Why is stability testing important?
As a formulator or product manufacturer you want to avoid the nightmare of precipitated pigments in a clear solution, suspended particles swimming on top of or settled at the bottom of the bottle, an emulsion separated in 2 or 3 phases, orange-brown spots on a soap bar or a shower gel with a water-like viscosity.
Even if you are not directly violate any health-related legislations, you definitely want to avoid customer complaints, product rejections and requests for refunds because your product has changed in appearance or functionality during its storage period and before the expiry of its shelf life.
The actual problem is, that the stability testing is not mandatory for cosmetics (in contrast to pharmaceuticals for instance) and because of this, there are no standards and no legal guidelines and instructions for stability testing.
Although not mandatory, it is a necessity for any cosmetic manufacturer, no matter whether you prepare bespoke 50 gram samples per order or 5000 tonnes of a certain product per batch to guarantee the visual, functional, toxicological and olfactory stability of the product.