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? Relative humidity (RH) of the drying room. RH directly controls how fast and in which direction moisture moves at the gummy's surface. Sure, moisture leaves 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 — steep gradient, moisture gets pulled 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. How fast? It's proportional to the vapor pressure deficit: the bigger the gap between surface and air, the quicker water escapes. 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 — gelatin or pectin, usually. 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. At KorNutra, we design curing rooms to hold RH steady because a 10% jump in RH can double the time to hit target moisture (typically 8–12% for a chewy gummy). Too high? The air gets nearly saturated, the vapor pressure gradient collapses, and the gummy holds onto moisture — resulting in stickiness, sugar crystallization on the surface (sweating), and uneven texture. Too low (say below 20%)? Evaporation is so fast the surface skins over while the inside stays wet, trapping moisture and causing defects.
Practical Implications for Curing
Manufacturers have to keep RH in a narrow sweet spot — typically 30–50% 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. That means dehumidifiers, HVAC, and moisture sensors aren't optional. Bottom line: 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.