Preservative challenge testing for cosmetic products
Because they often come in direct contact with the human body, cosmetic products are subject to a range of testing to protect users from microbiological and chemical contamination and from other possible toxic effects. In this article, we’ll discuss the specifics of one type of testing applicable to cosmetic products, that is, preservative challenge testing.
What is preservative challenge testing?
Cosmetics and personal care products are typically subjected to a number of different tests prior to being placed on the market for sale. Specific testing usually includes evaluations for product stability, and for microbiological or chemical contamination. Cosmetic products are also subject to testing to gauge the effectiveness of preservative agents that are added to many cosmetic products. These preservatives help prevent the growth of microbiological contaminants after production, and protect the product from deterioration that might otherwise compromise its intended effect.
The effectiveness of a given preservative or combination of preservatives is dependent on its interaction with a product’s unique ingredient formulation. This factor makes it impossible to select an appropriate preservative based solely on a list of ingredients contained in a cosmetic products. Instead, the actual effectiveness of a preservative can only be determined through testing a complete cosmetic formulation, including product packaging.
In preservative challenge testing, samples of cosmetic products are injected with several varieties of bacteria and fungi, and then regularly evaluated during the testing period for levels of contamination. Cosmetic products that exhibit the growth of microbiological contaminants as a result of preservative challenge testing are reformulated, usually by adjusting the pH of the product or other characteristics to make the product unsuitable for microbial life.
Why is challenge testing important?
Preservative challenge testing is important for several reasons. First, challenge testing can help determine whether the chosen preservative system will be effective in controlling microbial contamination in a cosmetic product as formulated during its anticipated shelf life. Second, challenge testing can confirm the effectiveness of a product’s preservative system in dealing with contamination that might be introduced during and after the manufacturing process. Finally, challenge testing can provide assurances of consumer safety, even when a cosmetic product is subject to adverse conditions of use.
What does challenge testing involve?
Specific procedures for preservative challenge testing are detailed in three separate compendia. These include:
the United States Pharmacopeia (USP) National Formulary, Chapter 51, Antimicrobial Effectiveness Testing (typically referred to as USP 51);
the European Pharmacopeia (EP), Chapter 5.1.3, Efficacy of Antimicrobial Preservation; and
the Japanese Pharmacopeia (JP), Chapter 19, Preservative Effectiveness Testing.
In each of these compendia, the procedure for challenge testing involves inoculating multiple samples of a given cosmetic product with various microorganisms, with each sample inoculated with a different microorganism.
Using the procedures described in USP 51 as an example, challenge microorganisms applied in preservative challenge testing include:
Staphylococcus aureus (a bacterium)
Escherichia coli (a bacterium)
Pseudomonas aeruginosa (a bacterium)
Candida albicans (a fungus-yeast)
Aspergillus brasiliensis (a fungus-mold)
Cosmetic samples are first evaluated for the presence of any microorganisms or pathogens that may have been introduced during the manufacturing process. Then, the samples are inoculated, each with a different microorganism, and incubated for an extended period of time. For most cosmetic products (Category 2 under USP 51), the total incubation period is 28 days.
Samples inoculated with either of the two fungi are then evaluated at 7, 14 and 28 days following their initial inoculation. To pass challenge testing, fungi inoculated cosmetic samples must not show any increase in microbial concentrations in any of these evaluations over baseline concentration levels. For samples inoculated with any of the three bacteria, preservative challenge testing requires not less than a 2.0 log reduction in microbial concentration from the initial count by day 14, and no increase in microbial concentration levels at day 28 over those measured at day 14.
The specific challenge microorganisms, the test intervals and the passing criteria detailed in USP 51 vary somewhat from those presented in the EP and JP compendia. For manufacturers of cosmetic products subject to the testing requirements of more than one compendia, a preservative challenge testing plan should be developed that accounts for these differences and reduces the need for duplicative testing.
Summary and conclusion
Preservative challenge testing is just one of many different tests to which cosmetic products are subject. For further information about the different types of tests applicable to cosmetics, see our white paper, Cosmetics Testing: How You Can Ready Your Cosmetic Products for the Global Marketplace, which is available for download here.