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?
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:
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.
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.
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.
