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A cream that smelled exactly as it should a month ago smells a little different today. Its colour has changed, the texture is thinner and on the edge of the jar a stain that was not there yesterday has appeared.
Is it microbiology? Maybe. But it does not have to be.
The separation of water from the oil phase may be a consequence of physical instability of the emulsion, the darkening may be related to oxidation or a chemical reaction of one of the components and the change in odour for example to the oxidation of oils or fragrance. A visible coating or stain already resembles microbial contamination more, but without microbiological examination the origin of the problem cannot be reliably determined.
And the opposite also applies: a cosmetic product may look, smell and appear completely normal and yet no longer be microbiologically compliant.
That is precisely why, when formulating, it is not enough to deal only with the composition, fragrance, texture and physical stability. Microbiological safety is an equivalent part of a well-designed formulation.
This article follows on from our topic on preservatives in cosmetics and takes it a step further. We will not deal only with how to preserve the product, but above all with why cosmetics deteriorate, what microorganisms need for growth and why the safety and shelf life of the product is never determined by only one preservative.
Contents
Bacteria, yeasts and moulds need a suitable environment for growth and one of the basic conditions is sufficient available water.
Therefore, the microbiological risk of an emulsion, gel or toner differs fundamentally from the risk of pure oil or anhydrous balm.
If the formulation contains an aqueous phase – for example water, hydrosol, aloe juice, aqueous plant extract or another aqueous raw material – we have to deal with its microbiological stability.
However, this does not mean that it is enough to look at the percentage of water in the formulation. For microorganisms it is not only decisive how much water the product contains, but how much of it is actually available to them.
And this brings us to one of the most important concepts in the microbiological stability of cosmetics.
Water activity – aw (water activity) describes the amount of water that is available in the system for chemical and biological processes, including microbial growth. It is not the same as the percentage of water in the formulation.
Two products may contain a similar amount of water but have different water activity and thus different microbiological risk.
Water activity is expressed by a value from 0 to 1. Pure water has an aw very close to 1. The lower the value, the less water microorganisms have at their disposal.
Formally, aw is defined as the ratio of the vapour pressure of water above the product to the vapour pressure of water above pure water at the same temperature. However, for the formulator, the practical interpretation is more important: water may be present in the formulation, but it is not all equally available to microorganisms.
Some dissolved substances bind water and reduce its availability. These include, for example:
Therefore, for example, a concentrated glycerine or very salty system may have significantly lower aw than a classic emulsion containing a similar proportion of water.
We know this principle very well from food technology: jam may contain a considerable amount of water, but the high concentration of sugar reduces its availability. Salt acts similarly in highly salted products.
However, in cosmetics we have to be careful. A few per cent of glycerine in a moisturising cream will not turn it into a microbiologically stable product without preservation. Common concentrations of humectants usually do not reduce aw so much that a classic aqueous emulsion can be considered low-risk.
Reducing water activity is therefore one of the possible protection barriers, not a universal replacement for the preservation system.
Microorganisms also do not have the same requirements. In general, many bacteria need higher water activity, while some yeasts and moulds can grow even at lower aw values.
Therefore, reducing water activity may gradually limit the growth of individual groups of microorganisms, but there is no simple universal limit below which we can automatically label every cosmetic product as safe.
If low water activity is to form part of the professional microbiological argumentation of a product, aw should be measured on the finished product, not only estimated from the formulation. This may be particularly interesting for products such as:
When preserving cosmetics, people sometimes think too simply: there is water in the product → we add a preservative → finished.
Professionally designed microbiological protection is more complex. A principle is used that we can describe as a multi-barrier approach – hurdle technology.
Instead of basing the protection of the product exclusively on one preservative, we create several obstacles for microorganisms at the same time. Such barriers may be:
The individual barriers support each other. The aim is therefore not to use "the strongest possible preservative" or the highest possible amount of it. It is much more reasonable to design the product so that the preservation system works in the most favourable conditions possible."
In addition to classic preservatives, modern formulations often contain preservative boosters.
These are substances whose main declared cosmetic function does not have to be preservation, but which can support the antimicrobial protection of the formulation and increase the effectiveness of the entire preservation system.
They can work by various mechanisms. Some:
In cosmetic formulations we can encounter in this role for example the following substances:
Their strength lies mainly in combination with other barriers. For example, Ethylhexylglycerin is often used together with Phenoxyethanol. On its own, it cannot simply be regarded as a replacement for a classic broad-spectrum preservation system, but in combination it can improve its overall effectiveness.
Similarly, Glyceryl Caprylate or Caprylyl Glycol can contribute to antimicrobial protection, but their real effect depends on concentration, pH, type of emulsion and other components of the formulation.
This is an important point: the presence of a booster does not automatically mean that we can reduce the dose of the classic preservative. If we want to optimise the system or reduce the concentration of one of its parts, we must verify the functionality of the resulting combination on the finished product.
Another group that often remains in the background when preserving are chelating agents. Chelators bind metal ions – for example iron, copper, calcium or magnesium – which enter the product from water, plant extracts, mineral raw materials, trace impurities in raw materials or from production equipment.
They are interesting for the formulator for two reasons. Metal ions can catalyse oxidative reactions – faster oxidation of oils, colour change, degradation of sensitive active ingredients or the formation of undesirable odour – so binding them improves the chemical stability of the formulation. At the same time, many microorganisms need metal ions for the functioning of enzymes and metabolic processes; chelation makes them unavailable and, in some gram-negative bacteria, additionally affects the stability of the outer membrane and increases their sensitivity to other antimicrobial agents.
A chelator therefore is not a preservative and on its own will usually not solve the microbiological protection of the product. However, it is another barrier that can make the preservation system more robust.
For formulations oriented towards a more natural profile, reach for example for Sodium Phytate, a gentle alternative is sodium gluconate and a technologically efficient, biodegradable solution is Trilon M Max EcoBalanced. And again, the following applies: what matters is not the mere presence of a chelator, but its suitability for the specific formulation, the amount used and the pH. We discuss the selection in more detail in a separate article on chelating agents.
Microorganisms do not appear in the product by themselves. They have to get into it. From a practical point of view, we distinguish in particular primary and secondary contamination.
The sources may be:
With plant powders, extracts and some natural raw materials, the natural microbial load may be significantly higher than with highly purified synthetic raw materials. This is also why the preservative cannot replace good manufacturing practice.
The task of the preservation system is not to disinfect a poorly manufactured product. It is to protect a properly manufactured product during its storage and use. The higher the initial microbial load, the more demanding a task the preservation system has.
The second problem arises during use of the product. We scoop the cream with our fingers from an open jar. A body scrub stands in the shower and we use it with a wet hand. Water, droplets from the bathroom or impurities from the skin may get into the packaging.
A serum sealed in an airless package, where the user practically does not come into contact with the contents, is exposed to a completely different risk.
Therefore, packaging is not only a design or marketing issue. It is part of the microbiological design of the product. Airless dispensers, pumps and packages with a small dispensing opening can significantly limit secondary contamination. A wide open jar represents a higher risk.
Microbial contamination can cause:
However, based on these manifestations, it is not possible to say reliably whether we are dealing with bacteria, yeasts or moulds. Moreover, the same changes may also have a non-microbiological cause.
Separation of the emulsion may be a physical problem. Darkening may be oxidation. An unpleasant odour may arise from degradation of the oil. Therefore, the appearance of the product is not a microbiological test.
And this is particularly important: the absence of visible changes is not proof of microbiological purity. The product may be microbiologically non-compliant even before its appearance or odour changes.
If, however, visible mould appears on cosmetics, it is not enough to remove the affected part. The entire product must be considered contaminated and should no longer be used.
We do not deal with the pH of a cosmetic product only because of the skin or the stability of active substances. It can also fundamentally affect the preservation system.
A typical example is preservation systems based on organic acids, for example benzoic or sorbic acid and their salts. Their antimicrobial effectiveness is related to what proportion of the acid is present in the undissociated form. This ratio changes according to pH.
The result? The same concentration of preservative can work very well at one pH and significantly weaker at a higher pH.
Therefore, it is not enough to say: "I used 1% preservative." The right questions are:"
We measure pH on the finished product, and during development it is useful to also monitor its evolution during stability. Some formulations may slightly change pH over time.
A universal technological procedure for all preservation systems does not exist. The technical documentation of the manufacturer of the particular preservative always takes precedence.
In practice, however, one very common rule applies: many modern preservation systems are added towards the end of manufacturing, after the product has cooled to approximately below 40–45 °C. This is a good general formulation principle, not a universal law.
Some preservatives or components of preservation blends are sensitive to high temperature or prolonged heating, while other systems allow or require a different way of incorporation. We therefore always check:
For preservation systems sensitive to pH, it is advantageous to add them to an almost finished formulation, for which we know the approximate final pH. However, we perform the final pH check only after incorporating the preservative and other components, because the preservation system itself can also change the pH of the formulation.
A very important rule applies to preservatives: the recommended dosage of the supplier and the legislative limit are not the same.
The supplier of a commercial preservation system may, for example, recommend a dosage of 0.5–1.0%. This is a recommendation for a particular blend. However, European legislation lays down maximum concentrations of individual preservatives in the finished cosmetic product.
In the EU, permitted preservatives and the conditions for their use are listed in Annex V to Regulation (EC) No 1223/2009 on cosmetic products. The Regulation also stipulates that substances listed in this Annex and under the conditions laid down therein may be used as preservatives.
For example:
Specific limits and conditions of use are also laid down for parabens, formaldehyde donors and other regulated preservatives.
For the formulator this has a very practical implication. It is not enough to know: "This preservative is dosed at 1%." We need to know its INCI composition and the content of regulated preservatives in order to know what amount of individual substances will finally be present in the finished product."
The legislative maximum is not the recommended formulation dosage. It is the highest permitted concentration under specified conditions. And the opposite also applies: the fact that we are below the maximum legislative limit does not yet mean that the product is sufficiently preserved. Effectiveness must work in the specific formulation.
The preservative in the formulation does not exist in isolation. Its effectiveness can be affected by:
Particularly interesting is, for example, the distribution of the preservative between the oil and aqueous phase of the emulsion. Microorganisms need available water for growth, so we are interested in how much active preservative is actually available in the aqueous phase.
If a substance has a high affinity for the oil phase or is significantly bound into micelles formed by surfactants, its total concentration in the product may be correct, but its effective availability where we need it may be lower.
This is also why there is no simple equation: 1% preservative = safe cream.
Airless, pump, bottle with a narrow neck and open jar do not represent the same microbiological risk. In an airless system, the user practically does not contact the contents of the product. With a jar, at each use the following may get into the product:
The use of products in the bathroom or shower can be particularly risky. The choice of packaging can therefore influence how robust the entire preservation system has to be. From a microbiological point of view, we therefore design the packaging together with the formulation, not only after it has been completed.
Oils, oil serums, anhydrous balms or many body butters usually have a low microbiological risk, because they do not provide microorganisms with enough available water for growth. However, this does not mean: without water = automatically without risk.
Imagine an anhydrous sugar or salt scrub in a wide jar placed in the shower. The manufacturer did indeed fill it as an anhydrous product, but the consumer repeatedly reaches into it with wet hands and introduces small amounts of water. After several uses, the microenvironment in part of the product may no longer be the same as during manufacture.
When assessing the risk, we therefore look at:
An anhydrous product in a pump represents a different situation than an anhydrous scrub in an open jar in the shower.
This is one of the most frequent misunderstandings in home cosmetics making. Tocopherol – vitamin E – is an antioxidant, not an antimicrobial preservative. Its role is primarily to slow down lipid oxidation.
It can therefore help to:
However, it does not provide sufficient protection for an aqueous product against bacteria, yeasts and moulds. Oxidative stability and microbiological stability are two different things. A product can be excellently protected against oxidation and microbiologically non-compliant – or vice versa.
We cannot find this out with certainty from the formulation. We can select a suitable preservative, set the pH correctly, use a chelator, a booster and airless packaging – but we are still working with an assumption until we verify the antimicrobial protection of the finished product.
For this purpose a preservation efficacy test, commonly referred to as a challenge test, is used. In the test, the finished product is inoculated in a controlled manner with defined microorganisms and the evolution of their number is subsequently monitored over a specified period.
The aim is not to prove that cosmetics are sterile. Cosmetics do not have to be sterile. The aim is to verify whether their antimicrobial protection system can sufficiently suppress the microbial load.
The reference standard is ISO 11930:2019 – Cosmetics — Microbiology — Evaluation of the antimicrobial protection of a cosmetic product. The standard includes the preservation efficacy test as well as a procedure for evaluating the overall antimicrobial protection of the product. At the same time, it assumes that the classic challenge test is not intended for products with low microbiological risk, whose risk has been assessed as low on the basis of the relevant assessment.
In 2026, a third edition of ISO 11930 is being prepared, but for the time being this is a draft and the currently published standard remains ISO 11930:2019.
This is an important difference. "Contains water" does not automatically mean "add 1% preservative". And "does not contain water" does not automatically mean "I do not need to deal with microbiology". We always assess the specific product.
Instead of the question: "How many per cent of preservative should I add?" it is more useful to proceed systematically."
And this is precisely where preservation ceases to be a question of one raw material.
If you are making an emulsion, gel, toner, serum or other cosmetics with an aqueous phase, do not select the preservative only according to whether it is labelled as "natural
Look at the whole system: pH, product composition, water activity, type of emulsion, active substances used, recommended dosage, method of incorporation and also the packaging.
In our range of preservatives you will find various systems suitable for different types of formulations. For each of them we recommend working with the technical documentation of the manufacturer and respecting the recommended concentration, incorporation temperature and pH range.
When designing a more robust system, it is also worth looking at preservative boosters and chelating agents that can support a suitably chosen preservation system.
If you still do not know which preservative is suitable for your formulation, continue with our article on preservatives, where we discuss the individual types and their use in more detail.
The microbiological safety of cosmetics does not come into being at the moment when we add a preservative to the formulation. It starts with the quality of water and raw materials, continues with manufacturing hygiene and the design of the formulation itself and ends with suitable packaging, method of use and verification of the finished product.
Each barrier acts in a slightly different way and it is precisely by combining them that we can create a robust product without relying on a single substance.
Therefore, the question: "Which preservative is the best?
