Why does softgel production need high-viscosity gelatin?

Beyond Biopharma | Pharmaceutical Gelatin FAQ

Short answer: Softgel production needs high-viscosity gelatin because the shell is cast as a continuous warm ribbon that must stretch, seal and hold a liquid fill without leaking or deforming. Higher viscosity supports a thicker, more even and more elastic film at practical process conditions, which improves ribbon stability and seal integrity and reduces rejects; gelatin viscosity is measured on a standard 6.67 % w/w solution at 60 degrees C and is normally reported on the certificate of analysis in milliPascal seconds (mPa-s).

How a softgel shell is formed

A softgel is made in one continuous operation. A warm liquid mass of gelatin, water and plasticiser is cast onto a chilled drum to form a ribbon of controlled thickness, two ribbons are brought together between rotary dies, and the fill is injected as the dies close and seal the shell around it. Everything depends on the ribbon: it must be uniform in thickness, strong enough to carry its own weight from the drum, and elastic enough to wrap around the fill and seal without pinholes or weak points.

Why viscosity is the ribbon property

Viscosity describes how the liquid gelatin mass flows, and flow behaviour is what controls the ribbon. A mass with low viscosity spreads too thinly and unevenly on the drum, producing ribbons that are hard to control, weak in places and prone to leaking seals. A mass with higher viscosity casts into a thicker, more even film that holds its shape better, seals more completely around the fill, and resists the deformation that creates leakers. In practical terms, high viscosity is what keeps a large, fast softgel line stable and its reject rate low.

How gelatin viscosity is measured

Gelatin viscosity is a standardised measurement, not an everyday process check. The usual method dissolves the gelatin at a defined concentration, commonly 6.67 % w/w in water, holds the solution at 60 degrees C, and measures the flow time or resistance under defined conditions. The result is reported in milliPascal seconds (mPa-s), and it appears on the certificate of analysis together with the test conditions. Because viscosity depends so strongly on concentration and temperature, two certificates are only comparable when the conditions are the same.

Viscosity and Bloom work together

Viscosity is often discussed together with Bloom, because the two describe different things. Bloom measures the strength of the cooled gel; viscosity measures the flow of the warm solution. Softgel grades are typically chosen with a moderate-to-low Bloom for a supple, elastic set, combined with a high viscosity so the warm mass still behaves well on the machine. That pairing, rather than either number alone, is what gives a softgel shell its combination of strength, stretch and seal quality.

Getting the balance right

Higher viscosity is not automatically better. If the mass is too viscous it becomes difficult to de-aerate, harder to pump and cast, and prone to trapping air, and the ribbon may be thicker than the target. The practical choice is a grade whose viscosity suits the ribbon width, line speed, fill type and die design of the specific machine, which is why softgel producers usually qualify a gelatin by trial rather than by certificate alone. The COA confirms the numbers; the line confirms the behaviour.

What to check when you buy

When comparing softgel gelatins, ask for the viscosity with its test conditions, the Bloom, and the moisture range on the certificate, and confirm that the grade is sold for softgel use with the viscosity band you need. Note that the same nominal Bloom can be offered at different viscosities, so state both when you request a grade. Your own process data, collected over a few production runs, is the final arbiter of whether the viscosity is right.

Related reading

See also what Bloom strength is best for hard capsules versus softgels and how to read a pharmaceutical gelatin specification (COA).