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Revision - Cooking of food and heat transfer - Coggle Diagram
Revision - Cooking of food and heat transfer
Why is food cooked?
To make food safe to eat
Some foods must be thoroughly cooked to destroy the food poisoning bacteria they are likely to contain.
To develop the flavours in food
Cooking develops flavour by causing chemical reactions to take place in food for example: in cakes, the fat melts, proteins in eggs and flour coagulate, sugars caramelise and starch gelatinises.
Cooking concentrates and intensifies flavour by causing water to evaporate.
To improve their texture and appearance
Cooking causes starch granules to swell, gelatinise and thicken or soften food. It tenderises meat so that it is easy to chew and digest. It develops a crisp texture on the outside of foods.
To improve its shelf-life
Cooking destroys harmful micro-organisms such as bacteria and moulds which preserves the food.
To give people a variety of foods in their diet
Foods can be cooked in different ways to give variety - potatoes can be cooked as boiled, chips, in a salad, jacket potato, roasted, crisps, rostis etc.
Methods of heat transfer
Conduction
Cooking pans and baking tins are usually made from any of these metals: iron, steel, copper or aluminium. These are good conductors of heat because of their atomic structure. Heat insulators are poor conductors of heat and examples are wool, wood.
What happens?
When we heat a metal pan on the hob or a baking tin in the oven, it quickly heats up and transfers heat energy to the food inside. This is by the process conduction. The atoms that make up metals are tightly packed together in a lattice. As the heat energy from the cooker hob or oven is transferred into the pan, it makes the metal atoms start to vibrate and knock against adjoining atoms and pass on the heat energy, then they pass it on to others and so on.
Convection
Convection is the name given to the transfer of heat energy by the movement of molecules in a liquid or in the air from a warm area to a cold area. Two examples of this are heating water in a pan and heating the air in an oven.
What happens?
In the oven
In the oven, convection is the molecules in the air that move upwards as they receive heat energy, then fall back down as they cool. This sets up convection currents of hot air. In gas ovens, the temperature is hottest at the top of the oven, and coolest at the bottom of the oven. this results in 'zones of heat' in the oven.
Some electric ovens are fan assisted. This means that there is a fan at the back that blows hot air around the oven so that the temperatures on each shelf is the same and there are no zones of heat. Because of this, foods tend to cook quite quickly in fan ovens.
On the hob
When you heat a pan of cold water on the hob, the heat energy passes through the metal pan by conduction and is then transferred to the water molecules in the pan. As the amount of energy going into the water increases, the water molecules start to move upwards where the water is cooler, and collide with other molecules as they do so, thus passing on the heat energy into the food cooking in the water, such as vegetables/pasta.
The more heat energy that passes through the water, the faster the water molecules move and the more they collide with other molecules. When they reach the surface of the water, the molecules start to slow down and sink back down again. As they reach the bottom of the pan, they receive more heat energy and start to move upwards again. This sets up convection currents and eventually when there is enough heat energy in the water, it will boil.
Radiation
When food is grilled, it is heated by radiation. the heat energy in radiation is in the form of infrared heat rays.
What happens
Infrared rays travels through the air and when they come into contact with a solid object, they are absorbed into the surface of the object and heat it up.
The temperature becomes very hot and intense (hotter than an oven) so it is important not to place the food to close to the grill element as it may easily burn on the outside but not be fully cooked on the inside.
Different cooking methods
Using moisture to transfer heat
Stewing
Method of heat transfer
Conduction --> Convection
Suitable foods
Meat, poultry, sausages, casseroles, fruit.
What is it?
Cooking food by simmering gently in a covered pot either in the oven, on the hob or in a slow cooker.
Steaming
Method of heat transfer
Conduction --> Convection
Suitable foods
Green vegetables, white fish, sponge puddings, rice.
What is it?
Cooking food in the steam rising from a pan of boiling water beneath.
Simmering
Method of heat transfer
Conduction --> Convection
Suitable foods
Vegetables, soups, stews, fruit, meat sauces, curries, fish chowder.
What is it?
Cooking food in a liquid just below boiling point so it bubbles gently.
Poaching
Method of heat transfer
Conduction --> Convection
Suitable foods
Fish, eggs, fruit.
What is it?
Cooking foods in a shallow pan of water or wine at just under boiling point.
Braising
What is it?
Sealing meat in hot fat, then cooking it slowly in a covered dish with a little liquid.
Suitable foods
Meat, poultry, vegetables.
Method of heat transfer
Conduction --> Convection
Boiling
What is it?
Cooking food in water at 100C
Suitable foods
Eggs, rice, pasta, vegetables, joints of meat, beans, peas, vegetable or meat stock.
Method of heat transfer
Conduction --> Convection
Methods using oil to transfer heat
Sauteing
Method of heat transfer
Conduction
What is it?
Frying food gently in a little oil to soften the food and develop the flavour.
Suitable foods
Onions, leeks, peppers, meat/poultry.
Shallow frying (pan frying)
Method of heat transfer
Conduction
Suitable foods
Eggs, fish, bacon, burgers, sausages, meat cuts, pancakes, flat breads, onions, bananas.
What is it?
Frying food in a shallow frying pan with a little oil
Stir frying
Method of heat transfer
Conduction
Suitable foods
Finely cut vegetables and other foods.
What is it?
Frying food for a short time in a wok using very little oil
Roasting
Method of heat transfer
Convection --> Conduction
Suitable foods
Meat/poultry joints, root vegetables, some fruits, nuts.
What is it?
Cooking food in some oil or fat in a hot oven.
Deep fat frying
Method of heat transfer
Conduction --> Convection
Suitable foods
Fish, scotch eggs, chicken joints and pieces, battered vegetables, spring rolls, doughnuts, fritters, potato croquettes.
What is it?
Frying food in a deep pan of very hot oil, so that the food is fully immersed in the oil
Methods using dry heat to transfer energy
Baking
Method of heat transfer
Convection --> conduction
Suitable foods
Cakes, breads, biscuits, cookies, scones, pastries, potatoes, pizzas.
What is it
Cooking foods in a hot oven
Grilling
Method of heat transfer
Radiation
Suitable foods
Meat and poultry joints, fish, sausages, burgers, tomatoes.
What is it
Cooking foods by intense radiant heat on a metal grid or grill rack, underneath a heated grill elements in a cooker or above the flowing charcoal/flames in a barbecue.
Toasting
Method of heat transfer
Radiation
Suitable foods
Bread, buns, nuts, seeds.
What is it
Cooking starch based foods with dry heat from a grill or flame
Dry frying
Method of heat transfer
Conduction
Suitable foods
Minced meat, nuts, seeds, flat breads.
What is it
Cooking food that naturally contains oil or fat in a frying pan without adding oil.
Other methods of transferring heat energy
Microwaving
Method of heat transfer
Radiation.
Suitable foods
Sauces, cake and sponge pudding mixtures, scrambled eggs, vegetables, fruits, fish, soups, melting chocolate and butter.
What is it
Cooking food by electromagnetic waves called microwaves in a microwave oven.
Induction cooking.
Method of heat transfer
Conduction
Suitable foods
Any food that is cooked in a pan on the hob.
What is it
A method of cooking where heat energy is transferred quickly to a pan through a specially designed ceramic cooking surface over an induction coil that creates a magnetic current. Pans used on these hobs must be made from metals that contain iron.