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.
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).
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