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For reasons of time, I like to cook more than one portion of a meal and then reheat a portion in the microwave whenever it's needed.

Aiming for maximum energy efficiency, I wonder if it's a good idea to cover the food with a big ceramic or glass bowl?


Just in case this is somehow relevant, here are my two main thoughts which led me to ask this question:

  • Covering might be good because the developing steam is concentrated in a small volume and gives off more of its heat to the food.

  • Covering might be bad because the bowl partly reflects the microwaves, and therefore a part of the energy never reaches the food.

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You do get some reflection of microwaves off a glass or plastic cover (Table 1 here)

    Material    Reflection  Transmission
    Metal       0.88         0.00
    Glass       0.38         0.60
    Plexiglas   0.16         0.83

Don't use metal lids. Neither glass nor Plexiglass plastic absorb much of the beam.

The question then becomes: Does reflection off a lid slow heating more than retaining heat in an enclosed vessel speeds it? Anecdotal, but experience tells me that using a plate over a bowl or a casserole dish cover routinely gives hotter, more uniformly heated food. Probably true for plastic wrap too, but I don't use that in a microwave since I had a piece of it melt on me.

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    Wouldn't you lose a bit, though due to the energy having to go towards heating the container as well? I often find that ceramics in the microwave come out hotter than the food I'm trying to heat...eg, the container can't be touched it's so hot but the food is still practically cold. :( – Catija Jan 31 '15 at 22:18
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    Both glass and plexiglass, as tested, absorb about 2% of microwaves ( 1-(.38+.60), 1-(.16+.63) ). If you've got old ceramic which has absorbed water; as you say, the container gets hot, but the food doesn't. Best to avoid dishes that like that altogether. I have some of those in my kitchen. They get used for making cole slaw or cake batter, not microwaving. – Wayfaring Stranger Jan 31 '15 at 23:04
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    Ah. I guess I should get some new glassware, then... I usually just use the bowls we eat out of to heat stuff up (mostly to reduce cleaning) but clearly that's not a good option. – Catija Jan 31 '15 at 23:06
  • @WayfaringStranger Thanks for that table, very interesting. I just searched for "microwave cover" in an online shop and found that literally all of them are made of some kind of plastic. Knowing the reason now, I definetly have to get such a thing :-) – MaxD Feb 1 '15 at 9:50
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I use the cover because it's easier to clean than the microwave if something splatters.

  • Doesn't try to answer the question. This would be better as a comment. – MaxD Feb 4 '15 at 9:48
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If I am reheating something like rice, than having saran wrap over it in the microwave adds moisture and aids in the reheating and subsequent moisture of the rice. I guess I have always had better luck having saran wrap over anything that may dry out a bit; rice, chicken, beef etc. Don't see how it could really do any Harm. Personal preference more than anything.

  • According to this article, plastic wrap (if labeled "microwave safe") is not dangerous: health.harvard.edu/staying-healthy/… The article warns that the wrap can melt if comes in contact with food, but it seems complicated to keep the wrap from touching the food especially when using a flat plate. How do you solve this? – MaxD Feb 1 '15 at 10:09
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I always cover food when reheating it in a microwave, for several reasons:

  • it prevents mess in the event of an explosion (particularly so with soup).

  • it prevents the food from drying out.

Lay the cover on lightly (perhaps with a small non-metal weight on top to hold it in place) or puncture it (if using cling-film): this allows air/steam to flow out of the container if pressure develops when heating (which helps avoid said explosions), and flow into the container when you remove it from the oven (cling-film is particularly adept at shrinking back down onto the food).

For the lid, experiment! Try a saucer, plate, glass, plastic, whatever until you get something that won't itself get too hot to let you lift it out. Try ANYTHING BUT METAL. I usually use a plastic lid saved from some old food container.

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no. this is becouse when food is been cover in a close container, the heat that is generated become too high. as it.become higher and higher, the vapour in the container will be force to move out and it lead to explusion.

  • Nobody said anything about a sealed container, loosely covering with a glass bowl will not create pressure. – Stephie Jan 6 '16 at 15:21

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