Thermal Conductivity of Food

Ice (k ≈ 2.22 W/m·K at 0 °C) conducts heat about 4× faster than liquid water (k ≈ 0.57 W/m·K). In crystalline ice, lattice phonon vibrations transfer thermal energy efficiently, whereas in liquid water, the less-ordered structure is less efficient at conducting heat.

Fresh apples have k around 0.40–0.45 W/m·K. Fresh tomatoes: 0.50–0.56 W/m·K. Potatoes: 0.50–0.55 W/m·K. These values reflect the high water content (85–95 %) of fresh produce and allow reasonably rapid heat penetration during blanching or cooling.

Lower thermal conductivity means heat penetrates more slowly from the surface to the centre of the product. For solid or semi-solid foods (blocks of cheese, canned vegetables), k determines whether the centre reaches the lethal temperature within the process time. Under-processing due to low k is a food safety risk.

The Choi-Okos model (1986) provides temperature-dependent equations for thermal conductivity, specific heat, density, and thermal diffusivity of each major food component (water, protein, fat, carbohydrate, ash, ice). Summing the compositional contributions gives total food thermal properties at any temperature.

Air has very low thermal conductivity (0.026 W/m·K). Aerated foods (bread, foam, marshmallow) have much lower effective thermal conductivity than their composition alone predicts. For bread, measured k is 0.07–0.18 W/m·K, far below the parallel model prediction for its wet dough composition, due to the insulating effect of the numerous air cells.

Thermal diffusivity (α) = k / (ρ × Cp), where ρ is density. It represents how quickly temperature equalises within the food. Typical values for most foods: 1.0–1.7 × 10⁻⁷ m²/s. Thermal diffusivity combines conductivity, density, and specific heat into a single parameter for heat penetration calculations.

Common methods: the line-heat source probe (transient hot wire method), the guarded hot plate apparatus (steady-state), or the modified Fitch apparatus for solid foods. The probe method (ASTM D5334) is most practical for food laboratories and gives results in minutes on a small sample.

Yes significantly. Cooking reduces moisture content (increases fat fraction in apparent composition), denatures proteins (changes matrix structure), and gelatinises starches. Cooked meat typically has slightly lower k than raw meat at the same temperature due to protein denaturation and moisture loss during cooking.

Use the lowest expected value for the product under the worst-case conditions (highest fat, lowest moisture, highest temperature) to ensure conservative (safe) process design. Under-estimating k leads to under-processing; using conservative low k values ensures adequate heat treatment is always achieved.