Vol. I · What Cooking Does Plate 3 of 10temperature, heat, energy, heat capacity, heat transfer coefficient, air, water, oil, steamH

Temperature Is Not Heat.

Temperature is how hot something is. Heat is energy flowing from hotter to colder, and the medium — air, water, oil, steam — decides how fast it flows.

Reach into a 200 °C oven for a second and you feel warmth; touch boiling water for the same second and you are burned. The air is twice as hot, so why is the water worse? Because temperature is only how hot something is. Heat is energy flowing from hotter to colder, and how much flows each second depends on what carries it. Water carries heat into your skin (or into a potato) many times faster than air at the same temperature difference, and condensing steam faster still.

There is a second difference. Different materials need different amounts of energy to warm by one degree — their specific heat. A kilogram of water needs about twice the energy of a kilogram of oil and nearly ten times that of cast iron. The workshop shows both: how long a burner takes to warm each material, and how fast each cooking medium pushes heat into a cold food.

Below: minutes for a burner to warm one kilogram of each material by the temperature rise you set; and the heat flowing into each square metre of a food at 20 °C from air, water, oil and steam at their usual cooking temperatures.

What you will learn
Workshop
Left: minutes for a burner delivering the power you set to raise 1 kg of each material by the chosen amount (energy = mass × specific heat × rise). Right: heat flow into a food surface at 20 °C, kilowatts per square metre, from each medium at its usual temperature (typical heat-transfer coefficients).

Hotter is not the same as faster

With 1,200 W reaching the pot, a kilogram of water takes 4.7 minutes to rise by 80 °C and a kilogram of cast iron only 0.5: water needs 9.1 times as much energy per degree. That is why a pot of water is slow to boil and why it holds its temperature so well when food goes in — the same property that makes a heavy cast-iron pan worth preheating (II.5).

The right-hand panel is the more surprising one. Still oven air at 200 °C pushes heat into a food at 20 °C at about 2.2 kW per square metre; boiling water at only 100 °C pushes about 30 times as much, because water is dense and touches the surface completely. That is why boiled vegetables cook in minutes and a baked potato takes an hour, and why steam (II.3, III.6) and frying oil (IX.6) are such fast ways of cooking. The number that carries this is the heat-transfer coefficient: watts per square metre for each degree of difference between the medium and the food.

The distinction between heat and temperature was made clear by Joseph Black in Glasgow in the 1760s, who measured that the same heat warms different substances by different amounts — specific heat — and that melting ice absorbs heat without getting warmer — latent heat.

Terms

Temperature
How hot something is; heat flows from higher to lower temperature.
Heat
Energy in transit because of a temperature difference.
Specific heat
Energy needed to warm one kilogram of a material by one degree; water about 4.19 kJ, cast iron about 0.46 kJ.
Heat-transfer coefficient
How many watts flow into each square metre of food per degree of difference; small for air, large for water, larger for condensing steam.
Temperature says how hot; the medium decides how fast. Boiling water at 100 °C cooks faster than oven air at 200 °C.

Sources

  1. 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.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.