
- Packaging compatibility.
When should a stability test be performed?
Stability tests for cosmetics should be performed when:
- A new formula is developed;
- An existing on-market product is reformulated;
- The production method or production site is altered;
- The raw material supplier is different;
- The packaging is changed, to ensure that the formula remains compatible with the packaging.
How can stability testing be performed?
Stability testing is not a “standard” process due to the variety and complexity of cosmetic formulas and packaging. Therefore, for each formula, the manufacturer should select the most relevant parameters to be evaluated based, for example, on the product’s vulnerabilities and its anticipated shipping, storage and use conditions.
The establishment of adequate protocols and procedures for stability testing should be based on guidelines provided, for example, by the FDA (Food and Drug Administration) and COLIPA (The European Cosmetic and Perfumery Association) and include:
- Tests that can “predict” the effects of regular storage and use conditions and, when relevant should include stress tests to allow the assessment of the integrity of a product under extreme conditions.
- Evaluation of aesthetic properties such as colour, fragrance, texture and viscosity during and after the exposure to the test’s parameters;
- Considerations of variations in process conditions;
- Packaging compatibility.
What to do during a stability test?
When a formula is first developed, preliminary stability testing can be made using test samples from laboratory or pilot batches to establish the reliability of the formulas and packaging, which can include more extreme protocols than those carried out during the stability testing of final products. However, as development continues, more precise determinations of the stability of representative batches of the commercial product are necessary.
Accelerated tests, are the most common form of stability testing and are designed to predict the stability of a formula since cosmetics have a short development cycle. Predictions attained from these studies are accepted provided that the producer carries out regular post-launch studies of samples stored at ambient temperatures. To predict the stability of a cosmetic product, appropriate conditions regarding, temperature, duration of the test and humidity should be selected according to the product category. Tests are often performed at set temperatures of 5 °C, 25 °C, 37 °C, 40 °C or 45 °C and at set relative humidity levels of 60%, 65% or 75% during 1, 3 or more months, depending on the cosmetic.
Packaging can affect final product stability due to interactions between the formula, the package and the external environment, therefore, stability testing should include packaging as similar as possible to the package in which the cosmetic will be marketed. Such interactions include adsorption and chemical reactions between the cosmetic and the container.
During a cosmetic’s shelf life, its properties may change as the product ages, hence during the stability testing the following parameters should be evaluated:
- Colour, odour and appearance;
- Packaging changes;
- pH;
- Viscosity;
- Weight changes;
- Microbiological quality.
Given the subjectivity adjacent to accelerated testing, it is advisable to conduct real-time monitoring to confirm stability testing’s results.
During shipping, storage, retail and use, cosmetics can be subject to extreme temperature and light conditions, which can be included in stability testing to ensure that the product reaches the consumer in the best conditions.
To assess a formula’s response to temperature extremes temperature cycles and or freeze/thaw tests can be applied. During the freeze/thaw cycle, the product should undergo three 24-hour cycles of temperatures testing from -10 °C to 25 °C. An even more rigorous test includes five cycles of temperatures testing from -10 °C to 45 °C, this test puts the formulation under tremendous stress revealing possible inadequacies in a short timeframe when compared to storage at a constant temperature. For all cosmetics marketed in clear packaging, light stability testing should be performed. Finally, to determine the effects of shipping movements on the formula or the packaging both mechanical shock testing or vibration testing can be carried. Among the problems uncovered by these tests are suspension problems, instability of emulsions and creams (phase separation, tendency to crystalize or cloud), package design issues (wrinkling or loss of labels) and corrosion of internal lacquers in aluminium tubes.
Conclusion
To sum up, stability testing aims to guarantee that a cosmetic product meets its intended physical-chemical and microbiological quality standards, as well as functionality and aesthetics. By carrying out suitable stability testing manufacturers can identify any physical and chemical vulnerabilities of their product early-on, allowing the producers to make needed adjustments to the formula or final product in order to avoid marketing improper products and consumer’s complaints. Therefore, cosmetic manufacturers benefit greatly from the incorporation of routine stability testing into the lifecycle of their products. By gathering critical information on different formulas and final products, manufacturers can develop better products, establish shipment and storage conditions, provide the customer with suitable application methods, and determine the appropriate packaging in order to provide consumers with the best cosmetic products.