A capsule may appear to be one of the simplest components of a dietary supplement. From the consumer’s point of view, it is primarily a convenient way to deliver a specific portion of the product. From a technological perspective, however, the situation is more complex. The capsule shell has its own physical properties, contains a certain amount of moisture, and remains in direct contact with the internal formulation throughout the entire shelf life of the product.
For this reason, the choice between a gelatin capsule and a plant-based capsule should not be based solely on marketing expectations. The properties of the formulation, powder behaviour, manufacturing conditions, packaging method and intended storage period should also be taken into account. In some formulations, an improperly selected capsule shell may make production more difficult or affect the stability of the finished product.
The Capsule Is Part of the Formulation, Not Just Packaging
The capsule shell remains in direct contact with its contents for many months. This means that the properties of both components should be analysed together.
If the fill contains a highly hygroscopic mixture, the possibility of moisture migration between the contents and the capsule shell must be considered. If the formulation includes ingredients that are particularly sensitive to moisture, the properties of the capsule material itself become even more important.
For this reason, capsule selection should be treated as part of the overall product development process rather than as a decision made only shortly before encapsulation begins.
Gelatin Capsules and HPMC Capsules – The Basic Difference
Traditional hard capsules are primarily made from gelatin. An alternative is provided by plant-based capsules, which may be made from hydroxypropyl methylcellulose, commonly known as HPMC.
The difference is important not only because of the origin of the material. The two types of capsule shells also differ in properties related to moisture content and moisture exchange.
In practice, this means that the same powder mixture may require a different technological approach depending on the type of capsule used.
The Moisture Content of the Capsule Shell Matters
The capsule shell material remains in equilibrium with the humidity of its surroundings. Changes in storage conditions can therefore affect its mechanical properties.
At excessively low humidity, some capsule shells may become more brittle. Excessive humidity, on the other hand, may cause them to soften and change their mechanical behaviour.
The issue is not limited to the capsule shell itself. Moisture may migrate between the shell and the formulation inside. This is why, particularly in formulations that are sensitive to water, the entire system should be analysed: the powder, the capsule, the packaging and the storage conditions.
What Happens When the Fill Is Hygroscopic?
Hygroscopic ingredients have the ability to absorb moisture. When they are placed inside a capsule, they may affect the moisture balance of the entire system.
Under certain conditions, the capsule contents may absorb moisture from the surrounding environment or from the capsule shell itself. This can influence both the properties of the powder and the behaviour of the capsule material.
In more demanding formulations, selecting the right type of capsule may therefore be one of the factors that helps reduce the risk of unwanted changes during storage.
Capsule Brittleness Can Already Become a Problem During Manufacturing
A capsule has to withstand much more than the transport of the finished supplement. During manufacturing, it is separated, positioned, filled, closed and then transferred between subsequent stages of the process.
If the capsule shell is too brittle, cracking, deformation or closing problems may occur. Even a small increase in the percentage of damaged capsules can translate into measurable losses when production volumes are high.
For this reason, environmental conditions in the manufacturing area and the way empty capsules are stored before use can affect the repeatability of the process.
The Capsule Itself Will Not Solve Problems Caused by a Difficult Powder
A common mistake is to analyse encapsulation problems only from the perspective of the capsule shell. In reality, the properties of the mixture inside the capsule are equally important.
The powder should flow properly and allow a consistent dose to be filled. If the mixture is highly dusty, tends to cake or has poor flow properties, it may become difficult to maintain a consistent fill weight.
The behaviour of the powder depends on factors such as:
- particle size and particle size distribution,
- bulk density,
- hygroscopicity of the ingredients,
- the proportions of individual raw materials,
- the tendency to cake,
- the flow properties of the mixture.
Capsule selection is therefore only one element of a properly designed manufacturing process.
Capsule Size Should Be Based on Volume, Not Only on Weight
Two powders with exactly the same weight may occupy completely different volumes. This is mainly due to differences in bulk density.
For this reason, it cannot be assumed that a given product weight will automatically fit into a selected capsule size. The properties of the finished mixture should first be assessed.
This is particularly important in formulations that contain high amounts of active ingredients. If the powder has a low bulk density, achieving the intended dose in a single capsule may be technologically difficult or may require a larger capsule size.
Why Is It Not Always a Good Idea to Fill the Capsule to Maximum Capacity?
The theoretical capacity of a capsule should not be treated as a value that must always be used to the maximum. The manufacturing process requires a certain margin that allows repeatable dosing and proper capsule closing.
Trying to fit the maximum possible amount of a technologically difficult powder into a capsule may result in greater fill-weight variation, closing problems and contamination of capsule surfaces.
In some cases, a better solution may be to change the capsule size, increase the number of capsules per serving or modify the formulation to improve its technological properties.
A Plant-Based Capsule Is Not Automatically Better Than a Gelatin Capsule
The growing popularity of plant-based capsules means that they are often presented as a more modern solution. From a technological perspective, however, the choice should not be reduced to a simple division between a “better” and a “worse” capsule.
HPMC capsules may be appropriate, for example, for products intended for consumers who avoid ingredients of animal origin. Their moisture-related properties may also be beneficial in certain formulations.
Gelatin capsules, on the other hand, have been widely used in manufacturing for many years and may perform very well in many standard formulations.
The most important factor is the compatibility of the capsule shell with the specific product.
Final Packaging Also Matters
After manufacturing, capsules continue to respond to environmental conditions. The choice of final packaging therefore affects how well their properties are maintained during storage and distribution.
A bottle, jar, blister pack or another packaging format creates a specific barrier between the product and the environment. For formulations that are particularly sensitive to moisture, it may be necessary to use packaging materials with suitable barrier properties.
The stability of the product cannot therefore be assessed solely on the basis of capsule type. The capsule shell and the final packaging should form a jointly designed protection system.
What About Capsule Colour?
Colour is usually seen as part of product identity, but capsule appearance may also have a practical function. Capsules may be transparent, opaque or coloured.
If the contents have an uneven colour or may naturally change shade over time, using an opaque capsule may provide a more consistent appearance of the finished product.
At the same time, all materials and colourants used must be appropriately selected for the intended application and relevant requirements.
Stability Should Be Assessed for the Finished Product
The fact that a particular raw material is stable does not automatically mean that the complete capsule will maintain identical properties throughout its entire intended shelf life.
A finished product contains many interacting elements at the same time: active ingredients, excipients, the capsule shell, moisture, oxygen and packaging materials. Interactions may occur between them that are not visible when an individual raw material is assessed on its own.
This is why quality control and appropriate product verification remain important even when the ingredients used are well known and widely applied.
Capsule Selection Should Take Place Before Full-Scale Manufacturing
Changing the capsule type at a late stage of product development may require several product parameters to be reassessed. The behaviour of the mixture, process conditions, product appearance, packaging method or storage requirements may all change.
For this reason, the type and size of the capsule should ideally be determined during formulation development and pilot-scale testing. This makes it possible to evaluate not only whether the intended serving fits into the capsule, but also whether the filling process can be carried out consistently.
There Is No Single Capsule Suitable for Every Formulation
The choice between gelatin and HPMC capsules should result from an analysis of the entire product. The properties of the ingredients, the hygroscopicity of the mixture, its density and flowability, the intended serving weight, consumer requirements and the expected storage conditions all matter.
Only by considering these parameters together can a solution be selected that performs well not only during development, but also during full-scale manufacturing and throughout the intended shelf life of the supplement.
In practice, a capsule is therefore not simply a neutral container for a finished formulation. It is one of the technological components of the product, and that is why its selection should be made just as carefully as the choice of the other ingredients and materials.
FAQ
What is the difference between a gelatin capsule and an HPMC capsule?
The main difference is the material used. Gelatin capsules are made from animal-derived gelatin, while HPMC is a cellulose derivative. These materials also differ in properties related to moisture content and moisture exchange.
Are HPMC capsules better for moisture-sensitive ingredients?
In certain formulations, their typically lower moisture content may be beneficial, but this should not be treated as a universal rule. Stability should be assessed for the specific mixture, capsule type and packaging system.
What determines capsule size?
Not only the serving weight. The bulk density of the mixture is also very important because two powders with the same weight may occupy very different volumes.
Can a gelatin capsule be replaced with a plant-based capsule without changing the formulation?
It should not be assumed that such a change is always technologically neutral. The behaviour of the mixture, filling process, product stability and packaging method should be reassessed.