Humidity is the variable that decides whether a capsule serum survives its shelf life, and it is the one least often discussed in a sourcing conversation. Capsules do not fail because the formula was wrong nearly as often as they fail because water moved where it should not have.
This article explains what is actually happening inside a capsule, which process points control it, and what to ask a manufacturer beyond "do you control humidity".
Water moves between the fill and the shell until equilibrium
A softgel capsule is two compartments separated by a membrane that water can cross: an anhydrous or low-water fill, and a shell that contains water as part of its structure. From the moment the capsule is formed, water moves between them until the two reach equilibrium.
That equilibrium is the product. If it lands in the right place, the capsule is flexible, seals well and stays stable. If it lands too wet, capsules soften, stick and leak. If it lands too dry, shells go brittle and crack. Everything in production is aimed at landing on target and staying there.
Water activity versus moisture content: the distinction that matters
Moisture content is how much water is present. Water activity is how available that water is - how much of it is free to move and to support microbial growth. The two are related but not the same, and confusing them leads to wrong decisions.
For cosmetics, both matter for different reasons. Microbiological safety is governed largely by water activity, because microorganisms need available water. Texture, hardness and brittleness are governed largely by how much water the shell holds and where it sits in the structure.
A very low water activity in an anhydrous fill is exactly why capsule serums often need no preservative in the fill. That is a real advantage of the format, and it is worth understanding before you accept a supplier who adds a preservative "just in case" to an oil fill that does not need one. Preservative strategy for water-based systems is covered separately in our preservative systems article.
Why the fill must stay nearly water-free
If the fill carries meaningful water, that water migrates into the shell, plasticising it beyond the design point. The capsule becomes soft and tacky, seals weaken, and leakage appears - sometimes weeks after packing, which is why the problem is often blamed on shipping rather than formulation.
This constraint shapes which actives are realistic in a capsule. Water-soluble actives at meaningful percentages are difficult. Oil-soluble actives, oil-dispersible powders, and encapsulated forms are the workable routes. It also rules out most botanical extracts supplied in water or glycerin unless a compatible anhydrous grade is sourced.
Gel mass water: the starting condition
The shell water is set when the gel mass is made. The ratio of film-former to water to plasticiser determines the starting moisture, and therefore how much has to be removed in drying.
Two things go wrong at this stage. A mass made too dilute means longer drying and a greater chance of over-drying the finished capsule. A mass with too little plasticiser produces shells that seal adequately but go brittle within months. Both are formulation decisions, not machine settings, and both are best fixed during development.
Drying: removing water to a target, not to a schedule
Drying is the slowest stage in capsule production and the one most often compressed under delivery pressure. It runs in two phases: an initial tumble phase that removes surface moisture and prevents sticking, then tray drying in controlled rooms where the bulk of the water leaves.
The correct end point is a target shell moisture, not a fixed number of hours. Ambient conditions change, batch sizes change, and the fill's own water activity changes how quickly equilibrium is reached. A factory that dries "for 48 hours" regardless is guessing.
Over-drying is as damaging as under-drying. Capsules taken too dry are brittle from day one, and they become worse as they age. Hardening behaviour over shelf life is a stability question, covered in our stability testing guide.
Room conditions: temperature and relative humidity
Two rooms matter: the drying room and the packing room. In both, temperature and relative humidity are controlled, and in both the risk is drift rather than a single excursion.
A packing room at high humidity is a common cause of capsules that leave the factory within specification and arrive soft. The capsules are dry when packed, then absorb moisture from the air before the pack is sealed - or absorb it through packaging that does not provide an adequate barrier.
Ask what the rooms are held at, whether it is logged, and what happens when readings go out of range. Logging is the point: a room that is "usually fine" is not controlled.
What happens when humidity control fails
| Symptom | Likely moisture cause | Where it came from |
|---|---|---|
| Capsules soft or tacky | Shell above target moisture | Short drying, or humid packing room |
| Capsules stuck together | Surface moisture at packing | Insufficient drying or humid packing room |
| Brittle or cracking shells | Shell below target moisture | Over-drying, or low plasticiser |
| Leakage appearing weeks later | Water migrating from fill to shell | Fill with too much available water |
| Dull or hazy surface | Moisture movement at the surface | Equilibrium shift after packing |
Measuring: what is actually checked
The measurements that matter are shell moisture at release, room temperature and humidity during drying and packing, and - for the fill - water activity where the formulation warrants it.
Moisture measurement methods vary, and results are method-dependent. What matters to a buyer is not the instrument but the record: is shell moisture measured per batch, is there a target with a range, and is the result available to you? A supplier who cannot produce batch moisture records is not controlling the variable.
Packaging as the second moisture barrier
Once a capsule leaves the drying room, packaging is what maintains the equilibrium. Blister packs with appropriate barrier film protect each capsule individually. Bottles are more exposed - every opening introduces ambient air, and a desiccant may or may not be appropriate depending on how dry the capsule needs to stay.
This is a compatibility question as much as a barrier question: the packaging should be tested with the actual capsule over time, in conditions that resemble the distribution route. The method is set out in our packaging compatibility guide.
What buyers should specify
- Target shell moisture at release, with an acceptable range.
- Fill water expectation, and confirmation the fill is anhydrous if that is the design.
- Drying end point defined by measurement, not by hours.
- Drying and packing room conditions, logged, with an out-of-range action.
- Packaging barrier requirement appropriate to your distribution climate.
- Batch records available on request.
If your distribution includes a hot or humid market, say so at the briefing stage. The shell, the packaging and the storage instruction on pack should be decided together, not sequentially.
Where our capsule capability fits
We produce capsule serums with defined shell moisture targets and controlled drying and packing conditions, across formats including moisturising oil capsules, ceramide essence capsules and brightening essence capsules. If your distribution includes a humid climate, tell us early and we will specify the packaging accordingly.
We work with brands on private label (your brand on a proven base), OEM/ODM contract manufacturing, and full custom formula development - whichever matches where your brand is today.
Tell us your target markets and storage conditions, and we will tell you what the capsule needs to survive them. Start at our inquiry page, email capsulebiotech@163.com, or reach us on WhatsApp at +86 158-7655-5319 (Capsule Series), +86 158-7655-5319 (Cosmetics Series).
This article is general manufacturing information, not regulatory or legal advice. Requirements vary by market, formulation and distribution route.