are fundamental to cookery. In the most substantial, and most readable, modern treatise on the scientific aspects of cookery, McGee (1984) puts the point thus:
Cooking can be defined in a general way as the transfer of energy from a heat source to the food. Our various cooking methods—boiling, broiling, baking, frying and so on—achieve their various effects by employing very different materials—water, air, oil—and by drawing on different principles of heat transfer.
An assumption that the cook will have access to fuel, devices for generating and controlling heat, and supports or containers for the food during cooking is implicit in the concept of a recipe. Geography and technology dictate availability of fuels; in turn, these have intrinsic characteristics which have influenced the development of cuisines. They may burn fast (dead wood) or slowly (dried cow dung); hot (charcoal) or cooler (peat); be easily controlled (a gas flame) or require skill in their management (coal and other solid fuels). Electricity is one step removed from fuel, but is also important for generating heat for cooking. Fireplaces, ovens, and stoves have evolved in different traditions to deliver optimum heat using available fuels.
There are three basic ways in which heat can be transmitted:
Radiant heat is a form of pure energy known as the infrared to physicists, and found on the electromagnetic spectrum at wavelengths between microwaves and visible light; atoms in a substance absorb microwave energy and then release it in the form of increased motion, which we sense as heat. To cooks it means a naked flame, glowing coals, or a hot electrical element. Food is cooked by being close enough to the source to absorb energy across the intervening space. Fire, the most basic source of radiant heat, has been known to man for many thousands of years, and was probably used to roast meat spitted on green wood far back into prehistory. Although an inefficient method of transmission, radiant heat is still utilized for grilling (broiling) and barbecuing. This is partly because it is very intense (coals and electric elements heat up to about 1090 °C/1994 °F; gas flames to 1650 °C/3002 °F), catalysing browning reactions which give attractive flavours on the surface of the food. This method is also utilized in a tandoor and in old-fashioned bread ovens, whose linings take up heat from fires lit inside, and radiate it back to the food being cooked.
Radiant heat is energy travelling at a wavelength which makes all molecules vibrate and so heat up. Microwave ovens (see microwave cooking) also use radiation, but at a longer wavelength, and make only polar molecules (carrying a strong positive/negative charge), such as water, vibrate, heating it up. Since water only heats up to boiling before becoming a vapour, the food is cooked at less intense temperatures than those from conventional radiant sources, without the browning reactions and the flavours produced by them. This is why some microwaved foods have a soft, pallid appearance. But microwaves penetrate much further into the food, cooking it on the surface and to a depth of 2–3 cm (1″) simultaneously.
In conduction the item to be heated is placed in contact with a heat source and heat moves directly from one atom to another. An example is an electric griddle heated by an integral element to give a hot surface, which in turn heats the molecules of, say, oatcakes placed upon it. The hotplates of electric or ‘Aga’-type stoves heat pans placed upon them by conducting heat from one surface to the other. Conduction is also exploited by cooks who skewer potatoes before baking them; the skewer heats along its length and helps to cook the potato from inside. In practice, conduction is most important when considering cooking utensils, as some of the various materials involved (e.g. copper) are much better conductors than others (e.g. ceramics). In cookery, transmission by conduction is mostly used in combination with convection, the third way of delivering heat to food.
Convection requires a fluid medium and can be divided according to this into moist heat (using water, stock, milk, wine, or other liquid) and dry heat (relying on air or oil). As molecules in the medium heat up they become less dense and rise, lose energy as they cool, then fall again. The same principle applies to hot air in an oven (where a fan may be used to force the air to circulate and to even out the temperature) and hot fat in a frying pan or deep-fat fryer.
When moist heat is used, the temperature achieved can be no higher than boiling point (100 °C/212 °F, or slightly less at high altitude), so browning reactions do not occur, and boiled foods lack the intense flavour of grills; but it is an efficient method of heating, as liquids provide a dense medium directly in contact with the food. Thin liquids heat faster than thick ones, so consommé heats more quickly than thick soup. Poaching is a gentler version of boiling, using water at just below 100 °C.
To steam food, it is held over boiling liquid and the heat is transmitted by water vapour surrounding the food, and condensing on it. The temperature is effectively no higher than that of boiling water, although a tightly lidded pan has the effect of raising the pressure inside very slightly. Only a specially designed pressure cooker can take the temperature of water vapour much higher, say up to 120 °C (248 °F), when it will cook food much faster than conventional boiling.
Baking using the conventional (western) ovens relies on transmission of dry heat by convection, the air inside the oven heating up from elements in the oven walls and circulating around the food. It is an inefficient method of transmission, and fans are sometimes added to force the movement of air around the oven and even out cold spots. A relatively high temperature can be achieved, giving a pleasant brownness and flavour to many foods cooked this way.
In practice, all methods of cooking involve combinations of heat transmission methods: for instance the base of a pan of water over a low gas flame is heated by a combination of convection (through the air) and radiant heat; the heat is transmitted by conduction through the fabric of the pan itself, with an efficiency depending on the material of which the pan is made; and the water it contains heats by convection.
There is a final point. One could say that the objective of most acts of cooking is to raise the temperature of the innermost part of the food being cooked to a certain level. The question then arises: what is the relationship between distance to the innermost part and time required for cooking? The answer is that the time required does not vary in correct proportion to the distance but in proportion to the square of the distance. This is explained at greater length under fish cookery since that is a subject where it is particularly relevant and where it has attracted recent discussion.
Laura Mason has written about several aspects of British food in books including Sugar Plums and Sherbet (1998), Farmhouse Cookery (2005), and Traditional Foods of Britain (1999), which she co-authored with Catherine Brown.
McGee, Harold (1984), On Food and Cooking, New York: Charles Scribner's Sons.
McGee, Harold (1985), ‘Heat’, Journal of Gastronomy, 1/4.
