Moisture moves both ways.
A gelatin shell holds 13 to 16 percent water and needs it to stay flexible. A hygroscopic fill, such as a dried extract or a mineral salt, pulls that water out of the shell, and the capsule becomes brittle and cracks in the bottle. A wet fill does the opposite and softens it.
HPMC shells carry far less water and give it up far less readily, so they tolerate a wider range of fills and a wider range of storage conditions. For anything moisture sensitive, that alone can decide the material.
Whichever shell is used, the fill should be conditioned to the same relative humidity as the capsules before filling, and the packaging should keep it there. A desiccant and an induction seal are cheap insurance.
Oxygen, light and the fill itself.
Oils, omega-3s, probiotics and some vitamins degrade with oxygen. Gelatin and HPMC both let some through; pullulan lets through very little, which is why it is chosen for the most sensitive fills despite its price.
Coloured and opaque shells protect light sensitive actives. A clear capsule shows the fill, which sells well, and exposes it, which does not always store well. Titanium dioxide free opacifiers are available where that ingredient is a concern.
Some ingredients react with gelatin directly. Aldehydes, including some flavourings and degradation products, cross-link the protein and slow dissolution over time. That is a known problem with a known answer, which is to test for it early.
Test the product, not the shell.
Shell specifications describe the empty capsule. Stability belongs to the filled product in its final pack, so a stability study on the finished capsule, in the bottle it will ship in, is the only test that answers the question.
Accelerated conditions give an early signal in weeks. They do not replace real time data for a shelf life claim, but they catch a shell and fill that do not get on before the first production run does.
Ask for the shell’s own documentation as well: moisture specification, dissolution data and a certificate of analysis for each lot. A capsule that meets its specification is one less variable when a result needs explaining.
