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evilhanz
07-02-2001, 09:05 AM
When cooking frozen ravioli, the directions are as follows:
1. Heat water to boiling
2. Add frozen ravioli to the pot. It will sink to the bottom.
3. Reduce heat to simmer for 5 minutes.
4. Ravioli will be finished when it rises to the top of the pot.

Hmmmmm. Sure enough, it's true. But why does frozen ravioli rise as it cooks? Since it's initially frozen, I would think it displaces more water than cooked ravioli. So, one would expect it float at first, and then sink as it cooks. That's not true, of course. My second thought is that the pocket containg the cheese must cook and expand, greatly increasing each ravioli's total volume - and *that's* why it rises.

Any thoughts?

Fiver
07-02-2001, 09:40 AM
I'd go with your second thought. The frozen ravioli is denser, and so doesn't float. When you cook it, the cheese pocket and its contents expand, including whatever gases were inside. It's like an inner tube or rubber raft: much more buoyant when inflated.

I don't understand why you think frozen ravioli displaces more water than cooked ravioli. It weighs the same, so its displacement would be the same (discounting any ice crystals on the frozen ravioli, which shouldn't be enough to matter).

DrMatrix
07-02-2001, 10:10 AM
Originally posted by Fiver
I don't understand why you think frozen ravioli displaces more water than cooked ravioli. It weighs the same, so its displacement would be the same (discounting any ice crystals on the frozen ravioli, which shouldn't be enough to matter). The displacement depends upon the volume, not on the weight, at least until it floats, then the displacement depends on the weight. Cooked ravioli displaces more water than frozen; we know this because it floats. (Acutally, I think the weight should increase as the ravioli absorbes water.)

Chronos
07-02-2001, 04:02 PM
Water at certain temperatures (between 0 and 4 Celcius) expands as it cools, but it's an exception. Most substances, including, apparently, pasta, expand when heated.

Giraffe
07-02-2001, 08:35 PM
Buoyancy is all about relative density. As Chronos said, most things are more dense when solid (frozen) than when liquid. If something floats in still water, the total average density must be lower than the density of water (1 g/cm3).

However, in my experience, my frozen ravioli/tortellini float because of the convection of the boiling water. If I turn off the stove and let them sit there, they will sink to the bottom. So the average density of your ravioli may not be less than that of the water -- it may just decrease to the point that the water convection can keep it afloat. My guess is that this primarily occurs due to the pasta absorbing water, although the cheese melting might contribute also.

choosybeggar
07-02-2001, 10:04 PM
The pasta retains pockets of steam and hot air. This contributes significantly to its bouyancy.