Cooking regime

Volumetric / dielectric

Every other cooking regime delivers heat to a surface and waits for it to conduct inwards. This one deposits energy through a depth of the food directly — which is why it is so fast, and why it is so uneven.

Energy through a depth

A microwave field makes water molecules rotate rapidly, and that motion is heat. Because the field penetrates a centimetre or two into the food, heating begins throughout that layer at once rather than at the outer surface only.

The consequence is speed. Conduction is slow — it is why an oven takes an hour to heat the centre of a joint — and skipping most of it makes a microwave several times faster than any other method for small quantities.

The consequence is also unevenness, and for three separate reasons. Penetration is limited, so the centre of anything thick is still heated by conduction. The field forms a standing pattern with hotter and cooler regions, which is what the turntable averages out. And different materials absorb differently — water heats readily, fat faster, ice barely at all, which is why defrosting is so uneven.

What it suits and what it does not

Excellent

Reheating, steaming vegetables in a covered bowl, melting, softening, warming liquids.

Good

Small quantities of anything that does not need a surface.

Poor

Anything thick, where the outside overcooks before the centre is warm.

Poor

Defrosting, which is uneven enough that edges start cooking while the centre is frozen.

Impossible

Browning and crisping. There is no hot surface and the environment is full of steam.

Related but different

Induction hobs also deliver energy without a hot element, but into the pan rather than the food, so the cooking that follows is entirely conventional.

Working with the unevenness

• Lower power, longer time. Most complaints about microwaves are solved by 50% power for twice as long, which lets conduction even things out as it goes.
• Stir at least once, halfway.
• Arrange food in a ring with the thickest parts outward and the centre empty.
• Cover loosely to trap steam and even out the heating.
• Let it stand. A minute or two of resting is genuinely part of the cooking, and it does more for evenness than more power.
• Pierce anything with a skin or membrane, which will otherwise build steam inside and burst.

Why it cannot brown, and why that is structural

It is tempting to think a more powerful microwave would eventually brown something. It would not, and the reason is worth stating because it explains the whole regime.

Microwave energy is absorbed overwhelmingly by water. As long as there is liquid water present, the food sits at or below 100°C — the energy goes into turning water to steam rather than into raising the temperature. Only once a region is genuinely dry can it climb higher, and by then it is usually a small, scorched patch rather than a surface.

The environment compounds it. A microwave cavity fills with the steam the food releases, so the surface stays humid throughout, which is exactly the opposite of what a crust needs.

That is why combination ovens exist: they add a conventional grill or fan element, because the browning has to come from a different regime entirely.

Common questions