Squeeze the bottle to prevent carbonation loss?

August 30, 2005 10:01 PM

Does squeezing a 2-liter Coke bottle before putting the cap on help keep the Coke from going flat as quickly, or not?

I was once told that a partially empty 2-liter of Coke can retain carbonation longer if it is squeezed before putting the cap back on. This seemed reasonable to me, as the amount of volume not taken up by the Coke inside the bottle is reduced, leaving less air for the CO2 to diffuse into. BUT, if the bottle isn't squeezed hard (resulting increases that would cause the bottle to retain its crushed shape even if the cap weren't replaced) maybe the bottle "wanting" to return to its original shape could produce a mini vacuum effect, causing the CO2 to diffuse into the space more quickly (not sure about this)? Even if the 2-liter were given a "hard squeeze" (the kind that doesn't need a cap replacement to "stick"), I'm thinking that it might actually encourage more diffusion of the CO2 because whereas an unsqueezed bottle is full of air and allows for some diffusion of CO2, a squeezed bottle can actually expand the bottle to its original shape, allowing for the release of more CO2.

This all boils down to one question: does squeezing a 2-liter Coke bottle before putting the cap on help keep the Coke from going flat as fast or not??

A Google search turns up a few answers, but they differ on this detail. Does AskMetafilter have a more definitive answer?

posted by msittig to food & drink (15 answers total)

The bottle will "want" to snap back to its original shape and create a mini-vacuum that "sucks" CO₂ out of the pop. Looked at another way, the CO₂ is going to come out of solution until the pressure equalizes, and since the pressure is starting out lower (i.e., the volume of trapped gas is smaller, which is the same thing in this case because the bottle will expand easily to its original volume), there is more "room" for the CO₂ to come out of solution.

What you want to do is pressurize the bottle before storing it. I've seen screw-on pumps for pop bottles that do this in various kitchen gadget catalogs.
posted by kindall at 10:24 PM on August 30, 2005

kindall has it.

Another example: When a diver travels far enough underwater, the water pressure forces the amount of nitrogen dissolved in their bloodstream to increase. Rapidly moving to an area of lower pressure (read: surfacing) decreases the solubility of blood, causing the nitrogen to quickly bubble out of solution, resulting in the bends.

For the same reason, if you create an area of low pressure (i.e. the partial vacuum of the mostly-evacuated bottle) outside of a fluid in which a gas is dissolved, that gas will come out of solution much faster than if the empty space in the bottle were at the same pressure as the fluid.
posted by Danelope at 10:32 PM on August 30, 2005

Another confirmation of what kindall and Danelope said. It's all about pressure.
posted by stavrosthewonderchicken at 10:55 PM on August 30, 2005

I can testify that the screw-on pumps are quite effective.
posted by Galvatron at 11:53 PM on August 30, 2005

Third concurrence with what kindall said sans anthropomorphising. My father, retired physicist, would have had a stroke if he read that.
posted by Jenga at 11:55 PM on August 30, 2005

Hey, at least he put quotation marks around it. Another vote here for not squeezing it. If you're feeling adventurous, you can make a horrible faux-cap out of a bike innertube that will let you pressurize it with a bike pump. I recommend making it out of metal ("the fixture on the the end of a garden hose" being what you want to go for).
posted by j.edwards at 12:59 AM on August 31, 2005

Actually, the air pump is not effective, either. There is plenty of space between the nitrogen and oxygen molecules for CO2 to emerge from the liquid and return to the air above the liquid. Anecdotal evidence aside, the pump serves only to make you believe that the soda retains its carbonation better. What's more likely is that the pump forces you to place the pump's cap on REAL TIGHT to prevent air loss through the cap's seal.

Most effective? Keep the bottle as cold as possible, pour your soda into a GLASS container when you drink it (plastic has all sorts of "nucleation sites", or imperfections off which bubbles seem to launch themselves, depleting your soda of precious CO2 rather rapidly), and if you're really concerned, there is one particular brand of soda bottle that lets you carbonate anything (not just water) with CO2 chargers.

Once a new bottle of soda is opened, a large quantity of CO2 (held in supersaturation while the bottle was, well, bottled, at the bottling plant) is released. There's just no going back to that crazy fizz of a new bottle without increasing the partial pressure of CO2 in the airspace above the liquid enough to cause CO2 to go back into the liquid.
posted by Merdryn at 5:42 AM on August 31, 2005

Most effective? Keep the bottle as cold as possible...

Now wait a second. I buy 12oz sodas at school all the time and rarely finish them. I noticed after a while that if I put them in the fridge when I get home they are guaranteed flat by the next day, but if I just let them sit on the counter they will still have carbonation when I open them two or three days later. Any thoughts?
posted by Who_Am_I at 6:41 AM on August 31, 2005

Partial Pressure!

It doesn't matter how much non CO2 you put in the bottle, it is the pressure of CO2 that determines the amount of boiling.
posted by Chuckles at 7:01 AM on August 31, 2005

What about dropping in chunks of Dry Ice?

Thoughts?
posted by delmoi at 7:44 AM on August 31, 2005

Who_Am_I, it is possible that at the cooler temperature the CO2 solution is not super saturated, so even though more CO2 is present, you don't notice it. There is also a PV=nRT aspect... If you put the cap back on while it is cold you trap a certain amount of CO2, as the temperature of the gas rises the pressure of CO2 goes up... You would have to run the numbers to see which factors outweigh the others.

I think Merdryn's advice needs to be revised a little. If you value fuzz more than cold you should keep the bottle as cold as possible until you reseal it, then you should store it warm until the second use (maximum trapped CO2, maximum super-saturation when served).

delmoi, that is an interesting idea. However, I assume it wouldn't take much to burst the bottle, which would defeat the purpose... As Merdryn suggests, I think people normally use CO2 gas cartridges to carbonate at home, probably much easier to control and store, and I wonder if dry ice is clean enough for food use (I guess it probably is, but it is worth considering).
posted by Chuckles at 8:00 AM on August 31, 2005

However, I assume it wouldn't take much to burst the bottle

A 2-liter bottle can hold enough pressure to provide ~300kJ, which can launch a pineapple for 11 seconds. Trust me on this.
posted by Aknaton at 8:44 AM on August 31, 2005

[offtopic]

Aknaton, post video!

[/offtopic]
posted by Vidiot at 9:51 AM on August 31, 2005

Dry ice if you make it yourself would be food safe I'd think as long as you washed the dry ice mold before hand.
posted by Mitheral at 11:12 AM on August 31, 2005

Great-uncle was a cowboy engineer with Union Carbide in the 50's. When he was a teen-ager, he made his own root beer in glass bottles, using cubes of dry ice for carbonation. When he didn't cut the cubes just the right size, the bottles blew up.
posted by sol at 1:38 PM on August 31, 2005 [1 favorite]