rotating and microwave ovens

February 25, 2005 12:00 PM

So, if your microwave oven has a carousel, is it necessary to stop the cooking halfway through and rotate the food, like most prepared food packages suggest? And do you place the item in the middle or slightly off to one side of the carousel?

I did a google search and came up with conflicting information.

I'm thinking that if you put something slightly off to the side, in the rotating, whatever cold spots there might be in the direct center of the oven would never be in the same spot.

posted by crunchland to technology (11 comments total)

No it's not necessary to stop and rotate the food.

Putting the item "slightly" off to one side wouldn't make a difference. If there is a cold spot in the direct centre of your oven, you'd have to put the item far enough to one side that none of it is in the direct centre of the oven -- otherwise you're just moving the cold spot to a different place on the item.

Whatever part of the food is in the direct centre of the oven will remain in the direct centre of the oven.

Now that I think of it... Maybe if you do have a cold spot in your oven that has caused you problems then you should stop to rotate/move the food partway through.
posted by winston at 12:07 PM on February 25, 2005

IME, most prepared/frozen food packages actually say to stir halfway through, which is still necessary with a carousel. Mainly to break up large semi-frozen pieces.
posted by smackfu at 12:12 PM on February 25, 2005

Even with a carousel, moving it would change the cold spots (these are in part the points where the microwaves are not intersecting the food), but I think generally the instruction to turn it is for microwaves with out carousels. You're correct that setting it off the middle would give a wider range of locations to move through and probably cok it better.
posted by abcde at 12:13 PM on February 25, 2005

Have you noticed a consistent cold spot in the middle of your oven? (I'm really asking.)

If that's a problem, then like winston said, it would help to move the food half-way through, since it doesn't matter how "off-center" you put it at first--the part's that not moving won't move, it'll just spin, whether it's really the exact center or the outer edge of your food plate.

If you've never noticed a consistent cold spot right there, then you don't have to stop and move it, since that's what the carousel is doing a much better job of, anyway.

The likelihood of there being a real cold spot in the microwave field is pretty small, I think, especially _right_ in the middle of oven. Most cold spots, as I understand it, are caused by the specific composition of whatever it is that you're cooking--whatever has more water in it gets hotter, and less water less. Unless you're heating up something really consistent like broth, or mashed potatoes, that's what makes for "hot spots", and so the carousel is definitely going to help by moving stuff around.

What makes a much bigger difference, in my experience, is _stirring_ the food partway through, for that very same reason. Even though my microwave has a carousel, I'll often nuke stew or noodles for a couple mins, stir it around, and finish nuking it.
posted by LairBob at 12:18 PM on February 25, 2005

the cold spots are predictable, known and fixed for any particular make of microwave oven. it would be a very stupid manufacturer who made the centre of the revolving tray a cold spot. but you can test this out - what is the distance of the centre of the rotating tray from the walls? if that distance is a multiple of 6.1cm (assuming a standard 2.45GHz oven and google giving me the right numbers) then it is cold and you should move the package around, i guess.
posted by andrew cooke at 12:29 PM on February 25, 2005

In truth, I've never gone to the trouble of testing my own oven. I don't know if it has cold spots, or where they might be. I was mostly wondering if the instructions on my lunch today, telling me to rotate it, even though there's a carousel, were another form of "lather, rinse, repeat."
posted by crunchland at 12:43 PM on February 25, 2005

oh, hang on. i may have the physics wrong. trying to check now...
posted by andrew cooke at 12:45 PM on February 25, 2005

false alarm. makes sense. you get standing waves, with nodes at the walls, and a half-wavelength of 6.1cm. the water molecule is jiggled around by the changing electric field, so the nodes are cold spots.
posted by andrew cooke at 12:50 PM on February 25, 2005

For those whose microwave doesn't have a carousel, it may have a rotating reflector next to the magnetron which serves the same purpose. The "size" of the chamber is constantly changing and standing waves, and thus cold spots, are averted. I have an older oven with one of these and have never really needed to rotate anything I've cooked.
posted by kindall at 1:22 PM on February 25, 2005

You could fill the bottom of your microwave with marshmallows and microwave them until they start to melt. This will reveal the hot and cold spots and let you measure the speed of light too.
posted by yarmond at 3:35 PM on February 25, 2005 [1 favorite has favorites]

The middle of my (cheap) microwave is a cold spot, so I move it directly across the carousel in the middle of cooking in order to change the spot that stays in the middle.
posted by rooftop secrets at 4:11 PM on February 25, 2005