Stability and Compatibility Testing

Stability testing establishes whether a cosmetic product will remain physically, chemically and microbiologically what it was on the day it was made, for as long as it is on the shelf and for as long as the consumer uses it. It is the data behind the date of minimum durability or the period after opening on the label, and it is a required section of the Product Information File.

Oxford Biosciences runs stability and compatibility studies in its own laboratory under the direction of our Head of Laboratory Sciences. Results go to the safety assessor preparing the CPSR, not to a certificate that the assessor then has to interpret at one remove.

Regulatory basis

In the EU and UK, Annex I, Part A, Section 3 of Regulation (EC) No 1223/2009 requires the safety information to include the physical and chemical characteristics of the cosmetic product and its stability under reasonably foreseeable storage conditions. Article 19(1)(c) requires either a date of minimum durability where the product’s durability is less than 30 months, or, where it exceeds 30 months, an indication of the period after opening during which the product is safe to use. The stability study is the evidence for whichever of these is chosen.

The Regulation does not prescribe a method. The industry reference is the Cosmetics Europe (formerly Colipa) Guidelines on Stability Testing of Cosmetic Products, published jointly with the CTFA in 2004, which sets out the principles of accelerated and real-time testing and the parameters to be measured. ISO 22716:2007 on Good Manufacturing Practice requires stability to be considered as part of product release, and the SCCS Notes of Guidance treats stability as a precondition for the chemical safety assessment: a product whose actives degrade, whose pH drifts or whose preservative partitions into the packaging is not the product that was assessed.

The same study satisfies the shelf-life substantiation expected under MoCRA in the United States, the ASEAN Cosmetic Directive, GCC registrations and Australian supply, where regulators ask the same question in slightly different words.

What stability testing has to detect

A cosmetic product can fail in ways that are visible to the consumer and in ways that are not. The study is designed to find both.

Physical instability. Phase separation and creaming of emulsions, syneresis of gels, sedimentation of suspended particles, crystallisation of waxes or actives, changes in viscosity, texture, spreadability and pick-up, colour change, odour change, and changes to the appearance of the pack.

Chemical instability. Drift in pH, oxidation of unsaturated oils and fragrance materials, hydrolysis of esters, degradation of actives such as retinoids, ascorbic acid and its derivatives, peptides and botanical extracts, loss of preservative through hydrolysis or adsorption to the packaging, and the formation of degradation products with their own safety implications, of which formaldehyde release from certain preservatives and the formation of nitrosamines in the presence of nitrosating agents are the two the safety assessor looks for first.

Microbiological instability. A preservative system that is adequate at the start of the study but degrades, is consumed by contamination, or is lost to the pack. The stability study does not replace the ISO 11930 challenge test, but a challenge test run on an aged sample at the end of the study is the confirmation that the preservation lasts.

Packaging interaction. Loss of product weight through permeable packs, absorption of fragrance and preservative into plastics, extraction of plasticisers and other migrants into the product, corrosion of metal components, swelling or stress-cracking of plastics in contact with oils and solvents, delamination of laminate tubes, and the failure of closures, pumps, valves and seals.

The programme

Preliminary stress tests at T0

Before any sample goes into an oven, three quick tests give an early warning of the failures most likely to occur.

Centrifugation. Emulsions and suspensions are centrifuged at 3,000 rpm for 30 minutes at ambient temperature. An emulsion that separates under these conditions will almost always separate in storage; one that holds is a candidate for the full programme. The test is repeated at 40 °C for emulsions intended for warm markets.

Freeze-thaw cycling. Samples are cycled between -10 °C and 25 °C, and separately between -10 °C and 45 °C, with 24 hours at each temperature, for three complete cycles. This simulates transport and storage extremes and is the test that finds emulsions that break on crystallisation of the water phase and gels that lose structure on thawing.

Light exposure. Samples in clear glass and in the final pack are exposed to daylight through a north-facing window and to a UVA and UVB source, with a foil-wrapped control, for the duration of the study. Colour and odour are assessed against the control at each time point. Products with photosensitive actives, natural colourants or high fragrance loads are the ones that fail here.

Storage conditions

The full programme places samples, in the final packaging where it is available and in inert glass where it is not, at the following conditions in parallel:

Condition	Purpose
4 °C ± 2 °C	Refrigerated storage; detects cold-induced crystallisation, clouding and viscosity increase
20 to 25 °C, protected from light	Ambient control; the reference against which all other conditions are compared
30 °C ± 2 °C	Sub-tropical shelf conditions where required for export markets
40 °C ± 2 °C	Standard accelerated condition; 12 weeks at 40 °C is the conventional predictor of 12 to 18 months at ambient
45 °C ± 2 °C	Higher stress accelerated condition; 8 to 12 weeks at 45 °C is used to support a 30-month claim or a PAO where the ambient data are not yet available
Cycled -10 °C / 45 °C	Transport simulation, three cycles, as above

The relationship between accelerated and real-time storage is an empirical convention, not a law. The Arrhenius approximation that a 10 °C rise roughly doubles reaction rate holds for simple chemical degradation but not for the physical processes, such as emulsion coalescence and crystallisation, that cause most cosmetic failures. Accelerated data support a durability claim; they do not prove it. Where a client needs a 36-month claim, or where the product contains an active with a known degradation pathway, a real-time study at ambient runs alongside the accelerated one and the claim is revised, up or down, as the real-time data come in.

Measurements at each time point

Samples are examined at T0, 2 weeks, 4 weeks, 8 weeks and 12 weeks for the accelerated programme, and at 3, 6, 9, 12, 18, 24 and 36 months for real-time studies. At every point, against a retained T0 reference stored at 4 °C in the dark, the following are recorded:

• Appearance: homogeneity, phase separation, sedimentation, surface condition, crystallisation, and any change in the pack
• Colour: visual comparison under standardised daylight, with instrumental colour measurement (CIE L*a*b*) where a colour claim or a natural colourant makes precision necessary
• Odour: assessed by a trained panel against the reference, with particular attention to rancidity, loss of top notes and the sharp note of aldehyde formation
• pH: measured with a calibrated glass electrode on a 10 percent aqueous dispersion for anhydrous or viscous products and directly for aqueous ones; a drift of more than 0.5 units is investigated
• Viscosity: rotational viscometer with the spindle, speed and temperature recorded, because a viscosity value without its measurement conditions cannot be compared to anything
• Density or specific gravity: by pycnometer or density meter at 20 °C
• Weight: of the filled pack, to detect loss through permeation or evaporation, and reported as percentage loss against T0
• Microscopy: for emulsions, droplet size and distribution under phase contrast at 400x, which shows coalescence weeks before it becomes visible to the eye
• Active content: where the product carries a quantified active or a claim depends on one, assayed by HPLC or UV-Vis at T0 and at the end of the study, and at intermediate points where degradation is expected

Packaging compatibility

Compatibility is assessed in the final pack, filled as it will be sold, with an inert glass control alongside. At each time point the pack is examined for deformation, panelling, swelling, stress cracking, delamination, label adhesion and print stability, closure torque and seal integrity, pump and valve function, and corrosion of any metal component including springs and ball bearings in pumps. Product removed from the pack is compared with product from the glass control; a difference between the two is a packaging interaction and is investigated.

For products in contact with plastics for extended periods, particularly oils, solvents, fragrance-heavy products and those containing essential oils or limonene, sorption of the product into the pack and migration of pack components into the product are assessed by weight change, by GC/MS headspace analysis of the pack wall, and by comparing the fragrance profile of the packed product against the glass control at 12 weeks.

Deciding the label claim

The study supports one of two labelling outcomes under Article 19(1)(c).

A date of minimum durability is required where the product is stable for less than 30 months. It is expressed as the hourglass symbol or the words “best used before the end of” and a month and year, and it is a hard date after which the product should not be sold or used.

A period after opening is used where the product is stable for more than 30 months in its unopened state. It is expressed as the open-jar symbol with a number of months, and it states how long the product remains safe once the consumer has broken the seal. The PAO is a judgement rather than a measurement: it combines the stability data, the challenge test result, the pack type and the use pattern. A 12M PAO on a wide-mouth jar of a lightly preserved cream and a 24M PAO on the same cream in an airless pump can both be right. We make and document that judgement in the CPSR.

Where a product qualifies as microbiologically low risk under ISO 29621 and is also chemically stable, a PAO may be unnecessary, and the CPSR records why.

Interpreting a failure

A failure at 40 °C is not the end of a product; it is information. The most common outcomes, in order of frequency, are: emulsion instability corrected by adjusting the emulsifier ratio or adding a co-emulsifier or stabiliser; pH drift corrected by buffering; oxidation of oils corrected by adding an antioxidant or changing the oil, and, for essential oils, by GC/MS confirmation of which constituents are degrading; and packaging interaction resolved by changing the pack. Where a product fails, the report says what failed, when, and what the likely cause is, and our formulation team is available to correct it.

What the report contains

• Product, batch, formulation summary and pack description
• Storage conditions and the schedule of examinations
• Results of centrifugation, freeze-thaw and light exposure
• Tabulated quantitative results for every parameter at every time point and condition, against T0
• Photographic record at T0 and at the end of the study
• Packaging compatibility findings
• Conclusion on stability under the conditions tested and the recommended date of minimum durability or period after opening, with the reasoning
• Recommendation on whether a real-time study should continue and at what interval

What to send

Six to eight units of the product in the final packaging, from a single batch, plus 200 g or mL in bulk for the tests that require glass controls and instrumental measurement. The full formulation with percentages and the pack specification, including resin type for plastics, are needed before the study is designed. Where an active is to be assayed, the supplier’s method or certificate of analysis for the raw material is needed to set up the analytical method.

How it fits with the CPSR

The stability data populate Annex I, Part A, Section 3 of the safety report and, with the ISO 11930 challenge test, are the basis of the durability and PAO decision in Part B. Where we prepare the CPSR, the stability study and the assessment are written together, so that the assessor’s conclusion on stability rests on data the assessor has seen in full. Where a client brings stability data from another laboratory or from in-house testing, we review the conditions, the time points and the parameters measured before relying on them, and we say if the study is not sufficient to support the claim on the label.