Gummy manufacturing has a make-or-break step: curing. That's when a soft, sticky blob turns into a stable, chewy final product. The single biggest environmental factor is relative humidity (RH) of the drying room. RH directly controls how fast and in which direction moisture moves at the gummy's surface. Moisture does leave through evaporation, but the real driver is the vapor pressure gradient between the gummy and the air.
Vapor Pressure Gradient: The Real Driving Force
Every gummy has free water molecules that create vapor pressure at its surface. The air around it also carries water vapor, with its own partial pressure. Moisture always moves from high vapor pressure to low. So when RH is low, the air's vapor pressure is far lower than the gummy's. That steep gradient pulls moisture out fast. When RH is high (say above 50-60% for many formulas), the air's vapor pressure is close to the gummy's, and evaporation slows way down or stops.
The Mechanism: Evaporation and Diffusion
It happens in two steps:
- Evaporation from the surface: Water molecules at the surface absorb energy and turn from liquid to vapor. That's a phase change, and it needs heat: the latent heat of vaporization. The rate is proportional to the vapor pressure deficit. The bigger the gap between surface and air, the quicker water escapes, and low RH widens that gap.
- Diffusion from the interior: As surface water evaporates, the area just below the surface gets drier. That creates a concentration gradient, pulling internal water molecules (still liquid) outward through the hydrocolloid matrix, usually gelatin or pectin. How fast depends on the matrix's porosity and temperature. The surface keeps acting as a sink, feeding the evaporation step.
Why RH Matters More Than Temperature Alone
Temperature boosts water molecules' energy and speeds evaporation, but RH can overrule it. The two are not independent: at a fixed moisture load in the air, raising the temperature drops the RH, so the room is controlled as a temperature-humidity pair. At KorNutra, we design curing rooms to hold RH steady because small humidity swings can stretch curing time well past the plan, and the target moisture window for a chewy gummy is narrow, typically 10–20%. When RH runs too high, the air gets nearly saturated, the vapor pressure gradient collapses, and the gummy holds onto moisture. The result is a sticky surface, sweating (moisture that migrates out and condenses on the surface), and uneven texture. When RH runs too low (below 20%), evaporation is so fast the surface skins over while the inside stays wet. That is case hardening, and it traps moisture and can leave a soft center behind a hard shell.
Water Activity Is the Number to Hit
Moisture percentage is only half the picture. Water activity (aW) measures the free, unbound water available for chemical and microbial reactions, the water that can move, crystallize sugar, or feed microbes. Two gummies with the same moisture percentage can behave differently if their water activity differs. Drying is done when aW lands in the target band. Typical targets for gummies sit around 0.60 to 0.67. A gummy cured only to a moisture number can still sweat or grow mold in the bag because its water activity stayed too high. Measuring aW closes that gap. For a contract manufacturer, this is the number that belongs in the production record, because it predicts shelf behavior better than moisture percentage alone.
Practical Implications for Curing
Manufacturers have to keep RH in a narrow sweet spot, typically 30–40% at around 20–25°C for standard gelatin gummies. The exact target depends on the formulation: gel strength, sugar content, hygroscopic ingredients like glycerin, etc. Air movement matters too, because it strips the humid boundary layer off each gummy and keeps the surface gradient steep. That means dehumidifiers, HVAC, air circulation, and moisture sensors aren't optional. RH decides whether the vapor pressure gradient can pull water out efficiently without wrecking the gummy's structure. Without that balance, curing is a crapshoot, too slow or too damaging. That's why RH is the linchpin of consistent quality.