Key takeaways
- Accelerated testing at 40°C for 12 weeks maps to roughly two years at ambient.
- Real-time testing is the reference and cannot be replaced.
- Pull points and test parameters must be defined before the batch is made.
Stability testing establishes that a product remains acceptable, in its pack, for the shelf life claimed on the label. It is not a single test but a protocol: defined conditions, defined pull points, defined parameters and defined acceptance criteria. Without the protocol, the result is an opinion.
1. Accelerated conditions
40 degrees at 75 percent relative humidity for twelve weeks is the standard accelerated condition, and it approximates two years at ambient for many formulas. It is a screening tool, not a substitute for real-time data, because some failure modes only appear at ambient over years.
2. Real-time
Real-time testing at 25 degrees for the claimed shelf life is the reference standard. A launch can proceed on accelerated data with a commitment to complete real-time testing, and the real-time result can later contradict the accelerated one. That risk is accepted by most brands and should be documented.
3. Low temperature
5 degrees for twelve weeks checks for crystallisation, phase separation and viscosity increase. A cream that thickens in the cold may not pump in winter. Cold testing is often skipped and it is the source of seasonal complaints.
4. Freeze-thaw
Three cycles between -10 and 25 degrees check emulsion stability under shipping stress. A formula that separates after two cycles will separate in an unheated container in winter. The cycle test is fast and cheap and catches a specific class of failure.
5. Pull points
Accelerated: week 0, 2, 4, 8 and 12. Real-time: month 0, 3, 6, 12, 24 and 36 as applicable. Each pull tests appearance, odour, pH, viscosity, colour and microbial count, plus any claim-specific parameter. A pull point without a defined parameter is a photograph, not a test.
6. In the pack
Test in the actual pack, upright and inverted, because the pack is part of the system. A formula that is stable in a glass jar can fail in a tube because of the headspace and the seal. The pack should be the commercial pack, not a lab substitute.
7. Acceptance criteria
Define the acceptable range for each parameter before the test starts: pH ±0.3, viscosity ±20 percent, colour ΔE below 2, no phase separation, no odour change. Criteria set after the results are in are criteria adjusted to the results.
8. Compatibility
A pack compatibility test checks the interaction between the formula and the pack: swelling, stress cracking, leaching, label adhesion, seal integrity. It runs alongside the stability test and uses the same pulls.
9. Claims
A claim about shelf life, about a preserved active or about a colour must be supported by the stability data. A vitamin C serum that browns at week eight cannot claim a twelve-month shelf life in a clear bottle, whatever the label says.
10. Records
Keep the protocol, the batch record, the raw data and the report. The report should state the conclusion and the shelf life assigned. A stability file that consists of photographs and emails is not a stability file.
11. Where it fits
Stability runs across the night cream range; the microbial side continues in the challenge test entry.
Reference: Stability protocols and accelerated test records, 2025
