What Is a Preservative Effectiveness Test (PET)?

About the Author

Hey, I'm Stefano, founder of Transparency Foods — and I genuinely love digging into the science behind what makes products safe and trustworthy. One of the things I'm most proud of is that we actually run the tests most brands skip. In this post, I'm excited to walk you through one of them: the Preservative Effectiveness Test, or PET — what it is, how it works, and why it matters.

What Is a PET?

A PET is a scientific test used to measure how well a preservative works in a specific formula. It's run in a lab where the product is intentionally inoculated with harmful microorganisms — bacteria, yeast, and mold.

The test then monitors microbial counts over 28 days (sometimes longer), measuring the reduction of microbial growth over time. To pass, the product must show significant microbial reduction in the first half of the test, and then sustained inhibition through the remainder of the test period.

In other words, a PET validates whether a preservative actually works — not in theory, but in reality.

Limitations

A PET is the gold standard for preservative testing, but it has its limits.

Passing the test proves that the preservative works against the microorganisms used in the test, which represent typical contamination threats. That's a strong indicator of preservative performance — but it doesn't guarantee protection against every possible microorganism, especially rare or highly resistant strains.

Identifying which microorganisms might still pose a risk after a successful PET requires additional, product-specific research. There are other tools available to address those edge cases, and we'll cover that in a separate post.

What Determines Product Stability?

You might be wondering: what determines whether a preservative is even necessary? You've probably seen products on the shelf with no preservatives at all — how is that possible?

Product stability depends on a variety of factors: water activity (the amount of water available for microbial growth), pH levels, the production process, and packaging and storage conditions. All of these can influence whether a preservative is needed.

For example, a low water activity and pH level might make a product inhospitable to bacteria, but it could still be vulnerable to certain yeasts and molds. In that case, adding a preservative effective against fungi would make sense. On the other hand, a powder product might have a water activity too low to support any microbial growth at all — in which case a preservative isn't necessary, but packaging becomes the main concern (making sure no humidity gets in).

Product stability is a fairly complex topic — I'll cover it in more detail in a separate post.

Example: Monk Fruit Drops

Here's a real example from our own line.

Our monk fruit drops are highly stable. But during testing, we discovered the formula was still vulnerable to certain types of yeasts and molds. We evaluated our options and decided that adding a natural preservative was the right move — it would add a meaningful layer of safety and let customers take the product anywhere without needing to refrigerate it after opening.

We did the research, found what we believed was the best preservative system available, and ran a PET to verify it worked. The results came back a month later: it passed. We transitioned to that natural preservative system shortly after.

Conclusion

A PET is one of the most reliable ways to verify that a preservative actually does its job. I wanted to share a bit of what goes on behind the scenes — not just in making great products, but in making safe ones. Products you can trust.