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Thread: Effect of vacuum on bouyancy of rigid-bodied containers

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    Zwiefel's Avatar
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    Effect of vacuum on bouyancy of rigid-bodied containers

    I have been learning about and intermittently experimenting with Sous Vide cooking for about 3 years now. I'm still trying to find the boundaries of the technique for my style of cooking. One of the keys to a successful experience with it, is to have a good vacuum sealer for your product. This ensures that there is no air trapped in your bag, which would make your bags float, reducing the evenness of heat transfer into the food, as well as creating an insulating layer on one or more sides of the food.

    However, with some softer foods, pulling a strong vacuum will cause the food to become misshapen or even damaged. So, I was thinking today, what if I could use a rigid-bodied container, such as a mason jar? Experiment time!

    I took two 1-pint mason jars, filled them with an equal amount of water, and pulled a -9Bar vacuum on one, and no vacuum on the other. Afterwards, I placed them both in a 2gal cambro and filmed the process. I was surprised to discover that I couldn't see any difference between the two!

    After some contemplation about the physics involved, I think I've figured out what's going on. Whether something floats or sinks is a function of the density of the item (as compared to the density of the water it displaces). With a standard plastic bag, pulling a vacuum keeps the weight the same while reducing the volume, thus increasing density significantly. With a rigid-bodied container, it has the opposite effect! It reduces weight (the weight of the air that is pumped out) while keeping the volume the same, DECREASING density.

    Once again, reality is under no obligation to be intuitive.

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    Here is a site that explains the basic bouyancy math: http://www.wikihow.com/Calculate-Buoyancy
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    David (WildBoar's Kitchen)

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    Senior Member MAS4T0's Avatar
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    The one with the vacuum should be slightly more buoyant as it's slightly less dense.
    You've got the theory down, the upward force is equal to the weight of the fluid displaced (Archimedes' principle).

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    Zwiefel's Avatar
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    Quote Originally Posted by MAS4T0 View Post
    The one with the vacuum should be slightly more buoyant as it's slightly less dense.
    You've got the theory down, the upward force is equal to the weight of the fluid displaced (Archimedes' principle).

    Oh, yeah...I knew all of that before...I just didn't THINK before I tried it out....
    Remember: You're a unique individual...just like everybody else.

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    I recently went through some of this real time. I wanted to make Chawan mushi (Japanese Egg Custard) with the SV. I prepared it in 8 oz mason jars, kind of following technique for making creme brulee. I pulled a vacuum on some of the jars but not all of them before dropping them in the bath. The vacuumed ones sat flat on the bottom of the bath and the custard set evenly. The no-vacuumed ones landed at angles and I could not get them to sit square on the bottom. I got in too much of a hurry and left them that way. When they set the custard was at an angle and some of the egg product was floating in the bath. Not pretty.

    With egg product I did not see anything that relates to deforming but will pull a vac on the jars in the future to ensure they sit flat and don't leak.

    As an aside there are a few native Japanese ladies in this food group. When they like something they will tell me in English. When I presented the mason jars they twittered a lot in Japanese and smiled at me. I'm not sure which was worse - fixing the traditional dish in a SV bath or serving it in Mason jars....
    Older and wider..

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    Senior Member MAS4T0's Avatar
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    Quote Originally Posted by Zwiefel View Post
    Oh, yeah...I knew all of that before...I just didn't THINK before I tried it out....
    I'm sorry if that came across as implying that you didn't know. It wasn't what I meant, more as a quick cliff note for anyone scrolling through.

    To tell you the truth I initially just read the bottom line and was sat here excitedly hoping for the non-vacuum sealed jar to be more buoyant. While waiting for the video to load I was trying to figure out how to explain the trick (I was going to guess that you'd put ethanol in the non-vacuum sealed jar).

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    Senior Member spoiledbroth's Avatar
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    Quote Originally Posted by daveb View Post
    I'm not sure which was worse - fixing the traditional dish in a SV bath or serving it in Mason jars....
    A quick note, for you and them :

    Sous in french means "Under," Vide "Vacuum", so Sous Vide actually only refers to the technique of "pulling the vacuum" and sealing in the bag, the "bath" has been around since the time of Antonin Careme, the bath part of the process is none other than a highly accurate method of poaching-- even the process of insulating the protein (or whatever) being cooked from the cooking liquid is not new. Apart from potentially damaging cell walls depending on how much of a vacuum is pulled, the application of this technique in a wide variety of preparations isn't even tremendously inventive. The Sous Vide technique is really just an extremely precise method of poaching something in its own jus or liquor (how I would call the jus from vegetables, ie. mushroom liquor). The introduction of other gasses once a vacuum has been pulled is I suppose another topic entirely, but I think people who look sideways at SV should be informed it's really quite old hat.

    Though I'm not a huge fan of the mason jar method for custards, at least not a fan of serving them in the damned things. Too millennial for me,

    Most vacuum sealers, by the by, do not actually remove all oxygen, most aim for below 0.05% oxygen level, however I believe only really high end sealers and industrial grade equipment can achieve this. This could explain the apparent failure of the experiment. I don't think outside of a laboratory supply catalogue you're going to find ANYTHING that will pull an absolute vacuum.

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    I bought a small brick/paver for like .99 at home depot and I plop it on the packages, definitely solves the floating problem :-)

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    Quote Originally Posted by gic View Post
    I bought a small brick/paver for like .99 at home depot and I plop it on the packages, definitely solves the floating problem :-)
    The first time I tried to SV cauliflower I had a floating problem...combination of air pockets in florets and copious EVOO....I filled ziplock bags with water to solve it...I like your idea much better!
    Remember: You're a unique individual...just like everybody else.

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    Z do you have a chamber sealer or a clamp style pump sealer?
    "God sends meat and the devil sends cooks." - Thomas Deloney

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