How microwaves heat food
Microwaves interact differently with different materials.
- They bounce off metal or fine metal meshes.
- They pass through glass, ceramic cookware and microwave-safe plastics.
- They are absorbed by the water molecules in food.
Microwaves work by energising the water molecules in food. The water molecules vibrate, and this causes the food to heat up. (Nearly all foods contain water.)
Other molecules, such as proteins, also absorb microwaves. They help heat your food too.
Safety of microwave ovens
Microwaves at the intensities used to cook food can be harmful if you’re exposed to them directly. But once the microwave oven is turned off, so are the microwaves. The microwaves are also turned off automatically whenever the door is opened. And the microwaves are instantly transformed into heat energy in the food.
There are no microwaves left in the food or the oven cavity once the oven’s off. It’s like turning off the light in a windowless room – it gets dark straight away.
If you've got a recipe which says to leave food standing for a few minutes after it’s been cooked, this is just to let heat dissipate evenly through the food.
Use your microwave oven safely
- Only use the oven if the door closes firmly and is undamaged.
- Do not use the oven if corrosion has caused holes to appear in the grille, oven cavity or outer housing.
- Do not tamper with or try to break the door safety switches.
- Do not insert any object through the grille or around the door seal.
- Do not place metal utensils in the oven cavity.
- Clean the oven cavity, the door and the seals regularly, with water and a mild detergent. Do not use an abrasive cleaner or cleaning method.
- Do not run the oven when it’s empty, unless specifically allowed in the manufacturer’s instructions.
- Make sure the door does not open more than a few millimetres before it is turned off by the safety switches.
Microwave ovens on sale in New Zealand must satisfy the Microwave Ovens Regulations 1982. The Ministry considers that Ovens which meet the requirements of the Electricity (Safety) Regulations 2010 comply with the intent of the Microwave Ovens Regulations
Keep metal out of the microwave
If you put something metal in the microwave oven, such as tin foil or metal cutlery, the microwaves can cause sparks to form around the edges. This may damage the oven.
In some cases, metal objects such as tin foil or cooking stands (trivets) can be used in your microwave oven. Always follow the manufacturer’s instructions.
The oven cavity and door opening are designed to stop microwaves leaking out of the oven. As long as the oven door of your microwave oven is undamaged and closes properly, you don't need to worry about leaks.
The Ministry of Health has tested leakage from many new and used microwave ovens.
- Old ovens don’t leak any more than new ovens.
- Any leakages have always been less than the maximum allowed by the Microwave Ovens Regulations 1982.
- Most ovens were 10 to 100 times better than required by law.