Most capsule briefs begin with the active. The question that decides whether a project runs smoothly arrives later, when a formulator asks about fill pH. A softgel is not an inert bag. Its shell is a hydrated polymer film in direct contact with the fill for two years or more, and pH is one of the few variables that can quietly destroy that film from the inside.
CapsulesBiotech produces single dose softgels for facial oil, eye serum, hair oil, body lotion, cream, and mask formats. Across those projects, shell choice has proved to be a chemistry decision before a marketing one. This guide covers each pH band, which fills cause trouble, and how to test before a batch is committed.
Why Shell Chemistry Reacts to pHGelatin is a protein. In water it behaves as an amphoteric material, swelling and holding a defined moisture level that keeps the shell elastic. Push the fill far into acid or alkaline territory and that balance shifts. Below roughly pH 3.5, acid hydrolysis shortens the protein chains, and the shell loses elasticity within months. Above roughly pH 8, alkaline hydrolysis produces a similar result from the other direction. The visible symptoms are brittleness, hairline cracks along the seam, and oil weeping into the blister.
Plant shells built on HPMC or pullulan behave differently. They tolerate a wider pH window, which is why acidic actives often land there, but they are more sensitive to moisture movement and need tighter control of fill water activity.
The Fill Variables Buyers Overlook- Water activity: shells hold roughly 5 to 8 percent water. A fill above 0.5 pulls moisture out of the shell and leaves it soft or tacky.
- Hygroscopic ingredients: glycerin, urea, and high solids niacinamide solutions migrate toward the shell and plasticize it.
- Aldehydes: many fragrance components cross link gelatin, forming a tough pellicle that slows disintegration.
- Reducing sugars: glucose and honey react with gelatin during drying, producing the same cross linked film.
There are three practical routes. The first is to adjust the fill: buffer the system, pre dry hygroscopic raw materials, and replace reducing sugars with sugar alcohols. The second is to change the shell: a plasticizer rich gelatin blend for mildly acidic fills, or a plant shell when the formula sits below pH 4. The third is to separate the two with a double shell or a lipid barrier layer, which costs more per unit but protects sensitive actives. A compatibility trial is cheap compared with a failed production run.
Three Tests Before You Commit- Accelerated stability at 40 degrees Celsius and 75 percent relative humidity for three months, checked at zero, one, two, and three months.
- Shell hardness and seam strength, measured on the same capsules to catch embrittlement before it becomes leakage.
- Disintegration and leak testing, including a visual and weight loss check after two weeks at elevated temperature.
Run these on the exact fill, shell, and blister. Moving from a lab sample to production changes shear, cooling, and drying time enough to shift results.
What This Means for Your BriefBring the pH target to the first conversation, not the third sample round. Tell us the target market, the positioning, and the actives you want to carry, and our team will propose a shell system, a fill pH range, a fill volume between 0.2 and 1.0 ml, plus a production schedule that fits your launch window. Single dose sealing, light protection, preservative free formulation, and precise dosing come with the format. The chemistry behind them is where the real work sits.