Conduction.
Heat crosses food layer by layer, each passing it on to the next, and food is a poor conductor — not much better than water, a thousand times worse than copper.
Drop a 2 cm slice of potato into boiling water. Its surface reaches nearly 100 °C almost at once, but the centre does not know yet. Heat gets there only by conduction: each layer, warmer than the one inside it, passes energy inward, and the rate depends on how steep the temperature difference is and on the material's thermal conductivity. That is Fourier's law: heat flow per square metre equals conductivity times the temperature gradient.
Foods are mostly water, and water is a poor conductor — about 0.6 W per metre per degree, against 16 for stainless steel and 390 for copper. Fat and air conduct worse still. So inside food heat creeps, and the temperature forms a curve from hot surface to cool centre that flattens slowly as the minutes pass.
Below: the temperature across a 20 mm slice of the food you choose, from face to face, after the time you set in boiling water; and the thermal conductivity of foods beside metals and air.
- State Fourier's law in words.
- Read a temperature profile across a piece of food.
- Compare the conductivity of foods, metals and air.
- Explain why the outside of food cooks first.
Layer by layer
After four minutes in boiling water the centre of a 20 mm slice of potato is only at 53.6 °C; after one minute it is 21.8 °C and after ten 86.3 °C. The profile in the workshop is steep at first, with the heat piled up near the surfaces, and flattens as the centre catches up. The conductivity of raw potato, computed from its water, starch and protein, is 0.55 W/m K — copper conducts 712 times better.
Two properties together decide how fast the centre warms: conductivity, which lets heat through, and heat capacity, which soaks it up. Their ratio, the thermal diffusivity, is about 1.4 × 10⁻⁷ m²/s for most moist foods (1.40 × 10⁻⁷ for potato here). Because water dominates every moist food, potato, beef and fish all heat at much the same rate for the same thickness; what changes cooking times is mostly thickness (II.2) and the heat supplied at the surface (II.3).
Cut two slices of potato, one 1 cm and one 2 cm thick, and drop them into boiling water together. After 5 minutes cut each in half. The thin one is cooked through; the thick one has a translucent, raw core — conduction has not reached it yet.
Terms
- Conduction
- Heat passing through a material from warmer parts to cooler parts without the material moving.
- Thermal conductivity
- How easily a material lets heat through, in watts per metre per degree.
- Fourier's law
- Heat flow per unit area equals conductivity times the temperature gradient.
- Thermal diffusivity
- Conductivity divided by heat capacity per volume; how fast a temperature change spreads.
Sources
- Y. Choi and M. R. Okos, "Effects of temperature and composition on the thermal properties of foods", in Food Engineering and Process Applications, vol. 1, Elsevier, 1986, pp. 93–101.