keeping fizz in pop bottles

Factual Questions
1

Ive been having an ongoing argument with my girlfriend recently regarding pop bottles. You know the type that hold a well known variety of brown fizzy vegetable extract drink (ie coke). The argument is regarding how best to keep the fizz in the drink once you have opened it and drank some. She thinks its best to squeeze the bottle so the coke is at the top then screw the cap on. I think its best to keep the bottle in its original shape (in the absence of pump for the top to keep the pressure in the bottle). Do you have any light to shed on this topic

Regards Matt Johnson

2

Squeezing the bottle reduces the internal pressure, allowing more of the gas to come out of solution; she’s wrong, but you’ll never convince her this is the case - people stick to their fizz-keeping methods with zeal surpassed only by television evangelists.

3

Don’t squeeze the bottle: About carbonation

4

I should have said “Squeezing the bottle to expel air, then closing the cap while the bottle remains in the squeezed state reduces the internal pressure…”

Obviously squeezing it with the cap on would increase the pressure, but that isn’t what we’re about here.

5

Soda carbonation is a finite deal, so the best answer is to drink it up - squeezing the bottle to reduce volume won’t do diddly squat. You could try decanting the half or 3/4 full 2 liter into previously clean, appropriately sized and sanitized smaller bottles to keep what carbonation level is left, but this is more trouble than it is worth.

6

No,the reason drinks lose carbonation is because the amount of carbon dioxide has to come in equilibrium with the air. If you squeeze the bottle, there is less air so less carbon dioxide needs te escape for the same concentration. Thus, squeezing it WILL be effective.

7

Wrong. PRESSURE keeps the co2 dissolved in solution. If you squeeze the bottle, co2 will have to come out of solution first to get the bottle back to it’s regular shape, then more will have to come out to build pressure in the bottle.

If you don’t squeeze the bottle, less comes out before pressure is attained.

8

Particularly as the agitation involved in decanting it causes the loss of a lot of carbonation.

9

Haha this is class - i had same argument going for a week and Mangetout is correct: There is just no telling some people.

Im with Eleusis every time - squeezing the bottle will flatten your drink quicker.

Just be told !

sinical

10

Hmm… would the pressure be significant enough to deform the shape of the bottle? I was operating under the assumption that a deformed bottle will remain deformed simply due to the tensional forces of the plastic. On second thoughs, even mild flexing of the bottle will reduce the pressure, however, I still maintain that its not a significant enough a factor to matter.

11

Squeezing the bottle is only a good idea if you like flat Coke.

The carbonation is carbon dioxide dissolved in water (or Coke). What “holds” the carbonation in Coke at equilibrium is pressure. Not overall pressure, but partial pressure of CO2 (carbon dioxide). The easy means of calculating partial pressure is to multiply the pressure by the fraction of CO2 in the atmosphere.

SCENARIO 1: Assumptions: 1/3 of a bottle of coke has been consumed, bottle is at room temp, and partial pressure of carbon dioxide in Coke at room temp is 2 atmospheres. Bottle has not been squeezed. “Air” in bottle is a combination of air and carbon dioxide. Once capped, CO2 leaves Coke until fraction of CO2 * pressure in bottle = 2 atmospheres.

SCENARIO2: Assumptions: same as above. Bottle is squeezed before capping. Carbon dioxide again diffuses from liquid until fraction of CO2 * pressure in bottle = 2 atmospheres. The difference this time is that the fraction will be almost 1 since there will be little air in the bottle. However, at anywhere near 2 atmospheres, the bottle will bounce back to its original shape. All the void has to be filled with CO2, then pressurized. (Try it yourself, but shake the bottle to get to equilibrium faster.)

Scenario 1 had a head start on 2. There was already CO2 in the void, so less had to come from the Coke, leaving more in it.

Better ideas for keeping Coke carbonated

Make sure bottle is cold BEFORE OPENING THE FIRST TIME. This has two effects. Unlike solids, gasses dissolve better in colder liquid than warmer (in general). If the bottle is colder when first opened, the partial pressure is lower, so less CO2 is in a gas form, and less hisses out. While open, the CO2 is no longer in equilibrium, but not all the CO2 bubbles out immediately. Colder temps seem to slow the rate of diffusion.

Make sure the bottle remains cold, for the same reasons above.

Don’t agitate the liquid in the bottle before opening or when pouring, as that speeds the diffusion of CO2 out of the liquid.

These tips, especially the first, make a huge difference in how long a 2-liter will last.

12

Even so, the partial pressure of CO2 in a freshly opened bottle of coke will be greater than the pp of a half flat bottle of coke. At some point, you will reach an equilibrium mark where squeezing the bottle releases the exact same amount of CO2 as not squeezing. Everything after than will favour squeezing. Personally, I believe this point to be towards the start if not before the start of the bottle so squeezing will be more benificial overall. However, without empirical evidence, nobody can really say.

Upon reflection, it seems the best compromise would be to drop something solid or non-reactive into the coke until the level reached the top. That way, you dont have to contend with bottle deformity but you still maintain the benifits of low volume of air.

Another factor worth considering is that you cannot pour a full bottle of coke “smoothly”, that is, with air going in at the same time as coke going out. Rather, you get the glug-glug effect which I suspect also greatly increases agitation and CO2 lost. How all these interrelated factors play into each other I have no idea.

13

So you know, there is a doo hickey I bought from REGAL that allows you to pump more air into a 2L bottle of pop to keep the carbonation.

Works well. About $5.

14

In the good old olden days, when fizzy drink bottles all had a cap that could not be replaced, sticking a tea spoon handle first in the bottle was said to be the way to hold the fizz in.

Oh now I feel old. Just put the lid on tightly.

15

eh? Could you explain this further? The only reason I can understand why the 1/2 bottle of coke would have lower pp is because it is not at equilibrium or the coke is already flat (probably because you squeezed the damn bottle :smiley: ).

Yeah, or if a baloon could be inflated in the air space (the barrier would reduce the air space that had to be pressurized with CO2 pp).

I can get a nice, smooth pour out of a plastic 2-liter bottle, with only minimum “glugging” when full. I don’t know how much of a difference it makes. Having the bottle cold before opening makes all the difference.

16

Be careful what you say PhilAlex. The alleged effectiveness of the Fizzkeeper was the subject of a highly emotional debate some months back. Eventually calmer minds were able to consider the matter. <<Thanks again to Squink.>>

17

I’ve heard the same ‘tip’ given in reference to the preservation of opened bottles of wine, but the claim has been empirically tested and found to be completely false.

18

What if I squeezed the bottle after pouring, then inflated the bottle like a balloon. That would increase the amount of CO2 in the bottle’s atmosphere and slow down the flattening process, wouldn’t it?

=8^)

19

I mean because exhaled air contains more CO2 than un-inhaled air.

20

There will be quite a bit of CO2 in the air you squeeze out of the bottle, moreso than you will replace with the CO2 expelled by your lungs (which is quite low, really). But if you did that with one of those little pressurized CO2 cartridge thingies, that would help.