Friday, 8 March 2013

Basics of Heat Transfer


Cooking is the act of applying heat to foods to prepare them for eating.  When foods are cooked, changes in flavor, texture, aroma, color, and nutritional content occur during the process.

There are three ways that heat is transferred to foods.  Conduction is the direct transfer of hear between adjacent molecules.  An example of conduction is cooking on a flattop range.  Heat is transferred from the molecules on the hot range surface to the molecules of the adjacent pan bottom, then from the pan bottom to the pan side and the food contained in the pan.  the pan must be in direct contact with the range for conduction to occur.

Some materials are better conductors of heat than others.  Generally, most metals are good conductors, while gases (air), liquids, and non-metallic solids (glass, ceramic) are not.  Because it relies on direct contact, conduction is a relatively slow method of heat transfer, but the slow, direct transfer of heat between adjacent molecules is what allows a food to be cooked from the outside in, resulting in a completely cooked exterior with a moist and juicy interior.

Convection is the transfer of heat through gases or liquids.  When either o these substances is heated, the portions of the gas or liquid closest to the heat source warm first and become less dense causing them to rise and be replaced by cooler, denser portions of the gas or liquid.  Convection, therefore, is a combination of conduction and mixing.

Convection occurs both naturally and through mechanical means.  Natural convection is at work in a pot of water placed on the stove to boil.  Conduction transfers heat from the stove to the pot to the water molecules in contact with the interior of the pot.  As these water molecules heat up, convection causes them to move away and be replaced by cooler molecules.  This continual movement results in convection currents within the water.  If a potato is added to the water, the convection currents transfer heat to the surface of the potato, at which point conduction takes over to transfer heat to the interior of the potato.

Mechanical convection occurs when stirring or a fan is used to speed and equalize heat distribution.  When you stir a thick sauce to heat it faster and keep it from scorching on the bottom of the pan, you are creating mechanical convection.  Convection ovens use fans to rapidly circulate hot air, allowing them to cook foods more quickly and evenly than conventional ovens.  (Natural convection occurs in conventional ovens as air in contact with the heating element circulates, but the majority of heat transfer in a conventional oven is the result of infrared radiation.)

Radiation is the transfer of energy through waves of electromagnetic energy that travel rapidly through space.  Radiation does not require direct contact between the energy source and the food.  When the waves traveling through space strike matter and are absorbed, they cause molecules in the matter to vibrate more rapidly increasing the temperature.  Two types of radiation are important in the kitchen: infrared and microwave.

Sources of infrared radiation include the glowing coals of a charcoal grill or the glowing coals of an electric toaster, broiler, or oven.  Waves of radiant energy travel in all directions from these heat sources.  Foods and cookware that absorb the energy waves are heated.  Dark, dull, or rough surfaces absorb radiant energy better than light-colored, smooth, or polished surfaces.  Transparent glass permits the transfer of radiant energy, so conventional oven temperatures should be lowered by approximately 25 F/5 C to offset the additional energy transfer that occurs when using glass baking dishes.

Microwave radiation, produced by microwave ovens, transfers energy through short, high-frequency waves.  When these microwaves are absorbed by foods, they cause food molecules to vibrate faster, creating heat.  Microwave radiation cooks foods much faster than infrared radiation because it penetrates foods several inches deep, whereas infrared is mainly absorbed at the surface.  Depending on their composition, foods react differently to microwaves.  Foods with high moisture, sugar, or fat contents absorb microwaves best and heat up more quickly.

Microwave cooking has a few drawbacks, however.  It is best suited to cooking small batches of food.  Meats cooked in a microwave oven lost greater amounts of moisture and easily become dry.  Microwave ovens also cannot brown food, and metal cannot be used in them because it reflects the microwaves, which can cause fires and damage the over.  There is also some conversation on the nutritional effect of cooking with microwaves.  Generally speaking, microwave radiation will alter nutrients faster than any other kind of heat transfer.

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