A material can change chemically without an obvious change in its appearance. Conversely, visible changes can have several causes and do not identify a particular degradation pathway. A clear solution or an intact-looking powder is therefore not evidence that a peptide has retained its identity, content or function.

Stability testing is designed to answer a more specific question: does a material continue to meet defined specifications over time under stated conditions? FDA's discussion of expiration dating emphasizes stability-indicating testing. FDA: Expiration dating and stability testing Such testing must distinguish the relevant material from degradation products rather than merely produce a reassuring-looking peak.

A useful investigation begins with a baseline. Without a suitable reference and initial characterization, a later result may be difficult to interpret. The method should be able to detect changes relevant to the experiment, including loss of the intended component or growth of impurities. Method selection should follow the chemistry and intended use, not a universal list of warning signs.

Storage and handling history provide context for the analytical findings. Temperature, light, container interactions and other factors may be relevant, but their effects are product-specific. ICH stability guidance describes a framework for developing that evidence. EMA: ICH Q1A(R2), stability testing An unsupported storage claim cannot be repaired by checking appearance alone.

For laboratory work, unexpected appearance or an unexplained excursion should be handled through the laboratory's quality procedures. Do not infer that filtering, freezing or otherwise manipulating a questionable sample restores its original properties. This article offers no rescue instructions or generic shelf life. Its central lesson is that degradation is an analytical question, and a credible answer requires a fit-for-purpose method, traceable history and predefined acceptance criteria.