Why does microwave defrost cook the edges of the food?

Microwave defrost overshoots at the edges because ice and water absorb microwaves completely differently — and there is a technique that largely avoids it.

Difficulty: beginner
Time: 8 min
Read: 4 min
Safety: caution

Short answer

Ice barely absorbs microwaves; liquid water absorbs them strongly. So the first thawed spot starts heating many times faster than the frozen part beside it and races ahead. Fix it by using low power, short bursts with standing time between, turning and separating the food, and shielding thin edges with small pieces of foil where your oven allows it.

This is not a fault in your microwave and no amount of choosing the right weight setting fixes it entirely. It is a consequence of the physics of how microwaves interact with ice, and once you know the mechanism the workaround is obvious.

Safety

Food defrosted in a microwave has warm patches that sit in the bacterial growth range, so it must be cooked immediately and completely — never defrosted in the morning and cooked in the evening. Do not refreeze anything that has been microwave-defrosted unless you cook it first.

Step by step

Understand why it runs away.Microwaves heat by making water molecules rotate. In ice those molecules are locked in a crystal and can barely move, so ice absorbs very little energy. As soon as a patch melts, that liquid water absorbs strongly and heats fast. The thawed area therefore gets hotter and hotter while the frozen core is still absorbing almost nothing — a runaway that starts at the thin edges and corners.
Use low power, not the defrost weight setting alone.Defrost programmes work by pulsing full power on and off. Setting 20–30% power manually does the same thing more gently. The pauses matter more than the power level, because they are when heat conducts inward from the thawed part to the frozen part.
Work in short bursts with standing time.Two minutes on, two minutes standing, repeated, thaws far more evenly than eight minutes continuous. During the standing time the temperature evens out through the piece. This is the single biggest improvement available.
Turn, flip and rearrange every burst.Take the food out, turn it over, move the outside pieces to the middle, and break apart anything that has loosened — mince, chicken pieces, prawns. Removing thawed portions as they free up stops them cooking while the rest catches up.
Shield the thin parts.Small smooth pieces of foil laid over the thin end of a fillet, the wing tips of a bird or the corners of a block reflect the microwaves away from the parts that thaw first. Keep the foil flat, smooth and away from the cavity walls, and only do this if your microwave's manual permits foil — some do, some do not.
Cook it immediately and completely.Microwave-defrosted food has warm patches in the bacterial growth range, so it goes straight into the pan or the oven. Do not leave it on the counter, do not put it back in the fridge for later, and do not refreeze it uncooked.
Plan ahead and use the fridge instead where you can.Overnight in the fridge is more even, safer and requires nothing of you but foresight. It is the right method for anything large. The microwave is for the evening you forgot, and for small, thin, evenly shaped items where it works well.

Tips

Freeze in thin, flat, even portions. A 2cm slab of mince in a bag thaws quickly and evenly in the microwave or in cold water; a fist-sized lump never will.
Cold running water in a sealed bag is often faster than a microwave for small items, and does not cook anything. Change the water every 20 minutes or keep it running.
Do not defrost anything large — a whole chicken, a joint, a turkey — in a microwave. The outside is well into the danger zone before the middle has moved, and most domestic ovens are not big enough to turn it properly anyway.
Some things cook better straight from frozen than defrosted at all: fish fillets, chips, peas, and most breaded items.

If it doesn't work

Edges are grey and cooked while the centre is solid

Cause: Continuous or high-power defrosting with no standing time — Fix: Stop, let it stand for five minutes to equalise, then continue at 20–30% power in two-minute bursts with equal standing time, turning each time.

Mince thaws into a hot outer shell round a frozen block

Cause: The thawed layer is absorbing all the energy — Fix: Scrape off and set aside the thawed mince every couple of minutes, so only the frozen block is left in the oven.

Chicken breasts go rubbery and white at the tips

Cause: Thin ends thaw and then cook — Fix: Shield the tips with small flat pieces of foil if permitted, turn every burst, and take each piece out as soon as it is pliable rather than waiting for the whole batch.

Questions people ask

Is microwave defrosting safe?

Yes, provided you cook the food immediately afterwards. The safety issue is not the microwave, it is that parts of the food are now warm, so it can no longer be treated as if it were still frozen.

Can I refreeze food I defrosted in the microwave?

Not raw. Cook it first, and then the cooked dish can be cooled quickly and frozen. This is the same rule as any other defrosting method, but it matters more here because parts of the food have been warm.

Why does my microwave ask for the weight?

It uses the weight to calculate a power-and-pause cycle. It helps, but it cannot know the shape, and shape is what causes uneven thawing. Turning and standing time still do more than the programme does.