The purpose of stability testing is to ensure that the cosmetic product maintains its intended physical, chemical and microbiological quality, as well as functionality and aesthetics when stored under appropriate conditions.
Due to the different nature of each cosmetic product, no mandatory Stability Test was established. Each manufacturer given its knowledge on the formulation and the product intended use, may draft and argue the best suited conditions to evaluate the stability of the product. The elements to be considered when drafting the protocol of a stability test:
- Identify the conditions that will “accelerate and predict” the effects of normal conditions of storage and use. Where relevant, consider stresses, including temperature that will enable assessment of product integrity under anticipated product exposure conditions.
- Consider evaluation of critical aesthetic properties such as color, fragrance, texture, and flow, particularly after exposure to conditions designed to stress each specific property.
- Consider variations in the process conditions.
- Consider the impact of the primary packaging on the formulation, as well as any effects which the product might have on the packaging.
Why:To ensure that the cosmetic product maintains its intended physical, chemical and microbiological quality, as well as functionality and aesthetics when stored under appropriate conditions.
When: The most common conditions for stability testing of cosmetic products are listed in several published references:
(✓) International Federation of Societies of Cosmetic Chemists, IFSCC Monograph, Number 2 (The Fundamentals of Stability Testing):
|Room Temp||20 – 25° C||–||Shelf life|
|Accelerated||37° C||–||3-6 months|
|Accelerated||37° C||80%||1 month|
|Accelerated||45° C||1-3 months|
(✓) Cosmetic Regulation in a Competitive Environment(Ch 11 “Stability Testing of Cosmetic Products”)
|Accelerated testing:||37, 45 and 50°C|
|Freeze-thaw cycling:||0 or -5°C|
|Elevated humidity:||80% RH|
How: Stability testing is an experiment in which samples of the formula are put at different environmental/ storage conditions for a set period of time to simulate what will happen to the product during its life cycle. At selected intervals, samples are evaluated for various physical, chemical and performance characteristics to see if and how they have changed.
- Selection of batches:
It is recommended to use at least one batch of the cosmetic product, the same batch as the marketed product.
- Container/ Closure system:
The container/closure system(s) shall be the same as the one used when placing the product on the market. Any other studies carried out on the cosmetic product outside its immediate container or in other packaging materials can form a useful part of the stress testing of the cosmetic product or can be considered as supporting information, respectively.
Appropriate controls (for example, product in glass containers) should be used.
- Stability evaluation parameters:
- Organoleptical properties: color, odor and appearance,
- Physico-chemical properties: pH value, viscosity, density, the monitoring of fomulation ingredients (whenever applicable)
- Microbiological stability: microbial count & Challenge Test of the preserving system made before and/or after the Accelerated Stability Test.
- The compatibility between the content & the container (Compatibility Test)
- Analytical data in relation to other parameters for specific product type
A preliminary test recommended is the centrifugation test. This test predicts emulsion creaming (for powder, liquid/cream products).
• A sample heated to 50°C is centrifuged at 3,000rpm for 30 minutes.
(✓) The product must remain stable and any sign of instability shows the need for reformulation.
If approved in this test, the product can then be submitted to the stability tests.
Preliminary stability test – it is also known as Screening test, Accelerated stability test or Short term test.
Why: help in the screening of the formulation.
When: is carried out in the initial phase of product development.
How: the samples can be submitted to:
- heating in ovens,
- cooling in refrigerators,
- to alternate cooling and heating cycles.
Duration of the study: 15 days
The parameters to be evaluated:
- Organoleptic characteristics: appearance, color, odor and flavor, whenever applicable.
- Physical-chemical characteristics: pH value, viscosity, density, or others.
Accelerated Stability Test
An accelerated stability test is also a possibility, none the less, the product will be required to be submitted to more-severe-than-usual conditions.
The accelerated stability is also known as normal or exploratory stability test. This test is a predictive study that can be used to estimate the expiry date of the product.
Duration of the study: generally 90 days.
The samples are evaluated against the standard sample (must remain unchanged during the whole life cycle of the product) and with products that are considered as ‘’references’’ submitted to the same test conditions. Generally limits of acceptance are defined for the evaluated parameters. The interpretation of the data obtained during the Stability Test depends on criteria established in accordance with the formulator’s experience.
|Criteria||Limits of acceptability|
|Appearance||The product must, maintain its initial appearance under all the conditions except high temperatures, freezer conditions or cycles in which small alterations are acceptable.|
|Color and odor||Must remain stable for at least 15 days when exposed to sunlight. Small alterations are acceptable at high temperatures.|
|Viscosity||The limits of acceptability must be defined by the formulator, considering the visual and sensorial perceptions caused by alterations. The possibility of the consumers also noticing them must be considered.|
|Compatibility with the primary packaging material||The integrity of the package and the formulation must be considered, evaluating the weight, the sealing and the functionality|
In cases where the monitoring of the active ingredient percentage is necessary, the quality and performance parameters of the product must be considered.
Other parameters can be established according to the formulator and the products’ specifications.
The Conclusion report on stability study shall contain:
The Conclusion report on stability study shall contain:
- – Identification of the lab conducting the testing
- – Identification of the product
- – Containing Primary packaging material used in the test
- – A description of the methodology used to determine the product’s minimum durability. If applicable, the specific preservation precautions should be mentioned.
- – Study conditions (sample storage conditions, test time period and periodicity of the evaluations). In addition, the following should also be provided:
- Evidence that the composition of the product used for stability testing corresponds to the product actually placed on the market;
- The results of the preservative efficacy study, e.g., challenge test, if applicable;
- When applicable, the period after opening (PAO) and its justification;
- – The signature of the person responsible for the study