A softgel looks like a single piece, but the shell is itself a small formulation — a film-former, a plasticiser and water, cast or dropped around a liquid, oil or suspension fill. Get that shell formula wrong for the fill inside it and the capsule either leaks, hardens, or stops dissolving on schedule months after it left the factory.
This guide covers gelatin versus plant-based softgel shells, the plasticisers behind both, fill compatibility, the cross-linking problem that shows up during storage, and the seamless capsule — a softgel variant developed in Japan.
Health supplements are foods, not medicines, and are not a substitute for medical treatment.
💬 Planning a softgel product made in Japan? We check the shell, plasticiser and fill together before quotation. See Japan-made supplement OEM.
Gelatin softgels or plant-based softgels
Gelatin remains the default softgel shell for cost and processability; plant-based shells built from modified starch and carrageenan (or similar hydrocolloids) serve vegetarian, halal and kosher markets without the animal-sourcing questions gelatin carries.
| Shell type | Film-former | Typical use | Watch-outs |
|---|---|---|---|
| Gelatin softgel | Bovine, porcine or fish gelatin | The default for oil-based fills (fish oil, vitamin E, CoQ10) | Not vegetarian; halal/kosher status depends on species and certification; prone to cross-linking over time |
| Plant-based softgel | Modified starch with carrageenan (or a similar plant/algal hydrocolloid blend) | Vegetarian, halal and kosher positioning; oil-based and some suspension fills | Different processing window from gelatin; not every gelatin-formulated fill drops into a plant shell unchanged |
A plant-based shell is not a drop-in replacement for a gelatin shell on the same encapsulation line without adjustment — the film’s viscosity, drying behaviour and set time differ, so a fill and process validated on gelatin should be re-checked, not assumed, when the shell changes.
Plasticisers: glycerin and sorbitol
Glycerin and sorbitol keep a softgel shell flexible; the ratio between them, not just their presence, decides whether the shell stays pliable or turns brittle over the product’s shelf life.
| Plasticiser | Role | Typical effect |
|---|---|---|
| Glycerin | Primary plasticiser in most softgel shells | More hygroscopic (draws in moisture); higher levels soften the shell further |
| Sorbitol (or a sorbitol/glycerin blend) | Secondary or co-plasticiser | Less hygroscopic than glycerin; often used to fine-tune shell hardness and reduce tackiness |
The plasticiser-to-film-former ratio is normally fixed by the factory’s shell formulation team against the target shell hardness and the climate the product will be stored in — a softgel destined for a humid market needs a different balance from one stored in a dry, temperate one, and this is a specification detail worth raising before the first trial batch.
Fill compatibility
Oils and lipid-soluble actives are the easiest fills for a softgel shell; water-containing suspensions and certain reactive ingredients are the ones that need a compatibility check first.
| Fill type | Compatibility with a standard gelatin or plant-based shell |
|---|---|
| Pure oil (fish oil, plant oil, oil-soluble vitamins) | Good — the standard softgel application |
| Oil suspension of a powder active | Generally good, but particle size and any water content in the powder need checking |
| Anhydrous liquid with reactive aldehydes, ketones or reducing sugars | Higher risk of cross-linking (see below) — needs a stability trial |
| Anything containing free water | Poor fit for a standard softgel shell; usually reformulated or shell-modified |
Cross-linking: the ageing problem
A gelatin softgel shell can slowly cross-link during storage when reactive substances from the fill migrate into it, forming a tough, poorly soluble film that delays or blocks the capsule from opening.
Cross-linking happens when reactive species — aldehydes formed from lipid oxidation, free amino acids, or reducing sugars in or near the fill — bond gelatin molecules together inside the shell, building a pellicle that resists the digestive fluid that would normally dissolve the capsule. It is a storage-time problem more than a manufacturing-day problem: a softgel can pass every release test at production and still develop reduced dissolution months later if the fill or its packaging environment supplies the reactive species. Formulation responses include controlling oxidation in an oil fill before encapsulation, choosing packaging that limits exposure to the reactive by-products of ageing, and in some cases modifying the shell itself with additional hydrophilic excipients to keep it more water-soluble as it ages. A plant-based (non-gelatin) shell avoids this specific ageing mechanism, since it has no free amine groups to cross-link in the same way — one of the reasons some manufacturers choose it for a fill known to generate aldehydes over time, independent of any vegetarian claim.
💬 Formulating a fill with a known oxidation risk (fish oil, certain botanicals)? Tell us the fill and we discuss the shell and stability plan before quotation.
The seamless capsule: a Japanese softgel format
The seamless capsule is a softgel variant developed in Japan using a multi-nozzle dropping method, forming a spherical, jointless capsule rather than the two-half, sealed-edge shape of a conventional rotary-die softgel.
Instead of casting two gelatin ribbons and sealing a fill between them on a rotary die (the standard softgel process), the dropping method ejects the fill and the film-forming solution together through a multi-nozzle head into a cooling and hardening bath, where surface tension pulls the droplet into a sphere that sets as it falls. The technique dates to the early 1970s in Japan and has since been extended to multi-layer seamless capsules — for example a two-layer capsule carrying an oil-based flavour, or additional layers separating incompatible ingredients within a single sphere. Because there is no seal edge, a seamless capsule can be made very small, which is one reason the format is used for confectionery-style, swallow-without-water supplement formats in Japan.
Before you ask for a quotation
A factory can quote a softgel accurately once the fill, shell type and target markets are fixed.
- Fill type: oil, oil suspension or other, and its oxidation profile
- Shell type: gelatin (and species) or plant-based
- Vegetarian, halal or kosher claims needed
- Target shell hardness and the storage climate of your market
- Rotary-die softgel or seamless capsule format
- Countries you will sell in
💬 Ready to specify a softgel made in Japan? Send us the fill and target markets and we match the shell and plasticiser to both.
Frequently asked questions
1. Can a fish oil softgel use a plant-based shell?
Yes — plant-based shells are commonly used for fish oil softgels aimed at vegetarian-labelled markets, though the oil itself is still fish-derived, so the capsule is vegetarian but not vegan unless the oil is also plant-sourced (e.g. algal oil).
2. Why does a softgel sometimes get harder to dissolve after a year on the shelf?
Usually cross-linking — reactive substances from the fill or its packaging environment slowly bond the gelatin shell into a less soluble film. It is a storage-time effect, which is why stability testing over real shelf life matters more than a release test on day one.
3. Is a plant-based softgel shell automatically halal and kosher?
Generally yes for the shell itself, since it avoids animal-derived gelatin, but the fill still needs its own check — an animal-derived oil or extract inside a plant-based shell does not make the whole product halal or kosher.
4. What is a seamless capsule, and is it only a Japanese product?
It is a spherical, jointless softgel made by a dropping method rather than a rotary die, developed in Japan in the early 1970s. Other manufacturers worldwide now use similar dropping technology, but the format is strongly associated with Japanese confectionery-style supplements.
5. Does glycerin or sorbitol change how a softgel feels or chews?
Yes — the plasticiser ratio affects shell hardness and tackiness, which matters more for a chewable or confectionery-style softgel than for a standard swallow capsule, and is worth specifying rather than leaving to the factory’s default.
Sources: general softgel shell and cross-linking pharmaceutical science — Naharros-Molinero et al., “Shell Formulation in Soft Gelatin Capsules: Design and Characterization”, Advanced Healthcare Materials, 2024 (advanced.onlinelibrary.wiley.com/doi/10.1002/adhm.202302250); cross-linking mechanism and mitigation strategies, Research Square preprint “Platform Technology and Alternative Strategies for Mitigating Cross-Linking in Soft Gelatin Capsules” and related ScienceDirect/PubMed record (pubmed.ncbi.nlm.nih.gov/42448177); seamless capsule dropping-method history and multi-layer development since the early 1970s in Japan, J-STAGE, “Capsule Technology” (jstage.jst.go.jp/article/isj/59/4/59_376/_article/-char/en); seamless capsule manufacturing method description, WO2005044174A1 (patents.google.com). Checked on 25 September 2026.