Preservative Efficacy Testing (PET/AET): USP & ISO 11930

Is Your Preservative System Effective?

Preservative Efficacy Testing (PET), also known as Antimicrobial Effectiveness Testing (AET) or a "Challenge Test," is arguably the most critical microbiological test for ensuring the long-term safety and stability of water-containing cosmetic, personal care, and pharmaceutical products.

Products packaged in multi-dose or multi-use containers are susceptible to microbial contamination introduced by the consumer during repeated use. An effective preservative system is essential to inhibit the growth of these contaminants, thereby protecting the consumer and maintaining product integrity throughout its shelf life.

Methodology

The principle of a challenge test is to intentionally inoculate, or "challenge," the product with a high concentration of a panel of specified microorganisms to simulate a contamination event. The standard panel typically includes Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa), yeast (Candida albicans), and mould (Aspergillus brasiliensis). The inoculated product is then held at a controlled temperature and sampled at specific intervals over a 28-day period (e.g., 7, 14, and 28 days) to determine if the preservative system is effective at reducing the microbial population to acceptable levels as defined by the relevant standard.

Navigating the "Clean Beauty" Preservation Challenge

The consumer-driven "clean beauty" movement has created one of the most significant technical hurdles for modern product formulators. The demand for "natural" and "preservative-free" products has led to the exclusion of many traditional, highly effective preservatives like parabens and formaldehyde-donors from brand and retailer "free-from" lists.

This shrinking palette of approved preservatives forces formulators to rely on alternative strategies, such as multifunctional ingredients (e.g., caprylyl glycol), pH manipulation, and other "hurdle technologies" to protect their products.

While these alternative systems can be effective, their performance is highly dependent on the specific formulation. Therefore, conducting a rigorous Preservative Efficacy Test is no longer just a final quality check; it is an indispensable tool in the R&D process. PET provides the critical data needed to validate that these novel and "clean" preservation strategies are sufficient to ensure product safety, preventing the risk of under-preserved products reaching the market.

Eurofins CRL partners with formulators to navigate this complex landscape, providing the reliable data needed to develop products that are both safe and aligned with modern market demands.

Preservative Efficacy Acceptance Criteria: USP vs. ISO 11930

The two most widely recognised standards for preservative efficacy testing are USP and ISO 11930. While they test a similar panel of organisms, their "pass/fail" acceptance criteria differ significantly. Understanding these differences is crucial for brands, as a formulation may pass one standard but fail another, impacting its suitability for different global markets. The following table compares the acceptance criteria for a topical product under USP against the more stringent Criteria A and the less stringent Criteria B of ISO 11930.

Note: “No increase” is typically defined as not more than a 0.5 log unit increase from the previous measured value.

FAQs – Preservative Efficacy Testing (PET/AET)

Q1. What is Preservative Efficacy Testing (PET) and why is it important?

Preservative Efficacy Testing, also called Antimicrobial Effectiveness Testing (AET), checks if a product’s preservative system can stop microbial growth. It ensures consumer safety and product stability during storage and use.

Q2. How is Antimicrobial Effectiveness Testing performed?

In Antimicrobial Effectiveness Testing, products are intentionally inoculated with bacteria, yeast, and mold. Samples are taken over 28 days to see if the preservative system reduces or prevents microbial growth as required by USP or ISO standards.

Q3. What is the difference between USP and ISO 11930 criteria?

Both USP and ISO 11930 use a similar microbial panel, but their pass/fail rules differ. USP focuses on log reduction by Day 14 and stability to Day 28, while ISO 11930 has stricter Criteria A and less strict Criteria B. Understanding these differences is key for global compliance.

Q4. Why is Preservative Efficacy Testing critical for “clean beauty” products?

With many traditional preservatives excluded from “clean beauty” lists, brands now use natural alternatives and multifunctional ingredients. PET provides proof that these systems are still effective, preventing under-preserved products from reaching consumers.

Q5. Do all products need Antimicrobial Effectiveness Testing?

Any water-based cosmetic, personal care, or pharmaceutical product in multi-use packaging should undergo Preservative Efficacy Testing. Without it, products risk contamination, recalls, and regulatory non-compliance.

Navigate Clean Beauty Challenges. Get Reliable Preservative Efficacy Testing Support.