A certain person near and dear to me believes that if you cut something up it will cook faster in the microwave. For example let’s say you want to warm up a piece of meat. The person believes that cutting it into smaller pieces will require less time to reach the desired temperature than if it were cooked whole.
I say baloney. If you cut it in half, and then slowly brought the pieces back together, at what point would it behave any differently? Would a one inch separation mean they cooked differently? A half inch? Quarter inch? Zero inches?
I am not going to offer an opinion on the quality or other merits of microwave versus conventional cooking, but the physics is of interest to me.
Won’t the outside of the food absorb more microwave radiation(and thus heat up faster) than the inside? I would expect that the smaller your pieces are, the smaller the temperature differential between the outside of the food and the middle.
By the very act of getting hot, the food is absorbing microwave energy. It is self-shielding. Smaller pieces combat this.
(Obviously if you cut a chunk of meat in two and then keep the two halves in contact, you gain nothing. The point is to reduce the average thickness of the food so that the microwaves needn’t penetrate the outermost (absorbing) material to get to the innermost material.)
The microwaves (depending on power of the oven, moisture content in the food, etc.) penetrate about 1/2 to 2/3 of an inch. If you have a chunk of meat 2" thick, the microwaves can’t get all the way to the middle, so the center of the meat has to cook by convection. Cut it into smaller pieces, and the microwaves can cook all the way through all at once–assuming, of course, there’s room in the microwave oven to spread all of the pieces out in a single layer.
you can take a pile of mashed potatoes out of the refrigerator. if you leave in a heap and microwave the outside could be hot (even too hot to eat) and the center still be at refrigerator temperature. spread the pile out thin and microwave and it will be evenly hot.
Yeah, frozen lasagna is the gold standard in demonstrating this. If you leave it in one bing lump, you get super hot corners that can melt through the plastic tupperware, and a still frozen solid center.
The ideal shape for microwave cooking, ime, is a tauroid (donut or ring). If you spread solid chunks out on the outer perimeter of a plate, or make a hole in the center of a container of mushy stuff like leftover pasta or mashed potatoes, you get the best, most even results.
This will depend somewhat on how big the two halves are. An analogous situation would be if you had two books next to each other. Obviously their touching covers are dark. Pull them 1/4 inch apart, and you can see in, but the covers are still darker than if they’re separated widely. As you pull them apart, the cover get brighter as more of the room light can reach the covers.
These sizes are only coincidentally close. The attenuation length in meat-like material is probably close to the 1/2 to 2/3 inch stated by Gary. The gap you need to call things separated, though, is, a function of providing line-of-sight access to the standing waves present in the enclosure. Since 1/2 inch is a large fraction of the total size of the the typical occluding body (e.g., the rest of the lasagna), it is a reasonable gap size for getting most of the previously hidden surface “out from behind” something. If you had bigger food in a bigger microwave, the attenuation length would remain 1/2 to 2/3 inch, but the required gap for satisfactory separation would increase proportionally.
Note that cutting things up may sometimes give you a surprising result. Cut vegetables, for example, can arc in a microwave. Whether or not veggies arc is mostly a function of the mineral content of the soil that they were grown in. Sharp corners on the cuts and a higher power microwave also make them more likely to arc.
Doesn’t this have to do with the peak of the microwave wavelength? That’s why the rotating tray is there. Theoretically the optimal strategy is to cut up and align all the food so that it lies along the peak wave trajectories of the microwave emitter.