A new cosmetic product can look perfect when it leaves the laboratory. But what happens six months later?

The cream may become thinner. A serum may change colour. An emulsion may start separating. Sometimes the formula remains perfectly stable, but the packaging begins to deform or interact with the product.

This is why stability and compatibility testing are such an important part of cosmetic product development.

The goal is not to prove that a product will never change. Instead, testing helps determine whether the changes that occur during storage and use remain within acceptable limits.

What is cosmetic stability testing?

Cosmetic stability testing looks at how a product behaves over time and under different environmental conditions.

Depending on the formulation, this can include monitoring:

• colour and appearance;
• odour;
• pH;
• viscosity;
• phase separation;
• crystallisation;
• weight changes;
• microbiological characteristics;
• and the stability of specific active ingredients.

The exact testing programme depends on the product. A lightweight serum, an emulsion-based cream and a sunscreen can have very different stability risks, so there is no single test protocol that works equally well for every cosmetic product.

Why does a formula become unstable?

There are many possible reasons.

Temperature, light, oxygen, pH and interactions between ingredients can all influence a formulation. An active ingredient may gradually degrade even when the product still looks completely normal.

Physical instability can also occur. An emulsion may separate, a product may become thicker or thinner, or crystals may form during storage.

This is why stability testing is more than simply looking at whether a cream still "looks good."

Accelerated stability testing

Waiting several years to see how a product ages is obviously not practical during product development.

For this reason, cosmetic laboratories often use accelerated stability testing. The product is exposed to selected stress conditions, such as elevated temperatures, to help identify potential problems more quickly.

Depending on the product, testing can also involve:

• low temperatures;
• freeze-thaw cycles;
• light exposure;
• and other relevant environmental stresses.

The Cosmetics Europe stability guidelines give elevated temperatures such as 37°C, 40°C and 45°C as examples, while emphasising that the appropriate conditions should be selected according to the product and scientific judgement.

Freeze-thaw testing can be particularly useful for identifying problems with emulsions and other formulations. Repeated freezing and thawing may reveal separation, crystallisation or even damage to the packaging.

Importantly, accelerated testing should not be treated as a simple formula for predicting an exact shelf life. Cosmetic formulations behave differently, and real-time storage data can also be important.

Stability Is Not Only About the Formula

This is where compatibility testing becomes important.

A formula may be stable in a laboratory glass container but behave differently in the packaging intended for sale.

Why?

Because the product and its packaging can interact.

Packaging materials can potentially influence the formulation through processes such as adsorption, migration, chemical interaction or inadequate protection against oxygen and water vapour.

The opposite can happen as well: the formulation may affect the packaging.

Depending on the product, this can result in:

• swelling or deformation of plastic;
• corrosion of metal components;
• deterioration of labels or printing;
• changes in colour or odour;
• loss of certain ingredients;
• or problems with dispensing.

The Cosmetic Formulation: Principles and Practice literature describes compatibility testing as an important part of evaluating the complete formulation and packaging system.

Why the final packaging matters

Testing a formula in a laboratory beaker or glass container is useful, but it does not tell us everything about the finished commercial product.

A serum placed in a plastic bottle may behave differently from the same serum stored in glass.

The type of plastic, the dispensing system, the closure and even the interaction between the formula and individual packaging components can matter.

For this reason, stability testing should, where possible, use the same packaging material intended for the final product. If several packaging configurations will be marketed, each may need to be evaluated.

Glass can also be used as a useful control. If a formulation remains stable in glass but changes in the intended plastic package, this can provide an important indication that the packaging may be contributing to the problem.