Convection occurs when heat is spread by the movement of air, steam, or liquid (including hot fat).There are two kinds of convection:
Hot liquids and gases rise while cooler ones sink.Thus, in any oven, kettle of liquid, or deep-fat fryer a constant, natural circulation distributes heat.
In convection ovens and convection steamers, fans speed the circulation of heat. Thus, heat is transferred more quickly to the food, and the food cooks faster.
Stirring is a form of mechanical convection. Thick liquids cannot circulate as quickly as thin ones, so the rate of natural convection is slower.This explains, in part, why it is so easy to scorch thick soups and sauces.The heat is not carried away from the bottom of the pan quickly enough, so it stays concentrated on the bottom and scorches the food. Stirring redistributes the heat and helps prevent this. (Using heavy pots made of a material that conducts heat well also helps prevent scorching because the pot conducts the heat more quickly and evenly across the bottom and up the sides.)
Convection is the process that carries the heat from the heat source to the food. Once the carrier of the heat (air or liquid) comes in contact with the food, the heat is transferred from the carrier to the food by conduction.
Continue reading here: Radiation
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