A standard soft gel capsule is a two-part system: an outer shell made primarily of gelatin, plasticizer, and water, surrounding an inner fill that is almost always a liquid or semi-solid, typically oil-based. That basic recipe has been around for well over a century, but the specifics of both the shell and the fill have become far more varied in recent years, with plant-derived shells entering the market and fill formulations growing increasingly sophisticated to improve how drugs and supplements are absorbed.
The Traditional Gelatin Shell
Gelatin is the classic gelling agent for soft capsule shells and remains the dominant one globally. It is an animal-derived protein, produced by breaking down collagen from the skin and bones of pigs or cattle (and occasionally fish). When dissolved in hot water and then cooled, gelatin forms a flexible, translucent gel that can be shaped into a sealed capsule around a liquid fill. This property makes it almost uniquely suited to encapsulation: it sets quickly, forms strong seals, dissolves readily in the stomach, and has a long safety record in food and pharmaceutical use.
But gelatin on its own would produce a shell that is too brittle and rigid to be practical. A soft gel capsule, by definition, needs to be pliable enough to squeeze slightly between your fingers without cracking. That flexibility comes from the other major components blended into the shell.
Plasticizers and Other Shell Ingredients
The softness in a softgel comes from plasticizers, most commonly glycerin (glycerol) or sorbitol, sometimes both. These small molecules work their way between the gelatin protein chains, increasing the space between them and allowing the chains to slide past one another more easily. The result is a shell that bends and flexes rather than snapping. Plasticizers also affect how much moisture the shell retains, its permeability to water vapor, and its melting behavior during manufacturing.
Water is the third major ingredient. A freshly manufactured softgel shell contains a substantial amount of water, which gradually decreases as the capsule is dried after production. The final moisture level in a finished capsule matters for shelf life and how quickly the shell dissolves when you swallow it.
Beyond these three essentials, shells often contain additional ingredients depending on the product:
- Colorants: Iron oxides, caramel, or synthetic dyes give capsules their characteristic amber, red, blue, or green tints.
- Opacifiers: Titanium dioxide has been the standard white pigment used to make capsules opaque. In the shell or coating of a capsule, it scatters visible and ultraviolet light, protecting light-sensitive contents from degradation and preventing the consumer from seeing what is inside.
- Flavoring or sweetening agents: Some chewable softgels include these in the shell.
Titanium dioxide also serves a less obvious function: it helps ensure that every capsule in a batch looks the same, which matters for product identification and patient confidence, and it aids in distinguishing genuine products from counterfeits.1Journal of Pharmaceutical Sciences. The Role of Titanium Dioxide (E171) and the Requirements for Replacement Materials in Oral Solid Dosage Forms: An IQ Consortium Working Group Review That said, the European Union banned titanium dioxide as a food additive in 2022 over safety concerns, which has prompted a search for replacement opacifiers in supplements and pharmaceuticals sold in Europe.
Plant-Based Shell Alternatives
Gelatin works well, but it creates problems for vegetarians, vegans, and people whose religious dietary laws restrict pork or beef products. Gelatin also raises concerns about bovine spongiform encephalopathy (BSE) risk, although regulatory controls have made this a negligible issue in practice. These pressures have driven a growing market for plant-derived softgel shells.
The most commercially successful plant-based shells use modified starch, carrageenan (a polysaccharide extracted from red seaweed), or combinations of the two. Pectin, derived from fruit peels, is another option. Beyond these, researchers have explored alginate, pullulan, cellulose derivatives, chitosan, gellan gum, and agar as shell-forming materials.2Macromolecular Materials and Engineering. Sustainable Shell Formulations as Alternative to the Conventional Soft Gelatin Capsules in Pharmaceutical and Nutraceutical Applications. A Review The shift to these materials has not just been about ethics and dietary compliance; plant-based shells can offer advantages in compatibility with certain fill ingredients, manufacturing flexibility, and sustainability.
Making a plant polymer behave like gelatin in a capsule shell is not trivial, though. Starch, for instance, tends to become insoluble again over time (a process called retrogradation) and does not handle heat or acidic conditions as well as gelatin. Chemical modification, such as hydroxypropylation of cassava starch, combined with a seaweed-derived gelling agent like iota carrageenan, can produce composite films with the right balance of mechanical strength, elasticity, and stability for a soft capsule shell.3International Journal of Biological Macromolecules. Synergistic functional properties of hydroxypropyl-modified starch and iota carrageenan for plant-based soft capsule shells The formulation work here is genuinely painstaking: plant-based capsules have to dissolve at the right rate, seal properly during manufacturing, and remain stable on a shelf for months or years.
What Goes Inside the Shell
The fill of a soft gel capsule is where the active ingredient lives, and it is almost always a liquid or a paste rather than a dry powder. The simplest fills are oils: fish oil in an omega-3 supplement, vitamin E dissolved in soybean oil, or coenzyme Q10 in a medium-chain triglyceride base. In pharmaceutical products, the fill can be far more complex.
A typical pharmaceutical softgel fill contains several classes of ingredients:
- Vehicle oils: Soybean oil, medium-chain triglycerides, olive oil, or specialty oils like pine essential oil serve as the base in which the active compound is dissolved or dispersed.
- Surfactants: Ingredients like polyoxyl castor oil help blend oily and watery components, forming microemulsions that improve how a drug dissolves in the gut.
- Co-solvents: Polyethylene glycol 400 (PEG 400) and similar compounds help keep the active ingredient in solution within the fill.
- Suspension agents: When a drug cannot be fully dissolved, it can be suspended as fine particles in the oil. Lecithin and beeswax are common stabilizers for these suspensions.
One increasingly popular fill technology is the self-microemulsifying drug delivery system, or SMEDDS. These are carefully designed mixtures of oils, surfactants, and co-solvents that, upon contact with water in the gut, spontaneously form extremely fine droplets. A study developing a SMEDDS for cyclosporine A (an immunosuppressant that is notoriously hard to absorb) used pine essential oil and medium-chain mono- and diglycerides as the oil phase, polyoxyl castor oil as the surfactant, and PEG 400 as the co-solvent, all encapsulated in a standard softgel.4Journal of Drug Delivery Science and Technology. Development of softgel capsules containing cyclosporine a encapsulated pine essential oil based self-microemulsifying drug delivery system The entire point is to present the drug to the gut lining already dissolved in microscopic oil droplets, rather than asking the body to dissolve a solid chunk of drug on its own.
Why Softgels Improve Absorption of Certain Drugs
This brings up one of the main reasons softgels exist beyond simple convenience. Many drugs and supplements are poorly soluble in water, which creates a bottleneck: a compound that will not dissolve in the watery environment of the gut cannot be absorbed efficiently through the intestinal wall. Softgels address this by delivering the active ingredient already dissolved in a lipid (oil-based) matrix.
Multiple studies have shown that lipid-based formulations in capsules significantly improve solubility, dissolution rate, and ultimately bioavailability of poorly water-soluble drugs compared to conventional tablets or dry-filled capsules.5PubMed Central. Lipid-based oral formulation in capsules to improve the delivery of poorly water-soluble drugs The lipid fill can also protect sensitive drugs from degradation by stomach acid or enzymes, improving stability on the way to absorption.
Softgels also solve a practical problem at the other end of the dosing spectrum. Some compounds are needed at very low doses, where measuring a tiny amount of powder accurately into a tablet is difficult. A liquid fill lets manufacturers dissolve a precise micro-dose into oil, then encapsulate a measured volume. This was highlighted in a review noting that the softgel form is advantageous for delivering ultra-low doses of a compound, as well as for encapsulating low-melting-point substances that would not survive tablet compression.6PubMed. Soft gelatin capsules (softgels)
How Softgels Are Manufactured
The dominant manufacturing method is the rotary die process, invented in the 1930s and still the industry standard. The process starts by melting the shell material (gelatin or a plant-based polymer, along with plasticizer and water) into a thick, warm liquid. This molten gel is cast onto rotating cooled drums, where it solidifies into thin, flexible ribbons. Two ribbons are fed from opposite sides into a set of rotating dies, and the liquid fill is simultaneously injected between them. The dies shape and seal the two ribbon halves around the fill, cutting out individual capsules in one continuous motion.
Temperature control during ribbon formation is critical. The gel needs to be warm enough to flow onto the drums smoothly but must cool and set quickly enough to have the mechanical strength for die cutting and sealing.7European Journal of Pharmaceutics and Biopharmaceutics. Temperature effects on ribbon characteristics in soft gelatin capsule manufacture If the ribbon is too warm, it sticks and tears; too cold, and it becomes brittle and does not seal properly.
The fill injection has its own challenges. When the fill contains suspended solid particles rather than a clear solution, the flow behavior of the suspension through the injection nozzle becomes important. One study examining calcium phosphate particles suspended in oil found that the viscosity and surface tension of the suspension affect how cleanly the fill stream breaks off at the injection point. If the stream does not break off cleanly, the trailing fluid tail can interfere with sealing the capsule, creating defects or waste.8International Journal of Pharmaceutics. Flow behavior of concentrated tricalcium phosphate suspensions in oil through injection for softgel encapsulation Adjusting surfactant concentrations (in this case, lecithin) helped optimize the viscosity to minimize these problems.
After the capsules are cut and sealed, they go through a drying phase, typically in tumble dryers and then on trays in controlled-humidity rooms. Drying removes excess water from the shell, bringing it to its final firmness and weight.
Cross-Linking and Shelf Life
Gelatin-based softgels face a well-known stability challenge called cross-linking. Under certain conditions, the amino acid side chains on one gelatin molecule form permanent chemical bonds with those on neighboring molecules, creating a tougher, less soluble network within the shell. The triggers include exposure to aldehydes (which can come from impurities in the fill ingredients, from flavoring agents, or from degradation of certain excipients), as well as storage at high temperatures and high humidity.9PubMed Central. Enzymes in the dissolution testing of gelatin capsules
Cross-linking is a real-world problem, not just a laboratory curiosity. A heavily cross-linked capsule can form a tough film on its surface that prevents the shell from dissolving properly in the stomach. In dissolution testing, this shows up as dramatically slower drug release, or sometimes no release at all. Research on the causes of retarded dissolution in soft capsule shells confirmed that both gelatin self-oxidation and aldehyde-driven cross-linking from additives are the main culprits.10Journal of China Pharmaceutical University. Studies on the cause of retarded dissolution of soft capsule shell For consumers, this means that storing softgels properly (cool, dry, sealed) matters more than you might think. A bottle of fish oil capsules left in a hot car for a week could develop enough cross-linking to noticeably slow how quickly the capsules dissolve after swallowing.
Moisture creates a different but related concern. Gelatin is hygroscopic, meaning it absorbs water from its environment. A study on gelatin shell leakage found that the equilibrium moisture content of soft capsules rises with temperature and humidity, and that leakage is actually caused by physical swelling of the gelatin rather than chemical breakdown. Under unfavorable storage conditions, the shell absorbs enough moisture to swell, lose mechanical integrity, and eventually leak the fill contents.11PubMed Central. How can the leakage phenomenon of gelatin-based soft capsule shells be estimated? The practical takeaway: the silica gel packet in your supplement bottle is there for a reason.
Enteric and Modified-Release Softgels
Standard softgels are designed to dissolve in the stomach, but some drugs are destroyed by stomach acid or cause stomach irritation. For these, an enteric coating is applied over the finished capsule. The coating resists acidic conditions in the stomach but dissolves once the capsule reaches the higher-pH environment of the small intestine.
One traditional enteric coating material is shellac, a resin secreted by lac insects. Shellac-coated softgels are gastric-resistant but have historically dissolved too slowly once they reach the intestine. Research found that adding pore-forming agents, particularly sorbic acid, to the shellac coating dramatically improved intestinal dissolution. The sorbic acid stays embedded in the shellac coating at low pH (the stomach) but leaches out at intestinal pH, creating pores that let the coating break apart quickly.12Journal of Controlled Release. Improvement in the disintegration of shellac-coated soft gelatin capsules in simulated intestinal fluid
On the plant-based side, researchers have developed entirely new shell formulations designed to be inherently enteric, not just coated after the fact. A capsule shell made from sodium alginate, carboxymethyl starch, and kappa-carrageenan was found to resist breakdown in simulated gastric fluid but degrade almost completely within about 15 minutes in simulated intestinal fluid, performing better than conventional gelatin softgels for enteric delivery.13International Journal of Biological Macromolecules. Sodium alginate/carboxymethyl starch/κ-carrageenan enteric soft capsule: Processing, characterization, and rupture time evaluation This approach is appealing because it eliminates the need for a separate coating step entirely, simplifying manufacturing and avoiding the dissolution-speed problems that coatings can introduce.
Does the Gelatin Source Matter?
Most gelatin for softgels comes from pigskin or cattle bones and hides. Fish gelatin is a smaller but growing segment, particularly for products marketed to consumers who avoid mammalian gelatin for religious or dietary reasons. The source of the gelatin is not just a labeling concern; it affects the physical properties of the shell.
The amino acid composition of gelatin varies depending on the animal source, particularly in the content of proline and hydroxyproline, two amino acids critical for forming the helical structures that give gelatin its strength. Mammalian gelatins and those from warm-water fish have higher proportions of these amino acids, which translates to more hydrogen bonding within the gel network, greater mechanical strength, and better thermal stability. Cold-water fish gelatins, by contrast, produce softer, weaker gels with lower melting points.14PubMed Central. Shell Formulation in Soft Gelatin Capsules: Design and Characterization This means a capsule made with tilapia gelatin will behave differently from one made with cod gelatin, and both will differ from pork-derived gelatin. Manufacturers have to adjust plasticizer levels, processing temperatures, and drying conditions to account for these differences.
For the consumer, the practical impact is subtle. You are unlikely to notice a difference between a pork-gelatin and a bovine-gelatin softgel. But fish-gelatin capsules, especially those from cold-water species, may feel slightly softer and could be more susceptible to sticking together in warm storage conditions. If you have ever opened a supplement bottle in summer and found the capsules clumped, the gelatin source and storage temperature likely played a role.
Softgels Versus Hard Capsules and Tablets
People sometimes wonder whether it matters that their supplement comes in a softgel rather than a tablet or hard capsule. The answer depends on what is inside. For compounds that dissolve well in water, a standard tablet works fine, and a softgel would offer no meaningful absorption advantage. The softgel format shines specifically for oil-soluble compounds, fat-soluble vitamins (A, D, E, K), omega-3 fatty acids, and drugs that are poorly water-soluble.
Hard capsules (the two-piece capsules you can pull apart) are usually filled with powder or pellets, though liquid-filled hard capsules do exist as a niche format. Softgels, being hermetically sealed, offer better protection against oxygen exposure, which matters for easily oxidized ingredients like fish oil. The sealed shell also means there is no headspace inside the capsule for air, further reducing oxidation. On the other hand, softgels are more expensive to manufacture than tablets, require specialized equipment, and cannot accommodate water-based fills because water would dissolve the gelatin shell from the inside. Plant-based shells have begun to loosen this restriction somewhat, but the vast majority of softgels still use lipid-based fills.
Another consideration is swallowing ease. Softgels are smooth, slippery when wet, and often oblong, which many people find easier to swallow than rough-surfaced tablets. This is a genuine compliance factor in clinical practice: patients who struggle with tablets may do better with softgels, and some manufacturers deliberately choose the format for this reason rather than for any bioavailability advantage.