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Explains conduction, convection and radiation, moist and dry heat cooking methods, and how cooking affects nutrients.
Heat can be transferred to food in three main ways. Conduction is the transfer of heat through direct contact between particles, for example heat passing from a hot hob through the base of a metal pan into food. Convection is the transfer of heat through the movement of liquids or gases, such as hot air circulating in an oven or boiling water moving currents of heat through a pan. Radiation is the transfer of heat through electromagnetic waves without needing direct contact, such as the heat from a grill element or the infrared waves used in a toaster. Microwave ovens use a form of radiation that causes water molecules inside food to vibrate rapidly, generating heat throughout the food.
Moist heat methods cook food using liquid or steam. Boiling cooks food in liquid at 100°C, while simmering uses a gentler heat just below boiling. Poaching cooks delicate foods, such as eggs or fish, in liquid held just below boiling point, helping to keep them tender. Steaming cooks food using steam rather than submerging it in liquid, which helps retain more water-soluble vitamins such as vitamin C and some B vitamins compared with boiling. Stewing and braising use long, slow cooking in liquid, which is ideal for tougher cuts of meat as it gradually breaks down tough connective tissue, making the meat tender.
Dry heat methods cook food without added liquid. Baking and roasting use hot, dry air in an oven, often causing browning on the surface of the food through reactions such as the Maillard reaction. Grilling uses radiant heat from above or below to cook food quickly, often used for thinner cuts of meat or fish. Frying uses hot oil or fat to cook food quickly at a high temperature, giving a crisp texture, though it adds extra fat and calories to the finished dish.
Food is cooked for several important reasons: to make it safe to eat by destroying harmful bacteria, to make it easier to chew and digest, to improve its flavour, texture, aroma and appearance, and to extend its shelf life. However, cooking can also reduce the nutritional value of food, particularly heat-sensitive and water-soluble vitamins like vitamin C and some B vitamins, which can be destroyed by heat or lost into cooking water.
To retain the most nutrients, methods that use less water and shorter cooking times are generally best, such as steaming or stir-frying, rather than boiling vegetables for a long time in a large volume of water. Cutting food into smaller pieces increases the surface area exposed to heat and water, which can increase nutrient loss, so keeping vegetables whole or in larger pieces where possible can help preserve nutrients.