Keeping 2 litre bottles of Coke fresh

It’s a long held belief of some friends of mine that one of the best ways to keep a half full (empty) bottle of cola fresh was to squeeze the bottle before putting the cap back on. Less soda exposed to air, etc. Personally, I’ve recently come to believe that anything you can do to STOP the gasses from expanding will help keep the soda fresh. So, squeezing the bottle before recapping it would give lots of expansion room, whereas just putting the cap on only gives a limited amount of extra space before maximum pressure buildup.

What do you StraightDopers think?


I think people should stop drinking Coke. Nasty stuff.

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Maréchal Lyautey

There is this device I’ve seen that is a replacement cap for 2 liters. After you originally open the bottle, you use this cap.

After you screw the cap on, there is a small button you pump that gets the air out of the bottle, and keeps it tasting carbonated till the last drop.

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I think Enright3 is correct, unless the squeezed bottle is stored in such a way as to prevent it from reexpanding. Like on its side, with a weight on top of it. But this is pretty risky; expanding gas can be pretty powerful, and may well knock that weight off.

Best device I’ve seen is a combination bottle-cap and airpump. You screw it onto the bottle, and pump air into the bottle, increasing the air pressure inside the bottle, so that less of the soda gas is able to escape out of the liquid.

It seems there are two different solutions here. I go with Keeves’.

Taking air out of the bottle will just require that more CO[SUB]2[/SUB] comes out of solution to fill the void. More pressure in the bottle will keep the CO[SUB]2[/SUB] in solution. Of course, too, more O[SUB]2[/SUB], N[SUB]2[/SUB], and other gases will be put into solution.

If you store the pop on the refridgerator door, here’s a way to keep it carbonated longer:

Store it on a shelf in the main part of the fridge instead.

It is too clear, and so it is hard to see.

Gas solubility in water tends to INCREASE as the temperature goes down (exactly the opposite is true for most solids). So keeping the bottle in the fridge is a good thing. Keeping it in the back (where it’s colder) is even better if your shelves are large enough to stand it upright (see below for why this is a concern).

The equilibrium point between gas in solution and gas in the “head space” over the liquid is where the chemical potential is the same in both locations. In the gas phase, chemical potential is roughly proportional to partial pressure; in the liquid/solution phase it’s roughly proportional to concentration.

You want the chemical potential to be high in the liquid, which means you need to keep it high in the gas above the liquid. (The “pump” mentioned above actually should be pumping air INTO the bottle; that increases the partial pressures of everything, including the CO2).

If you squeeze the bottle, the theory goes something like this: There is less volume in the gas above the liquid, so the partial pressure will rise more quickly and less CO2 will come out of the solution.

This is correct, but there are other factors involved.

First, the bottle has to hold its shape. If the bottle is trying to go back to the “normal” shape, that’s going to decrease the partial pressure of all gasses in the head space, including the CO2, and MORE CO2 will come out of solution than would otherwise be the case.

Second, you have to assume that the gas will reach equilibrium before the soda goes flat or you open the bottle again. This may or may not be the case.

Personally, I don’t squeeze the bottles, and my wife does. I don’t notice that squeezing has any significant effect either way.

If you’re the only one drinking from the bottle, and you DO (drink straight from the bottle, that is), you might as well blow into the bottle before resealing it. There’s a lot more CO2 in your exhaled breath than there is in normal air (somewhere around 5x as much, I think, but one of our MDs could probably give you a better number).

Leaving the bottle on its side increases the area of the interface between the liquid and the gas. This has no effect on the equilibrium concentration, but it DOES have an effect on how quickly the bottle comes to equilibrium.

I saw a little pump top in the grocery store and bought one. You put the top on and pump it to increase the pressure in the bottle. It kinda works. The problem is that the seal is not as tight as it should be and the air leaks out. It is better than nothing.

But I switched back to six packs anyway.

Soda is cheap. It stays fresh for quite some time just with the top on. So, not much of a debate.

Any one want to call Coke & ask them for their opinion?

A way to keep leftover pop in a 2 liter bottle from going flat? Easy, make sure you finish the bottle. I only use 2 liters for (1) parties, and I can usually finish off the bit that’s left or (2) marathon sessions on the Internet during which I need to constantly revive myself with the caffeine* (although since I seldom plan these ahead of time long enough to stock a 2 liter, I usually have to make do with cans). (3) That’s a two-item list, sorry.

*I bought a 2 liter bottle grocery shopping earlier this week, because they were on sale, so I may be having a marathon this weekend.

I was getting some 2 lt bottle from store, it was rather high up. Anyway, the thing fell & hit the ground [on its side], bounced up to about 5’ where I caught it & put it back.
A guy saw it and chucked. So would some of you try this & see if it works? Its a full bottle.

At one point, when my chemistry classes were fresh in my mind, I tried to calculate what percentage of the CO2 in Coke would be lost to one liter of air at 25C. (I know, that’s not the temperature you store the stuff, but I was trying to simplify.) I came up with a truly insignificant amount, BUT my guess to the concentration of CO2 and related species in fully carbonated Coke might have been off by a great deal.

Anyone know concentration of CO2 derived species in Coke, or alternatively the pH of Coke and whether there are other species that have a significant affect on pH?

(Better yet, does anyone know the right way to do this calculation?)