How do microwave ovens and combination microwaves work, and how can I cook effectively with them?

The basic science behind microwave cooking

Microwave ovens use electromagnetic waves at a particular frequency (a high frequency and short wavelength, hence the name "micro"waves) to agitate water particles in food.

Electromagnetic waves (of which radio, visible light, infrared and ultraviolet are all types of differing frequency and wavelength) are made up of alternating electric and magnetic fields. The alternating electric fields in microwaves are the parts that agitate the water molecules.

Water molecules (two Hydrogen atoms, one Oxygen) are positively charged on one side and negatively charged on the other. They react to the alternating electric field by trying to align themselves with it. Because the electric field keeps alternating (think of the shape of a wave - crest and trough, crest and trough), the water molecules spin one way and then the other. The molecules are moving, and the more the molecules move, the greater their heat. For example, when you boil water, the molecules move faster and faster and further apart until they are flying around as water vapour, or steam.

The agitated water molecules will also rub against neighbouring molecules, moving them as well, and heating them and the food or beverage of which they are a part.

Microwave construction

A microwave is effectively a high voltage power source, a transformer that takes alternating current mains electricity and turns it into direct current power, a steady stream of electrons.

This stream is fed into a cavity magnetron. The magnetron tube uses a magnetic field to guide the steady flow of electrons from the cathode in the centre of the tube towards an anode block that encircles it, in a random fashion - the electrons charge the anode where they strike it, and an oscillating electric current forms in the anode, constantly trying to equalise the charge.

The anode block surrounding the cathode has lots of cavities in it, and the oscillating current interacting with the electrons passing the entrances to these "cavities" causes microwaves to be created in them.

A waveguide, a hollow conductive metal pipe, directs the radio waves from this system into a metal cooking chamber that prevents the microwave energy (which can be dangerous) from escaping - it acts as a Faraday Cage, a shield against electromagnetic radiation. In some commercial microwaves the waveguide will vary the direction and flow of the microwaves to ensure all food is throughly cooked.

The transformer and magnetron arrangement is most commonly built with only one power setting (it being expensive otherwise) for a single intensity of microwave radiation, so power level control is likely to be achieved by the magnetron being turned on and off automatically.

However, there are some models that use something called pulse width modulation to achieve a constant temperature. In very simple terms, PWM involves switching power on and off faster than the device uses the power can register - in effect the device thinks there is still a constant (if reduced) flow of electrical power. Pulse width modulation units are sometimes refered to as "inverters".

What does this mean for cooking with a microwave?

Types of food

If your food has little or no water in it, it won't microwave well (or at all). If you do want to microwave dry foods, you may need to add a cup of water to the microwave to absorb excess energy, or alternatively use a low power setting (that will turn off the power intermittently and allow the microwave to rest).

If water molecules are well distributed throughout the food, and electromagnetic waves are applied uniformly too, then your food will heat up consistently up to a depth of around an inch from the surface (depending on food density).

It is a commonly held misconception that microwaves cook food from the inside or cook all parts of food simultaneously. Microwaves do penetrate some way into food, so for for thin foods this may be true, but for denser foods the microwave energy will be absorbed by the outer layers. Conduction (transfer of heat from one part of the food to another) or convection (the natural circulation of different temperatures of liquid, in the case of thin soups or milk, for instance) is still required to get heat all the way through your food.

Microwaves therefore work best when heating foods that are a uniform consistency - if you try to microwave a whole plate of different foods, you could find that some items are cooked ok, others are still a little cold, and some may have been blitzed of all moisture and have dried up. It is often better to microwave dish components separately and then assemble (which may require a number of microwaves, or good hot holding options).

To cook or reheat very large foods in a microwave, such as a whole chicken or a joint of meat, is very difficult. You will need to use a very low power setting for a long time to avoid dessicating the outer layers, and even then you may find the heat just does not conduct through to the centre.

Oil will not heat well in a microwave because its molecules do not have both a positive and negative charge. Butter can be melted because it does contain a reasonable amount of water.

Ensuring consistent results

For dense foods (including thick soups or sauces that cannot flow easily) you will get best results if you stir what you are microwaving one or two times during the cycle to encourage heat conduction. Loose foods without much moisture content can be challenging, as conduction doesn't work well. Sous vide style "in-a-bag" cooking in a microwave is one way around this problem.

The most effective microwaves are those that do a good job of distributing microwave energy. You'll notice that most commercial microwaves do not use the turntables that are the most common method of achieving this in domestic equivalents, instead using a "stirrer" system to distribute the energy around, effectively a microwave waveguide that changes direction, or more than one waveguide. Models with two or more magnetrons, usually top and bottom, mean that microwave energy is applied on more than one side for faster cooking and penetration above and below.

Understanding and using power settings properly

Commercial microwaves come in a great range of power grades, and most models have a variety of power settings. The more powerful the microwave in terms of Wattage, the more microwave energy is released into the oven chamber and the faster the food is cooked. Bear in mind, however, that the volume of the oven compartment affects the relative strength of your microwave - larger ovens will require more power to achieve the same strength of microwave energy achieved in a smaller oven, so direct power comparisons between models can sometimes be difficult.

A higher powered microwave will not penetrate more deeply than a lower powered microwave - radiation penetration is determined by wave frequency, not the amount of microwave energy being used. The net effect is usually the same however - more power will still cook faster because the outer layers will heat faster and begin conducting to the centre sooner.

In very general terms, lower power (up to around 500 watts) will cause more gentle warming, and unless you are defrosting something, there is a risk you could just dehydrate the outer layers without penetrating to the centre. By that token, however, low power is suitable for very thin foods.

You'll need something of at least 800 watts (in a standard sized microwave) to agitate water molecules in your food fast enough to steam or boil and begin conducting effectively to the centre, and power levels above this will do so much faster. Using too much power will just overcook the outside of your food though, which is why really high powered commercial microwaves are generally only that way because of their larger volume.

Generally, you don't need to worry too much about using too much power in your microwave for all but the most delicate foods - the maximum power setting is likely to be appropriate to your oven volume.

We mentioned that the majority of microwaves cannot truly modulate their power output. The power setting in most microwaves just turns the magnetron on and off in cycles - for example, a 2000w microwave set at half power over 10 minutes might only be emitting microwaves every other minute, provide five minutes of 2000w power and an average power over the time of 1000w.

This is perfectly fine for most foods, but for really delicate foods that don't respond well to temperature fluctuations, a model with one of the pulse width modulation or "inverter" power management systems (described above) may be better.

Do not run your microwave empty

If there is no food or liquid inside a microwave to absorb microwave energy, the microwaves can build up in the oven volume and reflect back into the magnetron, damaging it.

Avoid putting metal in a microwave

Metals in a microwave repel microwave energy. Electrons on the surface of metals move rapidly from side to side in response to microwaves, deflecting the waves that then build up into energy that can cause arcing (little lightning bolts) between the sharp edges or points of any metal you put in and the walls of the microwave or the magnetron itself. This can burn holes in the liner and generally cause damage to the microwave, possibly allowing microwaves to escape (and making the unit extremely dangerous to use).

There are various sources that indicate putting metal in a commercial microwave is perfectly fine, or at least less of a problem than a domestic model, but domestic and commercial microwaves work the same way so the same problems are experienced. True, they are often more durable, but you really don't want to test your liner or other microwave components against electric arcing.

The only metals that it might be safe to put in a microwave are those that are integral to specifically designed microwavable packaging - some foods may come with packaging that has a thin metallic layer to reflect heat back onto food in order to brown it (not usually possible in a microwave). These have been designed by professionals to avoid the arcing that tinfoil or other metals can cause. You must be extra careful to follow cooking power and timing advice with packaging of this kind.

Be wary of plastics

You certainly should not use any plastic in a microwave unless it has been designated microwave safe, and even then our advice is to opt for an alternative microwave safe glass or ceramic vessel if this is practical ("microwave safe plastic" has no commonly accepted legal meaning and is not associated with a defined safety standard). Microwave energy or even the temperatures that foods can reach in a microwave are enough to break down some plastics in contact with them, and even if this is not visible to the naked eye, chemicals can be released that are harmful in the long term if ingested.

What is better about combination microwaves and combination ovens?

Some of the flavour in food comes from the ingredients, and some of the flavour comes from the cooking method. One of the most common taste enhancing reactions in cooking is the Maillard reaction, between amino acids and sugars - effectively "browning". Most microwave cooking can't manage this as they don't achieve sufficient food surface temperatures, although in rare circumstances when oily foods are cooked the vessel and oil temperatures reach a point for a form of frying to occur.

In general, conventional microwaves are considered limited to cooking or reheating foods where flavour is solely a part of the ingredients or of previous cooking methods. However, when additional heat sources are added to microwave ovens, then browning or other reactions such as caramelisation can be achieved. These "combination microwaves" can be used to cook many more types of food, and are popular because the inclusion of microwave energy in the process allows a chef to get the same great result faster.

Combination microwave ovens use electric heating elements to achieve all over browning, and for even more control of the cooking environment, full gas or electric combination ovens can also introduce steam or otherwise regulate ambient moisture content.