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What not to put in the microwave

Evidence shows that microwaves do not affect our health but not everything can be popped into the microwave.   2015-09-01

You want a hot meal? Nothing is easier than to pop something in the microwave oven and lo and behold a few minutes later the food is piping hot. Be aware though that some things cannot be put in the microwave we bring you a brief list to remind you.

What happens in a microwave oven?

Waves of energy are used to cook food. These waves are remarkably selective, primarily affecting water and other molecules that are electrically asymmetrical — one end positively charged and the other negatively charged. Microwaves cause these molecules to vibrate and quickly build up thermal (heat) energy.


Metal is a great conductor of heat due to the free movement of electrons in its structure.

However when microwaves hit metal, the free electron on the metallic surface move rapidly from side to side preventing the metal from absorbing the heat. Instead they reflect the microwaves producing an electrical charge or spark to come from the metal object. Depending on what else is in the microwave, the sparks can damage the inside of the microwave and in extreme conditions cause a fire.

Food wrapped in aluminium foil will not cook as the foil reflects the microwaves rather than absorbing them also unabsorbed microwaves can damage the machine. 


Many plastics contain a chemical called bisphenol-A (BPA) which when heated can leach into the food.

BPA mimics hormones especially the sex hormone oestrogen in the body causing various health problems including heart disease in adults, reproductive disorders and male impotence.

However new research from the European Food Safety Authority (EFSA) state that “BPA poses no health risk to consumers of any age group (including unborn children, infants and adolescents) at current exposure levels”.

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Not all types of cling film should be used in the microwave, but there are heavy-duty cling films that are microwave safe. Make sure that the plastic is not laying directly on the food (because it can melt) and make a vent for the steam to escape.

Plastic containers that are suitable for microwave-use have it printed on them either in words or a symbol of a microwave however it’s better to be safe and warm up your dinner directly on a plate.


The best containers to use in the microwave are plain coloured heat-proof glass. Often dishes have the word “microwave safe” which means that you can microwave the container without damaging it or damaging the microwave. It doesn’t let us know if the containers get too hot (due to the food inside) and we are at risk of burning ourselves

Fancy china with metal trimming should not be used as even if the metallic trimming is miniscule, it may still react in the microwave.

Old and antique crockery may be glazed with materials that could give off radiation and may contain lead and other harmful heavy metals.

Porcelain and stoneware should be used instead of ceramics as these may have produced by “low fired”.

The dish may soak up some moisture into the clay body and if this moisture gets heated enough, it will turn to steam and expand, possibly causing an explosion.

Other things never to put in the microwave:

  • Paper bags, newspaper and plastic rubbish bags - the ink, glue and recycled materials in paper bags can emit toxic fumes when they are exposed to heat.
  • One-time plastic containers such as margarine tubs or yogurt containers
  • Styrofoam containers- these can release harmful chemicals into your food when heated


  1. Grapes will explode or burst into flames if put in a microwave. Grapes are full of moisture and when they are microwaved all that moisture turns to steam. If the stem is still attached to the grape and there is nowhere for the steam to escape, it will explode slightly and rupture the skin.
  2. However, if you place two grapes close together with their stem holes facing each other, some sparking may occur and they may catch fire.
  3. Eggs When an egg is put in the microwave oven pressure builds up within the sealed egg “container” (shell)  which can cause it to explode (even if the boiled egg is peeled. 
  4. Dried Hot Peppers – The chemical in peppers that make them hot is called capsaicin.  When put into a microwave it becomes volatile and can catch fire.
  5. Breast milk - Research has found that the heating method of microwaves may destroy some of breast milk's immune-boosting proteins and other nutrients. It is better and safer to heat a bottle of breast milk in a jug or bowl of warm water.

Operate on empty!

You should not use the microwave oven empty. If there is nothing in the oven to absorb the microwaves, they will just bounce around, creating a standing wave that will damage the machine.

Like all modern appliances the microwave has become an essential piece of equipment because it saves time, it is economical to use and easy to clean, makes heating food easier, there is no need to keep a watch and retains nutrients and vitamins in the food. Use the microwave correctly and enjoy your meal.