Cooking method

Microwaving

A microwave heats water molecules directly throughout the food rather than heating the outside and waiting for conduction. That makes it the fastest way to reheat something and a poor way to cook anything that needs a surface.

How it heats, and why it is uneven

Microwaves make water molecules rotate rapidly, and that motion is heat. Because the waves penetrate a centimetre or two into the food, heating starts throughout that outer layer rather than only at the surface — which is why a microwave is so much faster than an oven for small quantities.

The unevenness comes from three things. Penetration is limited, so the centre of anything thick is heated by conduction from the outside, exactly as in an oven. The waves form a standing pattern inside the cavity with hotter and cooler regions, which is what the turntable exists to average out. And different materials absorb differently — water heats readily, fat heats faster still, ice barely absorbs at all, which is why defrosting is so uneven.

The practical answer to all three is the same: stir, rotate, and rest. Standing time lets heat conduct from the hot regions into the cold ones, and it does more for evenness than any amount of extra power.

What it is genuinely good at

• Reheating, faster and with less drying than an oven for most things.
• Steaming vegetables, in a covered bowl with a splash of water. Fast, and it retains more than boiling does.
• Melting butter and chocolate, in short bursts at low power.
• Softening, and cooking potatoes quickly before finishing them in the oven.
• Blooming spices in oil, briefly, and toasting nuts at low power with attention.
• Warming milk, stock and liquids.
• Reviving stale bread for a few seconds — although it goes hard again quickly afterwards.

What it is bad at

• Anything that should be crisp or browned. Microwaves cannot brown, and steam trapped under food makes it soggy.
• Large or thick items, where the outside overcooks before the centre is warm.
• Anything with a skin or membrane — egg yolks, sausages, tomatoes, chestnuts — which build steam inside and burst. Pierce them.
• Defrosting, which it does unevenly enough that edges begin cooking while the centre is still frozen. The fridge is better where time allows.
• Even reheating of a full plate, unless it is arranged in a ring with the thickest parts at the outside.

Using it properly

• Use lower power and longer times. Almost every microwave complaint is solved by 50% power for twice as long.
• Cover it, loosely, to trap steam and heat more evenly. A plate over the bowl is enough.
• Stir halfway, at least once.
• Arrange food in a ring, with the thickest parts towards the outside, and leave the centre empty.
• Let it stand for a minute or two afterwards. This is genuinely part of the cooking.
• Never heat sealed containers, and pierce anything with a skin.
• Do not superheat water. Very smooth containers of clean water can heat above boiling without bubbling and then erupt when disturbed. Put a wooden spoon or a non-metal object in the cup, or heat in a rough-sided mug.
• No metal, except where a manufacturer specifically permits it.

Common questions