Why Food is Cooked and Heat Transfer

Why Do We Cook Food?

Most food can be eaten raw, but cooking transforms it in ways that make it safer, tastier and more nutritious. There are five core reasons why we cook food, and the written paper expects you to know all of them.

First, cooking makes food safe to eat. Raw chicken can harbour dangerous bacteria. When a chicken breast reaches a core temperature of 75°C, the heat kills pathogenic microorganisms and the meat becomes safe. Second, cooking develops flavours. A raw onion is sharp and pungent; fry it slowly and the sugars caramelise, creating a sweet, golden depth that no raw onion can match. Third, cooking improves texture. Raw potato is starchy and hard; boiling it gelatinises the starch and softens the cell walls, giving it the floury texture we expect. Fourth, cooking improves shelf life. Blanching vegetables before freezing destroys the enzymes that cause spoilage, extending how long the food stays usable. Fifth, cooking gives variety in the diet — the same chicken breast can be poached, roasted, grilled or stir-fried, each method producing a different colour, flavour and texture.

How Heat Is Transferred to Food

Heat does not simply “jump” into food — it travels by one of three distinct mechanisms: conduction, convection, and radiation. Understanding which mechanism is at work in a given cooking method is a high-tariff skill.

Conduction

Conduction happens when heat passes through direct contact, molecule to molecule. Place a frying pan on a hob. The burner heats the metal base of the pan; the molecules at the bottom vibrate faster and pass their energy along to neighbouring molecules. The heat travels upward through the pan until the cooking surface is hot enough to fry. When you lay a chicken breast into the hot pan, conduction transfers heat from the pan surface into the meat. Metals are good conductors — this is why a metal frying pan heats quickly. Food itself is a poor conductor, which is why the centre of a thick piece of meat takes much longer to cook than the outside.

Convection

Convection relies on the movement of a fluid — either liquid or gas. When you heat a pan of water, the water at the bottom nearest the heat source warms first, becomes less dense, and rises. Cooler water from the top falls to replace it, setting up a circulation known as a convection current. This is why a sauce heats unevenly if you leave it still — only the bottom is directly heated by conduction, and without stirring, the top stays cool. Agitation — stirring with a wooden spoon or whisk — forces the hot liquid to mix evenly with the cooler liquid, distributing the heat throughout the sauce. In an oven, hot air rises and circulates in the same way; a fan-assisted oven speeds this up by moving the air artificially.

Radiation

Radiation transfers heat as infrared waves, which do not need any medium — they travel through air just as sunlight travels through space. The grill in an oven is the clearest classroom example. When you place halloumi or tomatoes under a grill, the glowing element emits infrared radiation that travels downward, striking the surface of the food and heating it directly. No air needs to move; no contact is required. This is why the top surface of grilled bread toasts and browns rapidly while the underside stays pale until turned over. Radiation browns and chars surfaces in a way that conduction and convection alone cannot.

Selecting the Right Cooking Method

Choosing a cooking method is not just about taste — it also affects the nutritional value and palatability of the finished dish. The spec groups cooking methods into three families:

Water-based methods include steaming, boiling, simmering, blanching, poaching and braising. These use water or steam as the heat-transfer medium. Boiling and simmering use convection currents to heat food evenly throughout. Steaming keeps food above boiling water, so heat reaches it by convection in the steam. Because the food is not submerged, fewer water-soluble vitamins (vitamin C and the B vitamins) leach out — steaming is therefore generally better than boiling for preserving water-soluble vitamins.

Dry methods include baking, roasting, grilling and dry frying. These use hot air (convection) or direct heat (radiation or conduction) without added water or fat. Grilling uses radiation from above; baking and roasting rely on convection in hot oven air. Because no water is present, browning reactions such as dextrinisation (bread crust) and caramelisation (roasted carrots) occur more readily, improving colour and flavour.

Fat-based methods include shallow frying and stir-frying. A small amount of oil is used in the pan; conduction transfers heat from the hot pan surface through the oil to the food. Stir-frying uses very high heat and constant movement over a short time, which preserves texture and much of the vitamin C content of vegetables because the cooking time is brief.

Marinades and Palatability

Cooking also affects the palatability of food — its overall pleasantness in terms of appearance, colour, flavour, texture, smell and mouthfeel. One specific technique in this area is the use of marinades. When a chicken thigh sits overnight in a yoghurt and lemon juice marinade, the acid in the lemon juice begins to denature the protein in the surface layer of the meat. The protein chains unravel, the texture softens, and the flavours from the marinade penetrate more deeply. When the chicken is then cooked, the denatured protein coagulates and sets in a more flavourful, more tender state than an unmarinated piece would. Acids in marinades (lemon juice, vinegar, wine) are not just for flavour — they are doing chemical work on the protein structure before the heat even begins.