Key takeaways
- Gelatin shells are plasticised with glycerin, which can migrate into an anhydrous fill.
- HPMC shells hold less water and are suitable for vegan and halal claims.
- Stability must be run in the final shell material, not a proxy.
The shell is not inert packaging; it is part of the formulation. Gelatin shells are plasticised with glycerin or sorbitol to stay flexible, and over time that plasticiser migrates into an anhydrous fill, changing its texture and, occasionally, its taste.
HPMC — hydroxypropyl methylcellulose — is plant-derived and holds less water, which makes it the default for vegan and halal claims. It is also less prone to cross-linking with aldehydes, a failure mode that makes gelatin capsules insoluble in hot climates.
The practical consequence is that stability data from a gelatin bead cannot be used to support an HPMC bead, even with an identical fill. The moisture and plasticiser exchange is different, and the failure modes are different.
When you brief a supplier, specify the shell in the brief and require stability data on that shell at 40 °C / 75% RH for six months, plus a real-time arm. Anything less is a guess dressed up as a specification.
Reference: Shell supplier stability summaries, 2024
