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| Methods of heat transfer |
Cooking is the application of heat to food. In most cultures throughout the world, food is cooked before it is eaten. Why then do we cook food? There are four important reasons why we cook food- these are:
· To improve the appearance, flavour and taste of food. Cooking also helps to develop, blend and alter flavours;
· To bring about structural changes in food. For example, meat fibres are tenderized and the cellulose in vegetables and fruits are softened so that they are easier to chew and digest;
· To stimulate the appetite and digestives juices. Cooking gives food an attractive appearance, aroma and taste;
· To improve the keeping quality of food. Cooking destroys some of the harmful micro-organisms such as yeasts, moulds and bacteria that are naturally present. Remember, though, cooking alone will not increase the life of foods for long, and care should be taken to refrigerate foods after they have been cooked.
Conduction cooking
The movement of heat from on item to another through direct contact. Most conduction heat occurs on top of the stove. Heat is conducted from the gas jet or electric element to the container holding the food, and the food is heated. Examples include:
- shallow frying
- sautéing
- stir frying
Convection cooking
Refers to the transfer of heat, through a fluid which may not be liquid or gas. In heating liquids or gases convection currents are produced which distribute the heat. Standard ovens require foods to be placed strategically within the oven. Ovens which are fan forced however do not require foods to be placed on certain shelves as the heat is dispersed evenly.
Examples of heat transfer by convection of gas include: roasting and baking.
Examples of heat transfer by convection of liquid include: boiling, simmering, poaching, stewing, braising, pot roasting and casseroles.
Radiation cooking
Is "the energy radiated or transmitted in the form of rays, waves, or particles." Most radiation occurs on the flat grill, salamander or open barbeque where direct rays of heat pass from the heat source onto the food and cook it by radiation.
How do microwaves cook food?
- Because microwaves are absorbed by fats, water, and sugars, it allows those molecules to produce heat.
- They produce heat because the microwaves cause the molecules to vibrate and twist which makes them hub together.
- This movement allows the molecules to absorb microwave energy and convert it into thermal energy (which is made when heat is produced).
COOKING METHODS
Foods can be cooked in air, fat, water or steam. These are collectively known as cooking media.
There are two general types of cooking methods: dry heat and moist heat.
Dry-heat cooking methods are those using air or fat. They are broiling, grilling, roasting and baking, sautéing, pan-frying and deep frying. Foods cooked using dry-heat cooking methods have a rich flavour caused by browning.
Dry-heat cooking methods are those using air or fat. They are broiling, grilling, roasting and baking, sautéing, pan-frying and deep frying. Foods cooked using dry-heat cooking methods have a rich flavour caused by browning.
Moist-heat cooking methods are those using water or steam. They are poaching, simmering, boiling, and steaming. Moist-heat cooking methods are used to tenderise and emphasise the natural flavours of food.
Other cooking methods employ a combination of dry- and moist heat cooking methods.
The two most significant of these combination cooking methods are braising and stewing. Each of these cooking methods can be applied to a wide variety of foods- fish, meat, vegetables and even pastries.
Broiling
Uses radiant heat from an overhead source to cook foods. The foods to be broiled are placed on a preheated metal grate, delicate foods will be damaged. The method of cooking produces grate crosshatch marks.
Simmering
A moist-heat cooking method that uses convection to transfer heat from a liquid to a food. When cooked properly, the food should be moist and very tender.
Sautéing
A dry-heat cooking method that uses conduction to transfer heat from a hot sauté pan to food with the aid of a small amount of fat. Heat then penetrates through conduction. High temperatures are used and foods are usually cut into small pieces to promote even cooking.
Roasting and baking
The processes of surrounding food with dry, heated air in a closed environment. Heat is transferred to the food's surface, and then penetrates the food by conduction. The surface dehydrates and the food browns from caramelisation, completing the cooking process.
Grilling
Uses a heat source located beneath the cooking surface. May be electric or gas, or they can burn wood or charcoal, which adds a smoky flavour to the food. Often identified by crosshatch markings.
Combination cooking methods
Some cooking methods use dry-heat and moist-heat cooking techniques. The 2 principal methods are braising and stewing. In both methods, the 1st step is usually to brown the main item using dry-heat. The 2nd step is to complete cooking by simmering the food in a liquid. These methods are often used for less tender but flavourful cuts of meat as well as for poultry and some vegetables.
Deep-frying
A dry-heat cooking method that uses conduction and convection to transfer heat to food submerged in hot fat.
Stewing
A combination of dry and moist-heat cooking methods. Usually small pieces of food that are first cooked either by browning them in a small amount of fat or oil or by blanching them in liquid or sauce. Have enough liquid added to cover them completely and are simmered at a constant temperature until tender.
Braising
Large pieces of food that are cooked in a high amount of fat at high temperatures. Heat is transferred from the pan through a process of conduction. Uses a moist and dry cooking method.
Poeleing
A cooking method similar to both roasting and braising. The food is cooked in an oven, but in a covered pot with aromatic vegetables and bacon fat or butter so that it steams in its own juices.
Stir-frying
A wok is used instead of a sauté pan; the curved sides and rounded bottom of the wok diffuse heat efficiently and facilitate tossing and stirring.
Pan-frying
A dry-heat cooking method in which heat is transferred by conduction from the pan to the food, using a moderate amount of fat. Heat is also transferred to the food from the hot fat by convection.
Boiling
A moist-heat cooking method that uses the process of convection to transfer heat from a liquid to a food. Uses large amounts of rapidly bubbling liquid to cook foods.
Steaming
A moist-heat cooking method that uses the process of convection to transfer heat from the steam to the food being cooked. Often associated with tender, delicately flavoured foods, which do not require long cooking times. Enhances a food's natural flavours and helps to retain its nutrients. When cooked properly, the food is moist and tender.
Poaching
A moist-heat cooking method, which uses convection to transfer heat from a liquid to a food. Most associated with delicately flavoured foods that do not require a lengthy cooking times to tenderize them.
Submersion poaching
The food is completely covered with the cooking liquid. There should not be too much excess liquid, nor too little.
Shallow poaching
The food is placed in just enough liquid to come approximately halfway up its sides. The liquid is brought to a simmer on the stove top. The pan is then covered with a piece of parchment paper or a lid; and cooking is completed either on the stove or in the oven. Combines aspects of poaching and steaming.
