The sticking of gummies during storage in humid conditions is a complex interplay of surface sweating and bulk softening, but the primary trigger is almost always surface sweating. Understanding the distinction is key to preventing the problem.
Surface Sweating: The Immediate Culprit
Gummy candies are hygroscopic; they readily attract and hold water molecules from the surrounding air. How humid the air must be before that happens depends on the formula. Each gummy has an equilibrium relative humidity (ERH), the level at which it neither gains nor loses moisture, and it takes up water only when the surrounding air exceeds that threshold. When that happens, the surface absorbs moisture and forms a thin film of water. This water dissolves sugars and other soluble components present at the surface, forming a concentrated, sticky syrup. The phenomenon is often called "sweating" because the gummy appears to exude moisture. That syrup acts like an adhesive, causing individual pieces to cling together, and even a small increase in surface moisture raises surface tack.
Bulk Softening: The Accelerant
Over a longer exposure period or under extreme humidity, moisture doesn't stay confined to the surface. It migrates along the water-activity gradient into the gummy matrix, raising the overall moisture content throughout the piece. This bulk softening reduces the structural integrity of the gel network, making the gummy more pliable and prone to deformation. A soft gummy on its own may not be sticky, but a softened gummy that is already sweating compresses under its own weight or stacking pressure, increasing the surface contact area and worsening the clumping. The two mechanisms feed into each other: surface sweating initiates sticking, and bulk softening makes the gummy more susceptible to it.
Why an Either/Or Answer Misses the Point
Treating the issue as simply "sweating vs. softening" overlooks the moisture migration gradient. The surface layer always has the highest moisture content when exposed to humid air, so the sticky interface between two gummies is a result of surface phenomena first. Bulk softening lowers mechanical resistance, but without the initial surface moisture layer, adhesion would be minimal. Mitigation has to address both at once. Drying to a target water activity, rather than a moisture content alone, holds the formulation at the right ERH. Polishing with carnauba wax or a wax-oil blend gives the surface a hydrophobic layer, while sanding with a hygroscopic acid blend such as citric acid can make sticking worse. Moisture-barrier packaging then keeps the finished piece at that equilibrium through distribution.
Water Activity: The Metric Behind Both Failure Modes
Water activity (aw) is the measurable property behind both failure modes. It equals equilibrium relative humidity divided by 100, so a gummy at aw 0.60 sits in balance with air at 60 percent relative humidity. Moisture moves from regions of higher aw to lower aw, which is why the damp surface feeds water into the drier center over time. For safety, manufacturers hold gummies below aw 0.85, a standard threshold for limiting microbial growth. Texture and anti-stick targets are tighter; manufacturer guidance commonly ranges from about 0.55 to 0.70. Specifying aw, and drying to it, gives QA a number that predicts both surface sweating and bulk softening before a batch ships.
At KorNutra, our manufacturing processes incorporate a deep understanding of these mechanisms. From precise control over gelatin or pectin setting and drying curves to the application of moisture-resistant finishing agents, we design gummies that resist sweating and maintain firmness, ensuring they stay free-flowing throughout their shelf life, even in challenging climates.