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Why is gelatin used to make hard capsule shells?
Beyond Biopharma · Pharmaceutical Gelatin FAQ
Short answer: Pharmaceutical gelatin is used to make hard capsule shells because it forms a strong, elastic, two-piece shell at low solids and dissolves quickly and predictably in stomach conditions. A shell filled with powder or granules is simply emptied in the body, so the capsule material mainly needs to protect, hold and release its contents reliably rather than taste or act itself.
What a hard capsule shell has to do
A hard (two-piece) capsule is basically an empty, dry container: a cylindrical body plus a shorter cap that locks over it. Its only jobs are to hold a measured dose of powder, granules, pellets or small tablets; to keep that dose stable and tamper-evident during storage and shipping; and to break open in the digestive tract so the drug can be absorbed. Because the shell is emptied rather than dissolved as a whole, formulators can pack several ingredients into one capsule, protect moisture- or light-sensitive actives, and mask taste — jobs that are hard for a single compressed tablet to do as easily.
Why gelatin is the classic shell material
Gelatin has been the default hard-shell material for many decades because its properties line up unusually well with what a shell needs:
- Strong, elastic films at low solids — a gelatin shell is typically formed from a ~14–20 % solution (commonly around 15–25 % solids in the process bath depending on the formulation) yet still goes through dipping, spinning and drying into a strong yet flexible shell. Very few film formers are as forgiving to machine as gelatin.
- High gel strength with controlled viscosity — different Bloom (gel strength) and viscosity grades let a capsule maker tune the shell for crispness, mechanical strength and machine speed. See the table below.
- Quick, reproducible dissolution — in simulated gastric fluid a conventional gelatin shell commonly disintegrates rapidly (often within minutes), which supports fast release of the fill.
- Thermo-reversible gelling — the shell sets when chilled and the pins of the dipping machine stay clean, giving clean demoulding at industrial line speed.
Gel strength and viscosity do different jobs
| Property | Role in a hard shell |
|---|---|
| Gel strength (Bloom) | Governs the firmness and physical stability of the dried shell; higher Bloom generally gives a stiffer, more dimensionally stable shell. |
| Viscosity | Controls film thickness and coating behaviour on the pins; film strength is also influenced by the molecular weight profile behind the viscosity. |
| Moisture | Too dry makes shell caps brittle and cracked; too wet makes them stick and lose shape. The shell moisture (commonly around 12–16 %) is balanced with plasticiser or storage humidity. |
Getting the moisture balance right
The most practical challenge with a gelatin capsule is not its strength — it is its moisture. Gelatin is hygroscopic, so shells are dried to a narrow moisture range and then kept in controlled, moderate humidity. If the environment is too dry the shells lose water and become brittle, cracking when the capsule is filled or dropped; if it is too damp they soften, distort and can stick to the filling equipment. Pharmaceutical gelatin grades for capsules are selected with good clarity and colour along with controlled gelation behaviour, and every lot is released against a certificate of analysis (COA) covering viscosity, gel strength, moisture, pH and microbiology, as required by the drug or excipient specification.
Quality-control language
Consistent capsule production rests on reproducible raw material. A pharma-grade shell gelatin is therefore tested lot by lot and the numbers quoted in a brochure — gel strength, viscosity, moisture range, dissolution behaviour, endotoxin and bioburden limits — should always be read against the supplier's COA and the pharmacopoeial or regulatory monograph that applies to the final product. Treatment parameters that are not described on the COA should be confirmed directly with the supplier or your regulatory contact rather than assumed from marketing text.
