The importance of stability in cosmetics
Each cosmetic product normally undergoes more or less marked variations due to product aging and use.
The stability tests purpose is to define a time within which these variations are recognized as acceptable to ensure the consumer the cosmetic product quality, safety and pleasantness.
The stability tests on cosmetics therefore allow to define the correct product duration (shelf-life), the product duration after opening (PAO – Period After Opening), the microbiological quality (Challenge Test), the product stability in final packaging (compatibility test).
Stability tests on cosmetics verify that the product meets all quality standards so that it does not undergo unacceptable changes during its life cycle under precise conditions of humidity, exposure to light and temperature.
In Regulation (CE) 1223/2009, Annex I, there are indicated the elements that must be reported in the Cosmetic Product Safety Assessment to which each cosmetic product must be subjected before being placed on the market. The Safety Assessment is divided into a PART A and a PART B.
Annex I, among the various elements, establishes that within PART A the following are indicated:
- Chemical / physical characteristics and the stability test result of the cosmetic product.
- The chemical / physical characteristics of the substances or mixtures.
Therefore, stability tests are a fundamental aspect that the Security Assessor will need to examine.
The process to follow for a correct evaluation
The stability protocol definition depends on various aspects, in particular on the product type, however it’s possible to list the conditions most frequently used for evaluating the cosmetic stability:
- Exposure to high temperatures;
- Exposure to high humidity;
- Cyclic tests in which the product is exposed to temperature and / or humidity conditions that are changed at regular intervals;
- Freezing / thawing tests for products that can crystallize or become opaque or for systems composed of several phases (emulsions) to evaluate their stability;
- Exposure to light;
- Mechanical stress test.
Cosmetics whose packaging is transparent and can therefore allow the product to be exposed to light must be subjected to light stability tests. The lighting used in the tests should likely simulate the intensity and duration to which the cosmetic will likely be exposed during its commercial life.
Accelerated and natural stability test: the differences
Accelerated stability tests are usually performed on cosmetics in order to obtain the necessary information in the shortest possible time and thus be able to assess whether a shelf-life of more than 30 months is attributable.
Stability protocols must accelerate any foreseeable changes in normal storage and use conditions. The parameters that are adjusted to perform an accelerated stability test are temperature, humidity, exposure to light and mechanical stress. The values obtained from this type of test must be evaluated and interpreted very carefully as there is also the risk of causing changes that would never occur under normal conditions. A product that is unstable at high temperatures may not necessarily be unstable in normal use conditions.
Natural stability tests consist in keeping the product monitored in normal storage conditions and assessing if any changes occur long after production. It’s a type of test that requires very long data collection times and that does not allows to test the product’s resistance in stressful conditions.
For these reasons, the natural stability tests are carried out during the cosmetic product life, collecting data after the development, while the product is placed on the market.