Convection: Movement in Fluids

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Convection is heat transfer through a fluid medium either as moist heat (e.g. steam or water, milk or wine) and dry heat (e.g. air or oil). The method of transfer occurs with heated fluids being less dense than colder ones, so the hot molecules rise, lose energy by transferring it to other fluids or solid food molecules, then as they cool, they fall again. The same concept occurs in an oven with air and in hot fat in a frying pan or deep fat fryer.

Because water boils only at 100oC (212oF) and can heat up no further, the browning of the food does not occur. Consequently, boiled food has a less intense taste than grilled food, but it is a very effective way of heating up and cooking ingredients, because liquids provide a dense medium in direct contact with your food. Thin liquids (e.g consomméé) heat more rapidly than thick liquids (e.g. thick soups). Poaching is a gentler version of boiling, using water at just below boiling point. Steaming is effectively the same as boiling because the steam is still at water's boiling point, i.e. 100oC (212oF), but because a lid is often on top of the pan the pressure is slightly higher and so the heat can be a little over boiling point. Pressure cookers boil water under pressure so the boiling point of the water is raised to, for example, 120oC (248oF), so your food cooks much more quickly than with normal boiling.

Baking in an oven cooks by convection with the air circulating on the basis of hot air rising, i.e. convection. Pure convection is an inefficient method of cooking, hence many ovens are fan-assisted to move the air around more rapidly and remove any cold spots. A decently high temperature can be achieved of up to 200oC (392oF). This results in food with a pleasing brownness to them and a rich flavour.

Further Reading

  • Davidson, A. (1999) The Oxford Companion to Food, Oxford University Press. ISBN 0192115790.