Microwave oven

A microwave oven is a sealed box or chamber with a fan to vent the chamber and a source of microwave radiation. Food is placed in the chamber and is bombarded with microwave radiation. This "excites" water molecules in the food, which become hot generating steam and heating the foodstuff around it. Microwaving is a similar process to steaming and food will not brown.

Illustration of a microwave

Microwaves penetrate deeper into foods than heat (infra-red) energy but do not cook from the inside out. If a block of butter is placed in a microwave oven for a short time, heat builds up in the centre and so the centre melts before the outside. Heat can build up in foodstuffs and is not able to escape and this can cause burning or other undesirable effects so it is important to follow manufacturer's instructions and test cooking processes carefully.

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Some products should not be placed in the microwave oven as this may damage the microwave generator. There are many materials that should not be placed into microwave ovens but many glass, ceramic, and some plastic containers are suitable. Melamine crockery should never be microwaved as it may catch alight and may cause a fire.

 

Microwave cookery

Microwave cookery involves the transfer of energy to the food in the form of electro-magnetic radiation. These microwaves penetrate the food and rapidly move the molecules of liquid it contains. This creates friction, which in turn creates heat to cook or reheat food.

Diagram of a microwave at work

Microwave ovens are designed so that the microwaves bounce around inside them, penetrating food from all angles. Microwaves can move through some types of containers as it is only when the waves reach the water molecules in the food that friction is created. Please see the chart below for a full description of the types of containers that can be used in a microwave oven.

 

Limitations

A microwave

Microwave ovens are convenient and useful appliances but they do have some limitations:

  • Microwave ovens don't produce the same results as conventional cooking methods. For example, microwaving foods does not produce the same browning effect that grilling or frying does. This is because microwaves do not use applied heat to cook foods.

  • Pastries and cakes tend to go soggy in a microwave oven because of the steam created by microwaving.

  • In general, microwave ovens are smaller and have limited capacity compared to a standard oven.

 

Advantages

Re-heating food in the microwave

Microwave cookery however is particularly useful for:

  • Defrosting frozen foods

  • Reheating pre-prepared dishes

  • Cooking foods that don't require browning

  • Foods that can be cooked fast at an even temperature

Foods suitable for microwave cookery

Ideally, only microwave foods that have high moisture content, such as:

A range of food suitable for microwave cooking
  • vegetables

  • fruit

  • fish

  • shellfish

  • eggs

  • herbs and breadcrumbs can be dried using a microwave.

Utensils and equipment for microwave cooking

Exposure to the radiated energy can be dangerous so it is important to take care when using microwave ovens. These appliances must be properly maintained - keep the unit clean and check doors and seals regularly to make sure that microwaves can't escape while the appliance is running.

 

Food containers

Food containers made from certain materials are unsuitable for use in a microwave oven as they prevent the waves of energy passing through and reaching the food.

Material

Reason for not using

metals

  • reflects microwaves

  • causes sparking which may damage the appliance

  • cause arcing when come into contact with the appliance's wall

lead crystal, antique glass containing glued handles

Microwaves will affect the glue

paper and cardboard

These containers absorb fat, which may lead to the absorption of undesirable elements from the radiated energy

porcelain or glass with a metallic rim

As for metals

Plasticised cardboard

The plasticised coating must be able to withstand temperatures of over 100°C or they will melt

Materials that are suitable for use in a microwave oven include:

  • porcelain without a metallic rim

  • plastic specially marked "microwave safe".

  • glass.

The shape of the container is also important in microwave cookery. A round shape is desirable as the food will absorb the microwave energy more evenly than in an oval, rectangular or square shaped container.