A microwave oven cooks food by causing the moisture molecules in food to vibrate, producing friction, which results in heat. The microwaves are reflected off the metal cavity of the oven, ensuring that the food absorbs waves from all directions. Although microwaves can only penetrate about 5 cm (2 in) into the food; the centre of larger pieces of food is cooked by the conduction of the heat produced near the food's surface.
Microwaves can pass through paper, plastic, china and glass, making them all suitable microwave containers. However, you should never use the following items in a microwave:
- Metal containers
- Dishes with gold or silver decoration
- Jagged pieces of tin foil
- Anything containing glue
- Pottery (it may have a metallic glaze)
- Melamine
- Thin-stemmed glasses
- Crystal glass
Microwaves are particularly useful for reheating, melting and defrosting food in small quantities. It is less useful for cooking or defrosting food in large quantities, as it often produces food that is frozen or uncooked in one area, whilst overdone in another. To prevent this, you must stop the microwave every so often and allow the food to stand so that heat can be conducted to the centre.
Microwaving is also a useful technique for cooking vegetables that would normally be boiled, such as peas and asparagus. They may also be used as a fast way to cook baked potatoes; however, you should put the potato in the a hot oven for 15-20 minutes after microwaving if you want the potato to have a crispy skin and a 'baked' taste.
