How products are tested to stay clean: the preservative challenge test

Every time you dip a finger into a jar of cream, you add a few bacteria and fungal spores. A well-made product is built to handle that. The way companies check is surprisingly direct: a lab deliberately contaminates the product, then watches to see whether it fights back. It’s called a preservative challenge test.

Why a cream needs to fight microbes at all

Water-based products, which include most lotions, creams, shampoos and cleansers, are a comfortable home for microbes. The US Food and Drug Administration says microbial contamination is a common reason for cosmetic recalls, and that contaminated eye-area products and lotions have caused serious infections.

Preservatives, the ingredients that stop bacteria and fungi from growing, are the usual defence. But adding a preservative doesn’t prove it works. It can be weakened by other ingredients, by the packaging or by the product’s pH (how acidic or alkaline it is). A 2018 review of cosmetic preservation in the journal Molecules notes that surfactants, water activity, the level of solids and even the container can all change how well a preservative performs. The only way to know is to test the finished product.

How a challenge test works

The idea is simple, even if the lab work is fussy. According to the Molecules review:

• A measured amount of the product is inoculated, meaning deliberately seeded, with known amounts of bacteria, yeast and mould.
• Whenever possible, the product is tested in its original packaging.
• The containers are kept away from light at room temperature for 28 days.
• At set checkpoints, samples are taken and the surviving microbes are counted.
• The drop in numbers is compared with the acceptance criteria of the method being used.

A good result means the microbe count falls sharply and doesn’t climb back up later. A poor one means the organisms survive or multiply, and the formula goes back to the bench.

The five usual suspects

Labs don’t use random germs. They use reference strains from official culture collections, chosen to represent the kinds of organisms that cause real trouble. The review describes the typical line-up:

• Staphylococcus aureus, a bacterium that lives on skin and in the nose. Finding it in a product can point to contamination from people.
• Pseudomonas aeruginosa, a bacterium found widely in the environment that the review describes as highly resistant to many preservatives.
• Escherichia coli, a gut bacterium used as a marker of fecal contamination.
• Candida albicans, a yeast that represents yeasts in general.
• Aspergillus, a mould and a major cause of spoilage. The review names Aspergillus niger; the study below used its close relative Aspergillus brasiliensis.

A 2024 study in Scientific Reports used exactly this set of five, following the ISO 11930 method, an international standard for evaluating the antimicrobial protection of a cosmetic.

ISO 11930 and the other rulebooks

There isn’t one single global method. The Molecules review lists several: the European, American and Japanese pharmacopoeias (official books of standards for medicines), methods from industry associations, and ISO 11930, the international standard written specifically for cosmetics. The Scientific Reports authors point out that there is no universally recognized method for running challenge tests and interpreting results, but for cosmetics, ISO 11930 and the closely related industry-association methods are the ones commonly used.

The methods differ in details such as which days samples are taken and how large the drop must be. They all rely on the same measure, the log reduction: the difference between the starting count and the count at each checkpoint, on a scale where each step means ten times fewer microbes.

The review also recommends testing at more than one point: on a fresh lab batch, after storage in the final container (to catch interference from packaging) and on the first production batch, to pick up anything the factory process changes.

Canada’s rules: the outcome, not the recipe

Canada’s Food and Drugs Act sets the ground rules for cosmetics in two short lines. Section 16 says no one may sell a cosmetic that contains a substance that may cause injury to the user’s health. Section 18 says no one may manufacture, prepare, preserve, package or store a cosmetic for sale under unsanitary conditions.

Those sections describe the result that matters, not a particular lab method. The challenge test is how a responsible company shows its product will stay safe to use.

Passing isn’t the end of the story

A challenge test tells you the preservation system was strong when the product was designed. It doesn’t freeze time. The FDA’s guidance on shelf life lists what wears a product down after you open it:

• Dipping fingers in adds bacteria and fungi, which preservatives then have to control.
• Preservatives can break down over time.
• Mascara wands and other applicators pick up microbes every time you use them.
• Bathroom humidity and heat make it easier for microbes to grow.

Packaging helps. The Molecules review notes that jars and wide-open bottles are harder to protect, while closed systems such as airless pumps and tubes with narrow openings are less accessible to contamination.

The same testing applies to products that skip classic preservatives. The Scientific Reports team made shampoos, shower gels and a lotion protected by a combination of multifunctional ingredients, meaning ingredients with another main job that also slow microbial growth. All passed challenge testing with results similar to conventionally preserved versions. Whatever the preservation strategy, the test is the same.

What this means when you shop

• A water-based product with no preservation system at all would be a risk. A well-tested one is not.
• Pumps and tubes are kinder to a formula than open jars, especially for products you’ll use slowly.
• Don’t add water to a product or share eye makeup.
• Throw out anything that has changed colour, separated, or started to smell odd.

If a product ever causes redness, pain or swelling, especially around the eyes, stop using it and see your doctor or pharmacist.

The thing to remember is that preservation isn’t a guess. Companies can deliberately contaminate their products with tough, well-known microbes and count what survives over four weeks. A product that passes is built to cope with everyday use, and your storage habits do the rest.