Microwave Oven: More Power Scotty?
October 20, 2014 12:58 PM   Subscribe

In a microwave oven of whatever wattage, is cooking something at 100% power for 30 seconds the same as cooking it at 50% power for a minute?

If it is not the same, why not and under what circumstances would I want to use the longer time but lower power method?
posted by 724A to Food & Drink (11 answers total) 4 users marked this as a favorite
Microwaves actually don't have "low power". To simulate it, they go through cycles that alternate between fully-on and fully-off.

This is useful when you want to heat something up more slowly/uniformly. Cooking something on high will often leave cold spots in the food, while a lower power will allow the food more opportunity to heat uniformly (as the heat moves to the colder spots via conduction).
posted by blue t-shirt at 1:04 PM on October 20, 2014 [7 favorites]

Your microwave operates using a "duty cycle" to achieve intermediate power levels. What this means is that the oven only outputs the wattage that it's rated for, just at different amounts of time. Most simply, 100% power means that the magnetrons are radiating for 100% of the time; 50% power might mean that over a ten-second cycle, the magnetrons will be on for five seconds and then off for five seconds.

Why you might want to do this: contrary to popular belief, microwaves do not cook "from the inside". The magnetrons emit radiation that can only penetrate a few millimeters into your food. So, the outside of the food heats up and then conduction carries that thermal energy inwards towards the center of your food. Conduction takes time, however, and if you're just blasting your food with energy constantly, you'll overcook the outside before the inside even gets warm. This is why you want to do things like defrosting on a low power setting.

Large, uniform chunks of things (like, say, chicken breast) would be best cooked or defrosted on a lower power setting. Smaller groups of fragmented pieces (broccoli florets?) have a much higher surface area-to-volume ratio and will heat more evenly at high power.
posted by backseatpilot at 1:06 PM on October 20, 2014 [3 favorites]

Lower power levels can let you cook using various techniques, such as frying, dehydrating, and steaming.
posted by Mr.Know-it-some at 1:10 PM on October 20, 2014

Penetration depth is 1 to 1.5 inches (from either side), so most foods are thin enough not to need that much heating by conduction.
posted by ambrosen at 1:12 PM on October 20, 2014 [2 favorites]

The way lower power modes work depend on the design of the microwave. While most microwaves do cycle between high power and no power to approximate power settings, Panasonic Inverter microwaves actually do heat consistently at the selected percentage of power to cook more evenly without crisping or drying out the edges of the food.
posted by eschatfische at 1:16 PM on October 20, 2014 [2 favorites]

I find it easier to do things that happen quickly (melting butter, for example) at low power levels for longer time, otherwise I turn my head to prep something else and I've got boiling, cooked butter, etc.
posted by jalexei at 1:20 PM on October 20, 2014 [1 favorite]

I am not sure I can describe what's going on in precise scientific terms, but when I cook something liquidy like pasta or rice in the microwave, I usually run the microwave at full power for long enough to bring the entire mass to boiling and then downshift to 50% for the remainder of the time needed to cook the item. For a dehydrated product like pasta or rice, the "cooking" process is not just going from raw to cooked, but going from dry to rehydrated. That process can only happen at a certain rate of speed, and pouring more energy into the equation is not going to get it to rehydrate any faster. So with rice, for example, microwaving a pot with 2 c water/1 c rice at 100% for 5 minutes and at 50% power for 15 minutes is just about right. If you microwaved the same amount of rice at 100% power the entire time, you'd use much more energy and your rice wouldn't be done any faster.

(I suck at rice and am a complete convert to microwaving it in a microwave rice cooker).
posted by drlith at 1:35 PM on October 20, 2014

A simple experiment:
Freeze two identical amounts of water (say 1 quart) in identical containers.
Put one in the microwave.
Microwave it on full power until you can tell the ice is melted. Note how long it took.
Replace the melted water with the other container of ice, taking care to place it in the same position.
Microwave on 50% power while watching to see when the ice is melted. Note how long it took.
Did it take 2x as long? If not, how big is the margin of error?

My assumption here is that the volume of water involved and the power of the microwave are both enough that heat absorbed from the air during the course of the experiment isn't going to be significant.
posted by Good Brain at 2:06 PM on October 20, 2014

My assumption here is that the volume of water involved and the power of the microwave are both enough that heat absorbed from the air during the course of the experiment isn't going to be significant.

I think you're confusing conduction and convection.

Radiation heat exchange is a surface phenomenon, not a volume phenomenon (grossly speaking). To exchange heat via radiation, such as in a microwave, the emitter needs to "see" the receiver, and for all intents and purposes air is "transparent" to the microwave radiation. Heat is exchanged at the surface (plus a penetration depth, but let's keep things simple here), and then conduction carries that energy into the colder parts of the solid body being heated.

The reason the air in the microwave heats up isn't from the radiation emitted by the microwave, it's because the body you're heating in the microwave is reemitting some of that radiation it absorbed via convection back into its surroundings. So your cup of water or your chicken leg is actually heating the air, not the microwave.
posted by backseatpilot at 2:20 PM on October 20, 2014

My main use case for lower power settings is trying to soften a stick of butter without melting it, because somehow I consistently don't realize that a cookie recipe isn't gonna want rock-hard butter straight out of the freezer until I'm halfway through the recipe and it tells me.

At high power, some of the better melts before the rest of it gets soft. At low power the heat diffuses through the butter more evenly and I get better results.
posted by aubilenon at 2:52 PM on October 20, 2014 [1 favorite]

is cooking something at 100% power for 30 seconds the same as cooking it at 50% power for a minute

You could consider it 'the same' in terms of the amount of energy you're imparting to the item you're heating, but in a lot of cases the outcome won't be the same.

If you tested both options with something like a cup of water, the results would probably be indistinguishable.

However if you're doing something like defrosting something frozen, dumping all the heat in at once will result in something that is overcooked on the outside & still frozen in the middle. Extending the same energy input over several minutes however, will give you something that is evenly defrosted.
posted by HiroProtagonist at 5:20 PM on October 20, 2014 [1 favorite]

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