Stability Testing

A formula that works on the bench has not been tested. What matters is whether it still works after a summer in a warehouse, a winter in a delivery van, three months on a windowsill and a year of being opened and closed. Stability testing is how a formulator finds that out before customers do.

The usual battery holds samples at several temperatures at once, cycles some through freezing and thawing, exposes others to light, and keeps a set at room temperature as the reference. At intervals the samples are examined for the failures that are visible — separation, colour, odour, thickness — and measured for the ones that are not, principally pH and the remaining content of anything prone to oxidation. Elevated temperature is used as an accelerator, on the reasoning that heat brings forward reactions that time would cause anyway. That makes it a proxy for shelf life rather than a measurement of it.

The container is part of the test. A formula can be perfectly stable in a beaker and still fail in its pack, because it interacts with the material or because the pack lets in enough air or light to undo it. This is why stability and pack compatibility are normally run together.

Preservation is checked separately, in a protocol where the product is deliberately inoculated with organisms and then watched to see whether it kills them.