Thermal shock cracking happens when a gummy's surface cools faster than heat can travel from its still-warm core, and the stress that builds up exceeds the gel's strength. The safe cooling rate depends on the formulation (gelatin, pectin, or starch-based), the piece's thickness, and how hard the cooling air pulls heat from the surface, so no single number applies across every gummy. In a production cooling tunnel, a gummy can drop from a deposition temperature near 70-80°C to a setting temperature of 10-15°C within a minute or two, and that steep drop is enough to open visible fissures in many formulations, especially in thick pieces whose core is still warm. Cracking indicates that the material's thermal diffusivity is too low to move heat outward quickly enough, leaving a steep temperature gradient between surface and interior.
Why the Temperature Change Rate Triggers Cracks
When a warm gummy meets cold air, the outer layer contracts while the inner mass stays warmer and expanded. The tensile stress that results at the surface can exceed the gel's fracture toughness. The surface cooling rate is set by the tunnel's air temperature and velocity. Whether that rate cracks the piece depends on the material's thermal diffusivity, a property that measures how quickly temperature equalizes within a body. Hydrocolloid gels have low thermal diffusivity, so they cannot redistribute heat fast enough, and even moderate cooling rates build up dangerous stress.
What Crack Thresholds Reveal About Thermal Diffusivity
If a gummy cracks at a moderate cooling rate, the material is not moving heat fast enough, which points to low thermal diffusivity. Gelatin and pectin gels with high water content sit on the order of 10⁻⁷ m²/s, close to water's own value of about 1.4×10⁻⁷ m²/s. In practice:
- Low diffusivity (below water's roughly 1.4×10⁻⁷ m²/s) makes a gummy prone to thermal shock and demands slow, staged cooling.
- Diffusivity close to water's value lets a gummy cool faster without failing. Raising sugar solids works against this: sugar lowers thermal conductivity and diffusivity, so high-solids formulations need gentler cooling rather than a faster tunnel.
Measuring the cooling rate at which cracking first appears lets a process team solve for thermal diffusivity using transient heat conduction models. Instrument a sample piece with surface and core temperature probes, cool it at a known rate, and fit the measured curve to the model. The Fourier number (αt/L²) combines diffusivity, time, and thickness, and the Biot number captures how much thermal resistance sits at the air-to-surface boundary. That estimate matters at scale-up: it tells engineers how fast a cooling profile can run before the gummy's own heat-transfer limit starts producing cosmetic defects and inconsistent texture.
How Cooling Tunnels Are Designed Around the Diffusivity Limit
Confectionery cooling tunnels avoid thermal shock by staging the temperature drop instead of applying one cold zone. A typical profile runs gentle at entry, coldest in the middle, and slightly warmer at exit, so the surface never cools faster than the core can follow. Air velocity matters as much as temperature: cooling studies on hard candy found that high air velocity produces a non-uniform temperature profile that increases fragility and drives rejects, so the first zones keep airflow low. Humidity is the third variable. A surface that dries out during cooling forms a stiff skin, and a stiff skin cracks at smaller thermal strain than a moist one, so tunnels hold relative humidity high enough to stop the outer layer from drying and locking in stress. For a thick, high-water gummy, that means a slower, longer tunnel rather than a colder, faster one.