The purpose of stability testing cosmetic products is to ensure that a new or modified product meets the intended physical, chemical and microbiological quality standards as well as functionality and aesthetics when stored under appropriate conditions.
Because the development cycle of cosmetic products is relatively short each manufacturer should design their own stability testing program such that it is economically reasonable and efficiently addresses the testing required.
Because of the wide variety of cosmetic products “standard” stability tests cannot be prescribed. Manufacturers require the flexibility to modify testing protocols and to build a sound scientific basis for assessing stability of their own products. Thus, specific tests may be developed in order to address new or unusual technologies, or to be adapted to products having extended shelf lives. Stability tests can be conducted in real time or under accelerated conditions and should address the stability of a product under appropriate conditions of storage, transport and use.
Basically, there are three forms of stability tests: physical and chemical integrity tests which evaluate color, odor / fragrance, pH value, viscosity, texture, flow, and emulsion stability (signs of separation); microbiological stability tests which evaluate the degree of contamination with bacteria, mold, and yeast; and packaging stability tests which evaluate the impact of packaging on the contained product.
Physical / Chemical Stability Tests
These describes approaches to predicting how well cosmetics will resist common stresses such as temperature extremes and light. Typically, manufacturers determine whether to perform such specialized testing based on the vulnerabilities of the particular cosmetic product and its anticipated shipping, storage display and use conditions. Common test procedures include:
- Temperature Variations: High temperature testing is now commonly used as a predictor of long-term stability. Most companies conduct their high temperature testing at 37oC (98F) and 45oC (113F). If a product is stored at 45oC for three months (and exhibits acceptable stability) then it should be stable at room temperature for two years. Of course, the product must be stored at 25oC (77F) for a period of one year. A good control temperature is 4oC (39F) where most products will exhibit excellent stability. The product should also be subjected to -10oC (14F) for three months.
- Cycle Testing: The product should pass three cycles of temperature testing from -10oC (14F) to 25oC (77F). Place the product at -10oC for 24 hours and place it at room temperature (25oC) for 24 hours. This completes one cycle. If the product passes three cycles then you can have a good degree of confidence in the stability of the product. An even more rigorous test is a -10oC to 45oC five-cycle test. This puts emulsions under a tremendous stress and, if it passes the test, indicates that you have a really stable product.
- Centrifuge Testing: The dispersed phase (of an oil-in-water emulsion) has a tendency to separate and rise to the top of the emulsion forming a layer of oil droplets. This phenomenon is called creaming. Creaming is one of the first signs of impending emulsion instability and should be taken seriously. A good test method to predict creaming is centrifugation. Heat the emulsion to 50oC (122F) and centrifuge it for thirty minutes at 3000 rpm. Then inspect the resultant product for signs of creaming. This test is an absolute necessity for those products that contain powders of any kind such as liquid/cream make-up.
- Light Exposure Testing: Both formulas and packaging can be sensitive to the UV radiation. All products should be placed, in glass and the actual package, in the window and if its available a light box that has a broad-spectrum output. Place another glass jar completely covered with aluminum foil in the window to serve as a control. All too often we will see significant discoloration of the product and sometimes of the package also. This discoloration may be due to the fragrance or some other sensitive ingredient. Usually all that is needed is the addition of a UV absorber (e.g. 0.1% of benzophenone).
- Mechanical Shock Testing: In order to determine whether or not shipping movements may damage the cosmetic and its packaging mechanical shock testing is often conducted. Vibration testing (e.g. on a pallet shaker) can help to determine whether de-mixing (separation) of powders or granular products is likely to occur.
- Monitoring: For all the above mentioned tests you should monitor the color, odor / fragrance, viscosity, pH value, and, if available, particle size uniformity and/or particle agglomeration under the microscope.
Microbiological Stability Tests
Microbial contaminants usually come from two different origins: during production and filling, and during the use of the cosmetic by the consumer. From the moment the cosmetic unit is opened by the consumer, a permanent microbial contamination of the cosmetic is introduced caused by contact with the consumers hands and body.
Microbial preservation of cosmetics is important to ensure the microbial safety of cosmetics for the consumer, maintain the quality of the product, and confirm hygienic and high-quality handling. Although only a small number of cases of microbial infections of the consumer has been reported, microbial contamination of cosmetic products may spoil them or seriously reduce the intended quality.
Therefore, it is necessary to carry out routine microbiological analysis of each batch of the finished product coming on the market. Pseudomonas aeruginosa, Staphylococcus Aureus and Candida Albicans are considered the main potential pathogens in cosmetic products. These specific potential pathogens must not be detectable in 0.1 g or 0.1 ml of a cosmetic product. The parameters examined, the criteria and methods used, and the results obtained per batch should be documented.
- Screening Tests: There are various easy testing kits available on the market (e.g. dip-slides or plate counts) which provide quick and semi-quantitative results whether a cosmetic product is significantly contaminated or not. Sampling and evaluation of the results is simple and can be performed also by personnel without any microbiological training.
- Quantitative Tests: Quantitative tests determine the actual count level of bacteria, mold and yeast in a cosmetic product. These tests are very sophisticated and laborious and can be performed only by professional microbiological testing laboratories. Typically, methods for isolation of microorganisms from cosmetic products include direct colony counts and enrichment culturing.
Packaging Stability Tests
Packaging can directly affect finished product stability because of interactions which can occur between the product, the package, and the external environment. For example, product constituents may be absorbed into the container or may chemically react with the container. In addition, the container may not fully protect the product from the adverse effects of atmospheric oxygen and/or water vapor, or volatile product constituents (e.g. fragrances) may evaporate through the container.
- Glass Tests: Glass is the most inert material and does not react with a cosmetic product in any way. For this reason all testing should be done in glass and the actual packaging. In this way you can determine if the cause of product failure is the formula or the package.
- Weight Loss Tests: To determine evaporation (water loss through the container wall or closure gaps) weight loss evaluation is one of the most important tests that must be conducted. This testing (performed in the actual package with the cap torqued to 100% of target torque) is done at room temperature and at 45oC (113F) for a period of three months. The weight loss should not exceed 1% per month for the package to be considered acceptable.
- Leaking Tests: It may be advisable to test the packaged product in various orientations (upright, inverted, on its side, etc.) to determine whether the packaging may leak (especially during transport).