Soft capsules continue to be one of the more popular dosage forms for dietary supplements. In fact, between 2016 and 2019, around 1,600 new soft capsule products were introduced into the U.S. supplement market.[i] It’s not hard to see why: they allow for easy swallowing, help mask unpleasant odors and tastes, and provide quick delivery of active ingredients. In addition, they offer brand owners a wide variety of product differentiation, from essential fatty acids, to vitamins and minerals, to botanical supplements and probiotics.
While soft capsule shells have traditionally been made using animal-based gelatin, many manufacturers and brand owners have begun to seek out plant-based alternatives to meet evolving consumer needs. Manufacturers looking for a vegan-friendly solution to soft capsule shells have several options, ranging from a variety of seaweed-based alternatives to modified starch. This article explores the history of gelatin-based soft capsules, emerging plant-based alternatives, and how manufacturers can incorporate vegetarian solutions into their own formulations.
A brief background
Soft capsules date back to the early 1800s, when one-piece capsules were handmade using mercury-filled leather molds. Within a year of being on the market, soft capsules had exploded in popularity and could be found throughout Europe and the United States.[ii] A century later, Robert Pauli Scherer invented the rotary die press to produce soft capsules, a manufacturing method that’s still used to this day. The process begins with the preparation of a molten gel mass, which is then cast into a thin film. The film is passed between two rotating dies and injected with a fill material, after which the dosage form is sealed.
Animal-based gelatin, the most traditional soft-capsule shell material, is typically sourced from bovine, porcine, or piscine skin and/or bone, and is available in a variety of gel strengths (i.e. 'bloom strength'). It forms a relatively low-viscosity gel that is molten at around 60°C with an acidic pH. Gelatin capsules are thermally sealed and, when wet, have a very high seal strength. However, under warm conditions (~30°C), the capsules can become tacky and fuse together; if left in excessively hot conditions (i.e. 40°C or greater), the capsules can melt completely within a few hours. The gelatin shell can also undergo interactions and crosslinking with the fill material, leading to discoloration or brittleness.
In the wake of mad cow disease (BSE) and African swine fever concerns, as well as the general consumer trend towards clean-label, non-animal products, the dietary supplements industry has begun to shift away from bovine and porcine gelatins in the last few years. Plant-based soft capsules made with modified starch or seaweed-derived carrageenan were invented to fill this void. They were quickly commercialized at production-scale to provide consumers with robust and stable soft capsules that satisfy vegetarian, vegan, non-GMO, Kosher and Halal diets.
Carrageenan, a naturally occurring polysaccharide extracted from red seaweed with a high molecular weight (200k-800k Da), provides an excellent vegetarian alternative for soft capsule shells. When manufacturing carrageenan soft capsules, a relatively high-viscosity gel is formed at around 85°C with a neutral pH. The wet seals of these capsules are initially weaker than gelatin capsules; however, after drying, they become strong and thermally stable at elevated temperatures without fusing or melting.
There are a variety of carrageenan types, and each one uniquely impacts capsule shell properties. The two most common types for soft shell capsules are Kappa and Iota. Kappa carrageenan forms strong and rigid gels, while Iota carrageenan forms rather soft, weak gels. A combination of the two, Kappa 2, has become a popular solution for improving soft capsule shell properties, as it yields superior soft capsule qualities and encapsulation efficiency.
Carrageenan has a long track record as a safe ingredient in food, dietary supplements and pharmaceutical products. However, it’s sometimes erroneously confused with poligeenan, which is an unnatural, low molecular weight (<20k Da) polysaccharide that cannot be produced biologically in the human body. Poligeenan is synthesised in the laboratory by subjecting carrageenan to extremely acidic conditions (0.9-1.3) at elevated temperatures (>80°C) for several hours.[iii] It is not authorized for use in any food application.
Another gelatin-alternative for vegetarian soft capsules is modified starch. Like carrageenan, starch is a naturally occurring polysaccharide extracted from plants such as corn, pea or tapioca. To form soft capsules of acceptable quality, starch is typically mixed with a certain amount of another gelation aid such as carrageenan, forming a high-viscosity gel that is molten (~90°C). However, the wet seals of those starch soft capsules tend to be relatively weak, as the capsules are not thermally sealed. Starch capsule shells are also not as elastic as gelatin or carrageenan, so they are limited to smaller capsule shapes to limit the level of stress on the capsule seals.
Compared to gelatin, plant-based solutions such as carrageenan or modified starches require higher temperatures to mix, transfer and encapsulate. Manufacturers can typically incorporate carrageenan or starch soft capsules into their existing gelatin operations with minimum capital investment by simply adding heat and temperature control systems to existing gelatin equipment. Once installed, the encapsulation process can easily switch between gelatin and non-animal capsule production as needs dictate, providing excellent product flexibility and allowing manufacturers to stay agile in this increasingly competitive market.
[ii] Podczeck F, Jones BE. Pharmaceutical Capsules. Second ed. 2004.
[iii] McKim JM et al. Clarifying the confusion between poligeenan, degraded carrageenan, and carrageenan: A review of the chemistry, nomenclature, and in vivo toxicology by the oral route. Crit Rev Food Sci Nutr. 2018:1-70.