Microwave mysteries

April 25, 2009 8:23 AM

Sometimes I use the microwave for a few seconds to soften hard-frozen ice cream (for spoonability), or to soften a cold stick of butter (for spreadability). In these cases why does the center of the food heat first but the edges stay cooler; but when I microwave leftovers or soup in a dish for lunch, the edges of the food get hot first and middle stays cool?

Just something I've always wondered. And does this qualify as a food or a science question?

posted by cuddles.mcsnuggy to grab bag (9 answers total)

Related might be how to measure the speed of light using marshmallows.
posted by flug at 8:43 AM on April 25, 2009

My guess would be that the dish is absorbing the heat first, as the plates that come out of my microwave are usually much hotter than the food.
posted by PhoBWanKenobi at 8:46 AM on April 25, 2009 [1 favorite]

I'd guess it has to do with the heat retention qualities of the container.
Both butter and ice cream are packed in paper, which retains little heat, while presumably you're heating your soup in a ceramic bowl, which does retain significant heat.
posted by Methylviolet at 8:50 AM on April 25, 2009 [1 favorite]

Side note on the ice cream thing--It's been a long "standard" that ice cream should be "scoop-able" if your freezer is at the right temp. Sounds like your freezer is too cold.

Carry on.
posted by 6:1 at 9:37 AM on April 25, 2009

Microwaves work mostly by exciting the bonds in liquid water and to a lesser extent by exciting the chemical bonds in some fats and sugars. In soup or other items with lots of liquid water the microwaves are absorbed quickly and don't penetrate the material very far. When you microwave frozen items the heating action (until defrosted) is mostly by the less effective excitation of sugars and fats which allows the microwaves to penetrate the food deeper.

Because microwave ovens radiate the food in all directions frozen food of just the right composition and size can experience heating from all sides in the centre but just one side on the edges as the microwaves penetrate the centre and beyond. This rarely happens in foods with liquid water because liquid water is much more efficient at absorbing the microwave radiation preventing the rays from reaching the centre much less overshooting the centre. This means foods with liquid water heat at the surface from microwave radiation and heat at the centre from conduction.

You see the same effect with foods that are drier on the surface than in the middle. An apple turnover can be mouth searingly hot in the centre while still cool on the surface. The high sugar, high water content fruit in frozen muffins with fruit chunks will thaw out and become scorchingly hot first; sometimes while the much dryer dough is still frozen.

6:1 writes "Side note on the ice cream thing--It's been a long 'standard' that ice cream should be 'scoop-able' if your freezer is at the right temp. Sounds like your freezer is too cold."

The solidness of ice cream at any given temperature is going to vary wildly as it mostly depends on the sugar content. More sugar = lower freezing point. At temperatures for proper long term food storage (-20F) practically all ice creams should be hard as a rock.
posted by Mitheral at 10:24 AM on April 25, 2009 [9 favorites]

Instead of a few seconds to soften ice cream, try a longer time (like a minute or two) at very low power. Turn the microwave down to like 10 percent power. Works great for nice, even softening.
posted by rokusan at 2:32 PM on April 25, 2009

Cheap ice cream freezes hard as a rock, in my experience, while better quality (fattier?) ice cream stays smooth even when very frozen.

Hm. I know too much about ice cream. I should join a gym.
posted by rokusan at 2:32 PM on April 25, 2009 [1 favorite]

Not sure, but I bet it has something to do with the ability of the food to absorb the microwave energy, the container and its reactivity to microwaves*, and the mass of the food. The food in the middle probably gets just as much energy as the edges, but that energy dissipates to the rest of the food. At the same time, because the air is a better insulator than the food is (probably), the food on the edges can't shed as much energy and gains a higher temperature faster.

* Cheap ceramics heat up tons, because of the lead and other nasty metals in them. And because of the porosity, which lets in water, which heats up.

(Ice cream temps- your freezer, for the purposes of keeping stuff fresh the longest, should be as cold as possible. And well-made ice cream should be scoopable down to a very low temp, however. Making ice cream is all about making the water crystals in the frozen milk as small as possible, and mashing them up between the fat "crystals". Ironically, the warmer ice cream is stored, the harder it eventually gets as the ice crystals migrate and form into larger ice crystals. Store ice cream at 25degF and it will be as hard as a rock in a week. Store it at zero and not so much.)
posted by gjc at 5:05 PM on April 25, 2009

*Uh, not so much.

Put some nice clean surgical quality non-toxic as you could ask for stainless steel in your microwave and let her rip. Put some fine china with gold leaf in your microwave. Etc. (OK, really, don't.)

Things that get hot in the microwave, unless they are designed to do so, share a special property that scientists describe as "not microwave safe." They may, or may not, be food safe.
posted by Kid Charlemagne at 5:48 PM on April 25, 2009