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I have noticed that whenever you buy franks (frankfurters), they come in the package straight. But whenever I boil them in water, they get wavy shaped. I believe the world is fundamentally quantum, so this should have something to do with wave nature of matter:), but I am happy with any kind of explanation, may that be classical or quantum.

enter image description here

If I boil the franks too for long, they get torn apart, but I do understand that to some level, because the internal kinetic energy (molecular vibration) inside will be too much for the outer layer (peal) to hold, and will get torn. Basically the internal parts will expand too much. But in the case of this wavy shape, I have no understanding of how this gets created. The only thing I can think of is that one part of the franks tries to curl one way, the other part the other way, like the franks is made up of sections.

Question:

  1. Why do franks (hot dogs) get wavy shaped when boiled?
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    $\begingroup$ I’m voting to close this question because this is more suited to Seasoned Advice $\endgroup$ Commented Nov 24, 2021 at 0:22
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    $\begingroup$ @StephenG The question is seeking an explanation for an observed physical phenomenon, based on physics. How could this be off topic here? $\endgroup$
    – Sal
    Commented Nov 24, 2021 at 0:31
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    $\begingroup$ @Sal thank you so much! Yes, interestingly, not all franks (but most) come out wavy when boiled. Only a few stay straight. The ones that are wavy, will look mostly the same. $\endgroup$ Commented Nov 24, 2021 at 2:26
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    $\begingroup$ @StephenG please do not close this. I have checked Seasoned Advice and that is not what I am looking for. I am looking for a physical answer, and so far it seems like there are pretty good ones here. $\endgroup$ Commented Nov 24, 2021 at 2:28
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    $\begingroup$ @ÁrpádSzendrei I'll withdraw my close vote as you at least checked out Seasoned Advice. $\endgroup$ Commented Nov 24, 2021 at 2:53

3 Answers 3

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Sausages are usually made with a particular type of casing (frequently collagen and cellulose, since early 2000s) and it's not often you will find a sausage whose casing is perfectly uniform at all points around the sausage. Obviously the sausage expands as it is cooked, and given some parts of the casing have different consistency to others (one point can handle stress/stretching more than another), then this expansion will not be uniform, hence the waviness.

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This is definitely not a wave nature of matter phenomenon. Nor, since you're cooking them in water and not a microwave, is it a wave nature of light phenomenon.

A hypothesis: since this doesn't happen when you grill them, it's probably a result of water infiltration and bubble formation. The sausage is full of good nucleation points for water to vaporize at, and as it boils it blows open more space for water to get in. This creates a bulge, which curves the sausage. Something about the curvature - maybe modeled as the sausage being like a spring, with a restoring force proportional to the curvature - makes it energetically preferable to expand in alternating directions. Since the possible nucleation sites are many and random, the ones that require the least amount of energy to grow are the ones most likely to develop into big bubbles.

You could test this by cooking some at a simmer, rather than a boil, and see if they still change shape like this.


A much superior hypothesis: flavor was holding them straight. By boiling all the flavor out of them like a joy-hating sadomasochist, you've removed the internal flavor magic holding the sausages straight. If you grill or sauté, the flavor will remain and so will the straightness, thus confirming the flavor hypothesis.

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  • $\begingroup$ Do you mean wave nature of water? $\endgroup$
    – OrangeDog
    Commented Nov 24, 2021 at 12:07
  • $\begingroup$ I suspect many sausages / hotdogs will curve when grilled, unless you regularly turn them to heat as evenly as possible. $\endgroup$ Commented Nov 24, 2021 at 15:12
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The inside of the sausage expands more than the casing. The casing can deform easily, but resists stretching. The inside resists bending. The sausage has to find a shape that minimizes the energy put into overcoming these resistances.

So it finds a shape that is bent as little as possible and stretches the casing as little as possible, while still fitting into the casing.

squiggly sausage diagram

In this diagram I considered a constant stretching of the casing. You can see how the increased bending can find a configuration that fits inside.

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    $\begingroup$ Great explanation, not valid though. Franks go wavy during boiling even if you peel the skin off (tested, my mom used to hate the skin texture, made me remove it) .. My best guess, but it is a guess: The gunk in a frank is extruded by a spiral compactor. The material nearer the core of the spiral is compacted less, thus wants to expand less on cooking. (i.e. it is not a zigzag, it is a constant spiral shape) $\endgroup$
    – PcMan
    Commented Nov 24, 2021 at 11:57
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    $\begingroup$ Yeah, I have no idea if this is true or not, just wanted to add another hypothesis. I'm also not sure how I would test it. Even with peeled franks, the inside could expand more than the outside. (The outside is cooked first.) Perhaps we could put a hot dog in a particle accelerator... $\endgroup$ Commented Nov 24, 2021 at 12:01
  • $\begingroup$ @DanielDarabos well maybe start with a microwave (limited penetration depth) before putting hot dogs on the SLAC target plate! $\endgroup$ Commented Nov 24, 2021 at 15:13
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    $\begingroup$ @DanielDarabos Also, this is essentially a statement of the Principle of Least Action. Now we just have to write the Hamiltonian for this system. $\endgroup$ Commented Nov 24, 2021 at 15:15

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