Cooking is the transfer of energy from a heat source to a food. To cook foods successfully, you must understand the ways in which heat is transferred: conduction, convection and radiation.
Most important, you must understand the cooking methods used to transfer heat: broiling, grilling, roasting and baking, sauteing, pan-frying, deep-frying, poaching, simmering, boiling, steaming, braising and stewing.
Heat is a type of energy. When a substance gets hot, the molecules have absorbed energy, which causes the molecules to vibrate rapidly. The molecules start to expand and bounce off one another. As the molecules move, they collide with nearby molecules, causing a transfer of heat energy.
Heat transfer can be transferred to foods through conduction, convection, or radiation. Heat travels through foods by conduction.
Conduction is the movement of heat from one item to another through direct contact. For example, when a flame touches the bottom of a pan, heat is conducted to the pan.
Generally, metals are good conductors. Copper and aluminum are the best conductors, while liquids and gases are poor conductors.
Conduction is a slow method of heat transfer because there must be physical contact from one molecule to another.
Convection is the transfer of heat through a fluid. The fluid may be a liquid or a gas. There are 2 types of convection: natural and mechanical.
Natural convection causes a natural circulation of heat because warm liquids and gases have a tendency to rise while cooler ones fall.
Mechanical convection causes heat to circulate more evenly and quickly through fans or stirring.
Radiation is energy transferred by waves of heat or light striking the food. Two types of radiant heat are infrared and microwave.
Infrared cooking is commonly used with toasters and broilers. These devices use an electric or ceramic element heated to such a high temperature that it gives off waves of radiant heat.
Microwave cooking relies on radiation generated by an oven to heat the food.
The Effects Of Heat
Coagulation is when proteins transform from a liquid state to a solid state. Examples: the firming of meat fibers and egg whites changing from a clear liquid to a white solid when heated.
When a mixture of starch and liquid is heated, starch granules swell. The liquid thickens because the starch granules swell to occupy more space. Examples: The thickening of sauces when starch is added.
As sugars cook, they turn brown and change flavor. Caramelized sugar is used in many sugars, candies, and desserts. In fact, caramelization is used in most flavors we associate with cooking.
All foods contain some water. The evaporation of water drys foods during cooking.
Fats are a greasy, smooth substance that do not dissolve in water. Oils are fats that remain liquid at room temperature. Fats melt when heated and then gradually liquefy. Fats will not evaporate.
More reading about effects of heat here.