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Cut orange kind of glues itself back together

  1. Feb 18, 2016 #1
    Many times my packed lunch includes an orange of the seedless variety cut into quarters, reassembled, and then wrapped in plastic wrap. About 5 hours latter it is eaten. On a regular basis, today for example, I have noticed that the orange tends to kind of "glue" itself back together again, that is there is some resistance to the orange coming apart where it was cut and I'm pretty sure the gluing occurs where the skin has been cut. This effect seems most noticeable with thicker skinned oranges.

    Could someone please explain what is going on. Is the orange trying to heal itself?

    Thanks for any help!
  2. jcsd
  3. Feb 18, 2016 #2

    Simon Bridge

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    Your first intuition is basically correct, the orange is gluing itself back together.

    When fruit is cut or otherwise damaged the ruptured cells release an enzyme that actively breaks down the tissue... its assuming it has fallen off a tree and is trying to make a new tree. It does not usually "try to heal" since its purpose for existing is to die. However, the thick skin contains oily resin, that can get sticky.
    The fruit pulp contains sugar and water, when the water dries you get a sticky syrup.
    You know how pancakes can stick together with molassas?
    The drying is only around the edges where air can get to it.

    Next time you cut some fruit, run your fingertips over the cut surface, then rub your fingertips together... compare different fruits for stickiness.
  4. Feb 19, 2016 #3

    Fervent Freyja

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    The orange is dead, it isn't gluing itself back together... What Simon says further is correct.

    This is what happens to roadkill(they have to use sharp shovels to remove their bodies). Most biological organisms will end up sticking to the surface after being dead for so long, especially if they have a lot of fluid expelled in that process.
  5. Feb 19, 2016 #4

    jim mcnamara

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    A more botanical answer: https://en.wikipedia.org/wiki/Resin
    Plants have resins which are an integral part of their defense system against pathogens entering a wound. Some people use the word "sap" to mean resin., e.g., pine sap.

    https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=define sap

    Sap moves down the stem from leaves to the rest of the plant. Resin lives in small pockets and does not move around much, unlike sap. You will see these terms misused in both directions, and by people who should know better.

    Anyway - the point is- what you see is resin attempting to seal a wound. Part of the bitter taste of citrus fruit rinds is due to those resinous compounds. For example, Seville oranges have more resins and the rind is quite bitter as a result. Orange oil and pine sap have antibacterial properties.

    FWIW - most fruits are alive for a time after they are picked off the tree. Respiration occurs (converting sugar to energy). Once fruit succumbs to either bacterial or fungal rots then the fruit dies.

    Example: Apples are kept crisp (and alive) in storage with low termperature (slows respiration) and compounds like DPA:
  6. Feb 19, 2016 #5
    So much to learn! Thanks to all.
  7. Feb 19, 2016 #6
    Of course fruits are alive.
  8. Feb 22, 2016 #7

    Simon Bridge

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    If a tube of glue sitting on a bench leaked a bit, then the leaked glue set, sticking the tube to the bench, I would say that the tube "glued itself" to the bench.
    So I guess it depends what you understand by the terms ... physicists tend to use the language of conscious motivation to describe unliving processes.

    As for if fruit being alive - even after being picked - consider:
    ... the consensus is "yes" - though it depends what you understand by "alive".
    I think it is fair to say that the orange under consideration is still alive during the observed events. It may be dying, but not dead yet.
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