Preservative Boosters and Multifunctionals: Can They Pass a Challenge Test?

Many modern formulas lean on ingredients that do two jobs: they soften, dissolve or condition, and they also make life harder for microbes. Suppliers call them multifunctionals or preservative boosters in cosmetics, and brands often ask whether a booster can replace a classic preservative or at least reduce it. The honest answer is that these ingredients can help, but only a challenge test on your actual formula shows whether the combination is enough.

The short answer

Preservative boosters and multifunctional ingredients can strengthen a preservation system, but their effect depends heavily on the formula, so they can pass a challenge test in one product and fail in another. A supplier’s recommended dose is a starting point, not proof. Test the finished formula, ideally comparing the planned dose against a slightly lower and higher one, and keep the signed report for your safety assessor. Also check the regulatory position of every ingredient you rely on, because a substance used mainly to preserve may fall under the EU list of permitted preservatives.

What are boosters and multifunctionals?

A booster is an ingredient added to improve the performance of a main preservative so that the system works at a lower dose or against a wider range of microorganisms. A multifunctional ingredient has a primary job, such as moisturising, solvent action, skin conditioning or fragrance fixing, and also shows some antimicrobial activity as a side effect.

Commonly used examples include certain glycols such as pentylene glycol, caprylyl glycol and hexanediol, glycerol derivatives such as ethylhexylglycerin and glyceryl caprylate, and some plant-derived or fermentation-derived blends. The list changes constantly, and each ingredient has its own strengths: some act mainly against bacteria, others against yeasts and moulds.

Read also What Is a Cosmetic Challenge Test? Preservative Efficacy Testing Explained

Is a multifunctional ingredient a preservative in law?

In the EU, the Cosmetics Regulation (EC) No 1223/2009 contains Annex V, a positive list of preservatives allowed in cosmetics. Preservatives are generally understood as substances added exclusively or mainly to inhibit the growth of microorganisms in the product. Annex V sets maximum concentrations and conditions for each one.

• If an ingredient is on Annex V and you use it to preserve, the listed limits apply.
• If an ingredient is not on Annex V, you cannot add it mainly to act as a preservative. It is generally accepted as a multifunctional only when it has a genuine primary cosmetic function at the dose used.
• The intended purpose, and not just the ingredient name, matters. If the dose is far above what the primary function needs and clearly aimed at preservation, an authority may question it.

Rules and interpretation can change, and the answer may differ by ingredient, so ask your safety assessor to confirm the position in writing for your formula.

Why formula context decides everything

An ingredient that works well in a light lotion may do little in a surfactant-rich shampoo or an oil-heavy balm. The result depends on several factors:

• pH. Many preservatives, especially organic acids, depend on pH; boosters can be less sensitive, but pH still shapes the whole system.
• Surfactants and emulsifiers. They can bind or partition preservatives away from the water phase where microbes grow.
• Oils and powders. Lipophilic ingredients, clays and starches can adsorb active molecules or provide nutrients.
• Water activity and solubility. A booster that does not fully dissolve will not work as designed.
• Packaging and process. Temperature at addition, mixing order and container materials can change the effective dose.

This is why a result from a supplier’s demonstration formula cannot simply be copied to yours. Technical data sheets are written for model formulas under ideal conditions, and your product, with its own fragrance, actives and pH, is rarely identical.

Designing a sensible test programme

1. Start from the formula, not the ingredient. Define the base, pH and packaging first.
2. Screen a few options. Prepare the formula with two or three combinations of main preservative and booster. Some laboratories can run these in parallel as an orientation, which saves reformulation rounds.
3. Pick the best candidate and run the full test on the near-final formula, using a recognised standard such as ISO 11930:2019 or USP 51. This is the report that goes into your product file.
4. Check stability. Preservation must hold through the shelf life, so link the result to a stability study in the final pack.
5. Verify the finished product from time to time with microbiological specifications on production batches.

Read also Preservative-Free Cosmetics: Are They Safe and Legal?

Which result do you need?

ISO 11930 describes acceptance criteria A and B. Criteria A reflect the recommended level of efficacy, and criteria B are a lower level that may be accepted when A cannot be met, with a justification. A reduced preservative system relying on boosters is exactly where an assessor will look closely at which criterion was met and why. Aim for criteria A, and treat any fall-back as a decision to be justified in writing. Keep the screening results too, including the lower dose that did not pass, because they show your assessor how much margin the chosen dose really has.

Typical weak points

• Moulds and yeasts. Some boosters are weaker against fungi, so the mould organism in the test is often where a lean system struggles.
• Gram-negative bacteria. Organisms such as Pseudomonas aeruginosa can be demanding in water-rich products with little preservative.
• Batch variation. Natural blends can differ between lots; the test covers the lot you sent.
• Sensory limits. Doses high enough to preserve may feel sticky, affect scent or irritate skin, which is a safety and a quality topic.
• Ageing. Efficacy at day zero does not guarantee efficacy near the end of shelf life.

Claims and labelling

If your marketing says “preservative-free” or “no preservatives” while the formula relies on ingredients added to protect it, you risk making a claim that is hard to support. Claims must be truthful and substantiated under Regulation (EU) No 655/2013, and authorities tend to look at the real function of the ingredients. Describe your product precisely, and discuss the wording with your assessor before printing.

Read also Challenge Test Cost: What Affects the Price

When to repeat the test

Treat any of these as a signal to ask whether the old result still applies: a different grade or supplier of the booster, a changed dose, a new fragrance or active, a pH shift, or a switch to another packaging type. A formula change normally means a new test.

What to hand over with your order

List all ingredients with percentages, the pH, the planned shelf life and the packaging. Say clearly which ingredients are meant as preservatives and which as boosters. This helps the laboratory choose neutralisers and avoid false results, and it gives your CPSR assessor the information needed to judge the preservation approach.

Frequently asked questions

Can a booster replace the main preservative?

Sometimes the combination works with less of a traditional preservative, but whether it can stand alone depends on the ingredient, the formula and the regulatory position. Only a challenge test on your formula answers the first part, and your assessor the second.

Does a natural booster need a challenge test?

Yes. Natural or plant-based origin says nothing about how well a system protects against microorganisms. Water-containing products need a preservation test whatever the ingredient source.

Is one passing result enough for all my products?

No. The result applies to the tested formula. A similar product with a different base, pH or fragrance needs its own evidence or a documented justification from your assessor.

Key takeaways

• Boosters and multifunctionals can support preservation, but they are not guaranteed to pass in every formula.
• An ingredient used mainly to preserve may need to be on the EU list of permitted preservatives; confirm the position with your assessor.
• Test the near-final formula, ideally comparing a few dose levels, with ISO 11930:2019 or USP 51.
• Keep the signed report with the product file and repeat the test when the system, supplier or formula changes.