What each shell is made of.
Gelatin is a protein, made from bovine or porcine collagen, and it has been the standard capsule material for more than a century. It is inexpensive, it dissolves quickly and predictably, and the machines, the documentation and the supply chain are all built around it.
HPMC, hydroxypropyl methylcellulose, is a plant-derived cellulose. It was developed as a vegetarian alternative and has become a first choice in its own right, because it holds up better in the conditions that damage gelatin.
Pullulan is the third option, a fermented polysaccharide with very low oxygen permeability. It is more expensive and used where that barrier is worth paying for.
Where the differences show up.
Moisture is the big one. Gelatin capsules contain around 13 to 16 percent water and become brittle when they dry out, so a hygroscopic fill, or a dry climate, can crack them. HPMC carries 4 to 6 percent and tolerates both dry fills and humid storage far better.
Dissolution differs as well. Gelatin dissolves fast and consistently in the stomach. HPMC is slightly slower and can vary with the ionic strength of the fill, which is rarely a problem for a supplement but is worth knowing for a product with a claim that depends on release.
Then there is the market. Vegetarian, vegan, halal and kosher positioning all point to HPMC, and many retailers now expect it for a premium product. Gelatin remains the cost leader and is perfectly suitable where none of those apply.
Choosing for your product.
Start with the fill. Oils, hygroscopic powders and probiotics all have a case for HPMC on stability alone. A dry, stable mineral or vitamin blend sold at a competitive price has an equally good case for gelatin.
Then check the market and the claims you intend to make. A shell change late in a project means new samples, new stability work and new documents, so the decision belongs at the start with the formula, not at the end with the label.
Both shells come in the same sizes, colours and finishes, so nothing about the look of the product depends on the choice. Ask for samples of both, run them on the line, and let the fill decide.
