Cosmetics are mixtures of chemicals that mostly aren’t supposed to react with each other.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, properties as well as functionality and aesthetics when stored under appropriate conditions.

Each manufacturer should design their own stability testing program such that it is economically reasonable and efficiently addresses the testing required. 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.

There are times when you need to do stability testing. Here is most important times to conduct a stability test. 
  1. New prototypes. When you make a new formula and are satisfied with the way it performs, you’ll want to do a stability test to ensure that it will stay together. 
  2. New raw materials. Eevery time you have to change the fragrance, color, or other raw material in a formula, you’ll have to do a stability test to make sure there aren’t unacceptable changes (Also for new raw material source/or supplier). 
  3. New manufacturing procedure. Every changing in manufacturing process (e.g mixing time, temperature drying), do stability test. It could affect your formula. 
  4. New packaging. Cosmetic products change their look almost yearly so packaging is constantly being modified. You’ll have to determine if the formula continues to be compatible. Stability testing helps ensure that it is.

How do you stability test a cosmetic?
This is the basic format you can follow for conducting a cosmetic formula stability test.

  1. Make your batch. Calculate how much to make based on the number of samples you’ll be using for the test. Ideally, make 30-40% more than you’ll need. 
  2. Fill your samples. Ideally, you’ll have the correct packaging but don’t count on it. When appropriate, fill glass jars with the product along with the finished package. In stability testing, you want to do both glass and packaging if possible. The number of samples depends on how much testing your doing but at minimum you should have 2 samples for each storage condition. 
  3. Take initial readings. Once you have a sample filled test it for all the characteristics you’re going to evaluate later.
  4. Put samples at different conditions. Stability testing requires different temperature and light conditions.
  5. Evaluate the product. Samples should be evaluated at the following intervals. 2 weeks, 4 weeks, 8 weeks, 12 weeks, and 52 weeks. Only the RT, 37C and 4C samples will be evaluated after one year. The highest temperature samples and the light exposed samples only need to be evaluated for the first three test intervals. The evaluation tests should be the same ones you conducted when taking your initial readings. 
  6. Determine stability. After 8 weeks you can confidently decide whether your formula is stable or not. 

Stability Testing to Perform in Cosmetic 

Stability protocols are designed to include testing of the attributes of the product that are susceptible to change during storage and are likely to influence quality, safety, and performance characteristics. The stability study includes the evaluation of product quality attributes at specific storage conditions and time intervals.
  • Organoleptic : appearance, color, odor/fragrance and texture
  • Physical-chemical : pH, weight, viscosity, flow, emulsion stability, assay of preservatives 
  • Microbiological parameters: evaluate the degree of contamination with bacteria, mold, and yeast and preservation efficacy test. 
  • packaging stability tests : Compatibility between product and container 

Physical / Chemical Stability Tests 
These describes approaches to predicting how well cosmetics will resist common stresses such as temperature extremes and light. Common test procedures include:
  • Temperature Variations: High temperature testing used to predict 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 (oil-in-water emulsion) has a tendency to separate and rise to the top of the emulsion forming a layer of oil droplets (creaming). It shows 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. 
  • Light Exposure Testing: 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: 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. Sometimes shipping movements may damage the cosmetic and its packaging. 
  • 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 ways, which is during production and filling, and during the use of the cosmetic by the consumer (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. 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 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. Product may be absorbed into the container, react with the container, or 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
    Weight Loss Tests: To determine evaporation (water loss through the container wall or closure gaps). 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).

If the changes are minimal according to your company standards, then your formula is said to have “passed” stability testing. This means you can have confident that when the formula is shipped to stores and ultimately customers, it will still be as good as when it was first manufactured.