Microwaves - Inside out or outside In!?

Question

I have heard varying reports as to whether microwave ovens heat from the outside in or from the inside out.
I always understood that heating from the inside is a myth. (I have certainly found that if food is not heated for a sufficient length of time, the centre can be cold whilst the edges can be hot/bubbling. Scientific answers please, rather than just opinions. Thanks

I realise that food with a centre with hight water content is going to get very hot in the middle (ie jam doughnut or mince pie)
I am referring to things like meat, which is the same texture all the way through.

Answer 1

Microwaves are a form of light which can penetrate food and impart motion to molecules containing hydrogen - like fats and water.
The microwaves are at their highest energy at the surface of the food, where they enter, but lose their energy as they penetrate deeper and are gradually absorbed by the hydrogen bonds they encounter in the food.

Because they are heating the first few millimeters deep instead of only the surface, more energy can get into the food quickly without overheating its surface layer.

The deeper layers of the food, where the microwaves penetrate less well, still have to depend on conduction (or convection in liquids) of the heat produced near the surface in order to cook too, so large pieces of meat or a deep plate of soup will take longer to cook or reheat in a microwave. Steam produced in one part of a microwave meal may also help impart heat to another part of the meal but this means that eggs can't safely be microwaved because the steam will explode the egg's shell.

Lightweight foodstuffs containing lots of hydrogen cook quickest (e.g. sponge puddings with a high fat content).

It's very important that all of the food gets piping hot to kill any microbes. Leaving a 'resting' time after microwaving allows cooler parts of the food to heat up by conduction from the hot parts. Bon appetite!

Answer 2

In an equally distributed mass with water volume the same through out it heats from the outside in. Microwaves heat by vibrating water, so if there is more water in the center of something it will heat first. I think the myth came from early microwaves where it would cook the inside of a human long before the burn is felt on the outer skin. The fact that water is exited and causes the heat is why you can touch the bowl of a hot soup and not get burned. IE no, or very little water in the bowls composition, but the soup is full of water. Of course the heat will eventually transfer to the bowl from the soup!

Answer 3

Outside in. Always. The fact that things like apple pies or doughnuts get very hot in the middle is due to the sugar content, not just the amount of water. Foods with high sugar contents get very very hot in a microwave, much hotter than foods that have a low sugar content.

Wonder what happens if you heat a bowl-full of sugar in a microwave ?

I've never come across any foods that are high in hydrogen, presumeably because they've all floated away by now.

Answer 4

Microwave ovens work by using radio waves which are absorbed by liquid water molecules. So the question becomes one of understanding the distribution of water within the food and the rate at which the microwave energy is attenuated by water.

If your food is really really wet then the microwaves will be absorbed rapidly and the food will cook from the 'outside'

If the object you wish to cook is relatively dry (like a potato) then the microwave energy penetrates the food much more evenly and the food cooks much more uniformly. It appears to be cooking from the inside. The inside is cooking but so is the outside. It is this possibility of uniform cooking that makes microwave cookers fast

The inside myth may have come about because the microwave energy is not uniformly distributed throughout the oven. There are hot spots. If your food has one of these microwave hot spots located at its interior then it will seem to be cooking from the inside out.

Ice doesn't absorb microwave energy well hence it is possible to cook a reverse baked alaska in a microwave oven. (Frozen on the outside and hot on the inside)

Answer 5

it depends on the structure of the food being microwaved, the energy for heating comes from a wave so the structure and orientation of molecules affects the rate at which they absorb the radiation, so in a pie the water in the filling absorbs microwave radiation more than the crust which is why it feels warm on the outside but you can still burn your tongue. This is because while the microwave can heat the internal as well as external molecules the heating is differential leading to superheating.

Answer 6

Microwaves excite water molecules causing them to heat up.
this process starts from the outside .
the heating from the inside thing is a myth.

Answer 7

It's the water molecules that heat up best so the higher in water the food the quicker it gets hot.

Answer 8

Both. However, try heating apple tart in the micro. The inside is much richer in water, so it gets much hotter.