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Conventional ovens

A conventional oven uses infrared radiation. Heat is transferred from a red hot heating element to food through an electromagnetic wave easily absorbed by objects. On absorbing infrared radiation, food surfaces are heated to a high temperature producing a crispy effect of roast chicken skin or bread crust, for example. However, food is usually a bad conductor of heat, and it may take quite a long time for the heat on the surface of the food to reach the interior. Cooking time is thus dependent on the speed of heat conduction of the food itself. In addition, if the oven's power is not carefully controlled, food can be burnt on the outside but remain uncooked inside. This is the major disadvantage of the conventional oven and cooking methods such as frying and baking.

Microwave ovens

Fig. 9   A household microwave oven

Microwave ovens are very popular in Hong Kong because they can cook or reheat food very quickly. They can heat food without heating up the food container.

A microwave oven generates microwaves inside its cavity to cook food. Microwaves are electromagnetic waves with wavelengths between about 1 mm to 100 cm. Domestic microwave ovens use microwaves of frequency 2.45 × 109 Hz (of wavelength 12 cm). Unlike infrared radiation, microwaves can penetrate much deeper into food (e.g. about 1 cm into beef). The energy of the microwaves spread out to fill a greater volume inside the food so the entire food item is more evenly cooked. This significantly speeds up the cooking process.

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But why are microwaves so effective in heating up food? This is closely related to the molecular structure of water (Fig. 10), a major component of food. Water molecules are formed from two hydrogen atoms and one oxygen atom sharing their outermost electrons. The oxygen atom exerts a stronger attractive force on the shared electrons, pulling them closer to the oxygen side of the water molecule. The oxygen side therefore has slightly more negative charge and the hydrogen side has slightly more positive charge. Water molecules are said to be 'polar' because of this small separation of charge. When microwaves pass through water, their energy is absorbed by the polar water molecules, and these molecules oscillate violently (Fig.11). As the water molecules collide with each other, this energy of oscillation is changed into the random kinetic energy of the molecules, resulting in a rapid increase in temperature.

Fig. 10   A water molecule is said to be polar because its positive and negative charges are slightly separated. Fig. 11   When microwaves pass through water, the oscillating electric field causes the water molecules to oscillate up and down.

The much shortened cooking time means a high energy efficiency.

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