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I was recently looking up a recipe for quick Chawanmushi and came across a recipe which used aluminum foil in the microwave.

First of all, I thought that aluminum foil, with it's jagged edges would cause sparks. Second of all, why can't you just use some plastic wrap?

I did not try it because I didn't want to cause some sort of fire or do something bad to break my microwave.

Is there a good reason to use this method?

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3 Answers 3

Personally for an application like the one in this recipe I would likely use a parchment paper.

If I really felt my desired application required aluminum foil in the microwave I might try Martha Stewarts Foil and Parchment wrap, with the foil side down.

I'm not even sure that using foil to protect food would work in the microwave the same as it does in the oven.

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See this wikipedia page, it states that if not crumpled aluminum foils is generally safe, so make sure you do not crumple it. Then it says

The USDA recommends that aluminum foil used as a partial food shield in microwave cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.

The reference states

However, small pieces of aluminum foil can be used to “shield” areas of foods, such as poultry drumsticks and wings, to prevent overcooking.

It also states

General Rules for Safe Use of Aluminum Foil:

  • Use new, smooth foil only. Wrinkled foil can cause arcing (sparks).
  • Cover no more than 1/4 of the food with foil.
  • Shape the foil smoothly to the food so no edges stick out
  • It makes no difference which side of foil (shiny or dull) is facing out.
  • Do not place the foil closer than one inch from the oven walls.
  • If the microwave oven has metal shelves OR a metal turntable, don’t microwave food in foil containers or metal pans, and don’t let foil used for shielding touch or be close to the shelves or turntable.
  • If you see arcing (sparks), immediately remove the foil shielding; transfer frozen food from foil container to a microwave-safe utensil.

My bolding.

So make sure you only cover the top of the cup and make the foil smooth, do not crumple it and it should be safe.

The reason to use this as stated in the blog is speed, a few minutes compared to much longer.

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This is just so weird. I would not want to cook with anything that sparks and explodes and, as a former electronic engineer, doing this makes my head spark and explode cause I'm aware of what's going on technically with the oven. –  Rob Jan 25 '13 at 13:36
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I'm sorry, but this sounds like horrible advice. It is "unwise." Period. Why would we recommend something that is "generally unsafe in microwaves" for an application it is entirely inappropriate for?! It's a scientific curiosity that you can make it work at all... until it doesn't. The penalty for getting it wrong is breaking the microwave (best case) or a fire in the kitchen. I suspect the voting is going for the "wow" factor, but this is not culinary advice. It's a science experiment. –  Robert Cartaino Jan 25 '13 at 21:24
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Because it is not as bad as you think. Why would USDA even say what they say? You could accuse them for being overly cautious, but the opposite?? Really. Think about it, Gas is dangerous, it explodes, the gas is dangerous to breath, why would anyone ever want to cook with it? Deep frying???? Caramel? All very dangerous, I say using foil in a microwave is safer than deep frying! –  Stefan Jan 26 '13 at 3:40
    
I hear ya, but it still sounds like describing "safe" ways to wash your clothes with gasoline. –  Robert Cartaino Jan 28 '13 at 19:43
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I think the first implied question is whether the use of foil in the microwave is safe. Per the USDA,

[...] small pieces of aluminum foil can be used to "shield" areas of foods, such as poultry drumsticks and wings, to prevent overcooking.

...

General Rules for Safe Use of Aluminum Foil:

  • Use new, smooth foil only. Wrinkled foil can cause arcing (sparks).
  • Cover no more than 1/4 of the food with foil.
  • Shape the foil smoothly to the food so no edges stick out.
  • It makes no difference which side of foil (shiny or dull) is facing out.
  • Do not place the foil closer than one inch from the oven walls.
  • If the microwave oven has metal shelves OR a metal turntable, don't microwave food in foil containers or metal pans, and don't let foil used for shielding touch or be close to the shelves or turntable.
  • If you see arcing (sparks), immediately remove the foil shielding; transfer frozen food from foil container to a microwave-safe utensil.

As to the question of whether there is good reason to use the method in this specific case, in the linked recipe, the foil is on top of the cup with holes poked in it. It is not in contact with the food as far as I can tell. The author gives no reasoning or background on why foil is used instead of, for example, plastic wrap or parchment with holes similarly poked into it. I suspect it is to slow the heating of the custard at the top surface (microwaves will not penetrate the foil), while still permitting steam to escape slowly so as to not water-log the custard. Steam in contact with the surface will also help cook the custard at the surface.

Still, the presentation of the recipe (including egregious punning) and lack of explanation don't leave me with any sense of why it would be needed or effective in this particular recipe.

Are you willing to take this risk with your microwave? If so, go ahead and try it... Truthfully, my best guess is that the foil makes little difference compared to say using plastic wrap to slow the escaping of steam and therefore help cook the top, since the microwaves will still penetrate the custard from the sides and bottom, but I have not experimented.

My own best guess is that plastic wrap, with holes poked in it to allow the slow escape of steam, would be equally effective.

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never tried, but would plastic wrap not melt? I guess not –  Stefan Jan 25 '13 at 8:42
    
It is possible to melt plastic wrap in the microwave, but very, very unlikely. It would have to be in direct contact with food that gets to its melting point (as the plastic wrap does not absorb microwaves itself), and that food would have to be completely dessicated to rise to the melting temperature. It is a normal practice to use plastic wrap to cover food being microwaved to prevent splatters, and so that the warm steamy air stays on top of the food to help it heat more quickly (much like keeping the lid on a pot). –  SAJ14SAJ Jan 25 '13 at 8:45
    
@SAJ14SAJ There are two types of plastic wrap, PVC and polyethylene. PVC is nice that it sticks much better, polyethylene is nice that it doesn't have the plasticizers & can take heat better. You can use the PE wraps in the microwave, wouldn't try it with a PVC one. –  derobert Jan 25 '13 at 16:15
    
I have managed to melt plastic wrap in a microwave, without it being in contact with the food. The wrap was sealing a container with food inside. Water vapour rose from the food as it cooked; and it was hot enough to eventually start melting the plastic. I would still far rather do this than experiment with aluminium foil. –  David Wallace Jan 6 at 11:41
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