Question for a physicist and/or chemist

I have some wild rice. The instructions on it say to boil some water in a saucepan, stir in the rice, cover, reduce heat to a simmer, and wait a while. When I stir the rice into the boiling water, I also stir in some butter and a spice mix (sugar, powdered soy sauce, onion, sesame seeds, garlic, and some other stuff). I mix everything thoroughly, cover it (so I can’t see what happens), reduce the heat, and wait.

When the rice is done, I uncover it, and all the sesame seeds are in a ring around the edge of the pan. The ring is maybe an inch thick; the pan is about 8 inches in diameter. The butter and sugar are mixed throughout and not clustered in any place, but the other spices, like the sesame seeds, seem to be in higher concentrations in the ring and lower concentrations in the center of the pan. I have a gas-powered stove, if that makes a difference.

Why does this happen?

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7 Comments

  1. krustad says:

    Have you watched the water flows while the rice is cooking? In my rice cooker, at least, most of the boiling water (especially when the water level is getting low) moves from center out. Presumably the sesame and other spices are light enough that they get caught by the flow, while the rice doesn’t (and the butter and sugar dissolve).

  2. sorethumb says:

    I look at my rice boil/cook, too. The edges of the boiling bubbles never have the highest point of the bubble rise up the side of the pan. I suppose this’d be physically impossible. Ergo, bubbles that form in all places will cause sesame seeds to be pushed to the center, or the outer rim.

    Also: I wouldn’t put your spice mix in when the boiling is taking place. I’d put them in after most of the boiling is done, but before the rice has completely finished cooking. That way you still have a bit of moisture outside of the rice grains with which to mix your spices and butter. (I suppose butter first, if you have no moisture, so it will at least melt and allow you do mix in the other ingredients into the melted butter.)

  3. hmcmodelt says:

    I have a clear lid to one of my pots. Next time I make rice I will try to watch.
    Interesting.

  4. ovary says:

    Convection currents are likely coming up the middle (hottest), rolling to the outside of the pan (cooling), before going back down again to heat up and come up the middle. (like krustad said.)

    If you’re finding this sort of thing happening without the heat being on (no currents), I would be more likely to guess it has something to do with surface tension. Fun thing to do: take a coffee cup, fill it with water, and sprinkle pepper on the surface of the water. Put your finger in soap, and touch the top of the pepper water. MAGIC!

    Exciting things:
    http://hmf.enseeiht.fr/travaux/CD0001/travaux/optmfn/hi/01pa/hyb72/rb/rb.htm

    This was fun too:
    http://www.curiouscook.com/cook/other_writings_detail.php?id=40

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