A liquid center gives a gummy a burst of flavor and a different mouthfeel, but producing one by co-extrusion introduces rheological and kinetic constraints. At KorNutra, we've honed this process to balance the demands of the outer gummy shell and the inner fill.
How Co-Extrusion Works for Liquid-Center Gummies
Co-extrusion simultaneously pumps two streams, an outer gummy mass and an inner liquid fill, through a concentric nozzle. As the dual-layered rope exits, a cutting or stamping mechanism forms individual pieces. Because the cut exposes the fill, the cutter has to pinch each end shut, or the shell has to close over the cut face before the piece leaves the line. The gummy shell must set quickly enough to trap the liquid, while the fill remains fluid until encapsulation is complete.
Viscosity Constraints
Viscosity is the make-or-break parameter. The outer mass needs a high enough viscosity to hold its shape immediately after extrusion. If it's too thin, it will sag or rupture before setting. The liquid center must flow easily through the inner nozzle yet not be so thin that it leaks back or squirts out during cutting, and oil-based centers run slightly thicker and thin less as they warm. Temperature differentials matter too: the gummy mass runs at around 75-80°C, hot enough to thin the fill if the two streams aren't managed separately.
Setting Time
Setting time is how fast the gummy shell transitions from a flowable sol to a self-supporting gel. For starch-moguled gummies, that can take hours to days in a curing room while the starch slowly draws out water. A co-extruded liquid-center piece can't wait that long; the shell has to set before the fill migrates or the piece deforms. Rapid-set pectin systems gel in minutes rather than the 24-48 hours typical of gelatin, so they're the usual choice for the shell. That speed comes from:
- Cooling tunnels: immediate chilling drops the temperature and accelerates gel network formation.
- Gelling agent selection: low-methoxyl pectin gels quickly in the presence of calcium ions, and rapid-set pectin or gelatin blends are matched to the line's cooling profile.
- Mold design: strips are cut and sometimes dropped into forming trays that provide temporary structural support during the first moments.
If the setting time is too long, the liquid center migrates downward, creating an uneven shell thickness or causing leaks. Too short, and the shell can set before the fill has fully taken shape, leaving voids or a center that isn't liquid enough.
Keeping the center liquid through shelf life
Setting the shell is only the first half. Over weeks in a warehouse, the fill and shell exchange moisture, and that exchange decides whether the product holds up. If the fill's water activity is higher than the shell's, water moves into the surrounding gel, softening it and thinning the walls or collapsing the piece; if the fill is drier, it pulls moisture out and leaves the shell hard or leathery. Matching the two phases' water activity, or loading the fill with humectants that hold its moisture in place, keeps the center liquid without damaging the shell. The fill is engineered as carefully as the shell; a plain flavored syrup in the middle would migrate into the shell or dry it out over time.
KorNutra's Approach to Tackling These Constraints
At KorNutra, we optimize formulations for both phases by precisely controlling the degree of crosslinking and the cooling profile. Our process often uses tailored pectin or modified starch systems that trigger rapid gelation without compromising the soft, gummy texture consumers love. We also employ inline viscosity monitoring to keep the outer mass within a narrow window, and our liquid fill recipes are designed to be heat-stable and non-reactive with the shell. The result is a center-filled gummy that stays sealed and delivers its liquid center on first bite.