Thermal shock cracking in gummies occurs when the rate of temperature change creates internal stresses that exceed the material’s strength. While the exact threshold depends on the gummy’s formulation-gelatin, pectin, or starch-based-most gummy matrices begin to risk cracking when the surface temperature changes at roughly 30-50°C per minute during cooling, especially if the gummy is thicker and the core is still warm. In rapid cooling tunnels, a drop from 80°C to below 20°C in less than one to two minutes can cause visible fissures. The cracking indicates that thermal diffusivity is too low to conduct heat away quickly enough, leading to a steep temperature gradient between the surface and interior.
Why the Temperature Change Rate Triggers Cracks
When a warm gummy is exposed to cold air, the outer layer contracts sharply while the inner mass remains warmer and expanded. The resulting tensile stress at the surface can exceed the gummy’s fracture toughness. The critical temperature change rate is inversely related to the material’s thermal diffusivity-a property that measures how rapidly temperature equalizes within a body. Materials with low thermal diffusivity, like many hydrocolloid gels, cannot redistribute heat fast enough, so even moderate cooling rates build up dangerous stress.
What Crack Thresholds Reveal About Thermal Diffusivity
If a gummy cracks at a relatively moderate cooling rate (say, a 40°C drop in under 60 seconds), it signals that the thermal diffusivity is low-typically in the range of 10⁻⁷ m²/s for gelatin or pectin gels with high water content. In practice:
- Low diffusivity (≤ 1×10⁻⁷ m²/s) makes the gummy highly susceptible to thermal shock, requiring slow, staged cooling to avoid cracking.
- Higher diffusivity (approaching 2×10⁻⁷ m²/s, as achieved by adjusting sugar solids or incorporating conductive ingredients) allows faster cooling without failure.
By measuring the exact temperature change rate at which cracking first appears, you can estimate the material’s thermal diffusivity using the Biot number and transient heat conduction models. Essentially, the crack point is the moment when the Fourier number indicates insufficient heat conduction to relieve thermal strain. This insight is vital for production scale-up: it tells process engineers how to design cooling profiles that respect the gummy’s inherent heat transfer limit, ensuring cosmetic integrity and consistent texture without defects.