What is the Best Way to Keep Soda Pop From Going Flat?

Your objective is to keep as much carbon dioxide gas in the bottle as possible, because that’s what the fizzy bubbles are made of. Keeping the bottle tightly stoppered certainly has to be your first line of defense.

But frankly, it won’t help very much. There are many kinds of stoppers on the market, including that fancy pump-up job that is sold in stores. It’s a miniature bicycle pump that you screw onto the bottle, and then you pump a plunger to compress the gas inside the bottle. Sounds good but, unfortunately, it’s a complete fraud. All it does is make you think your soda is livelier than it is. Let’s see why.

Soda fizzes when dissolved carbon dioxide gas emerges as bubbles. The gas wants desperately to escape from the liquid because the folks down at the bottling plant have pumped in much more carbon dioxide than would ordinarily dissolve under atmospheric conditions. As soon as you open the bottle, most of that excess gas escapes into the room, and there is absolutely nothing you can do about that. Your only problem is how to make the remaining gas stay in the liquid for as long as possible.

Three things determine how much of a gas can remain dissolved in a liquid: the chemical reactions of the specific gas, the pressure, and the temperature:

• Reactions: Gases that react chemically with water will generally dissolve more readily than inactive gases, whose molecules have nothing to do but cruise aimlessly around in the water. Carbon dioxide is one of those gases that reacts. It forms carbonic acid, which adds that nice little pungent taste to soda, beer, and sparkling wine. Air (nitrogen and oxygen) doesn’t react with water. As a result, carbon dioxide at room temperature is more than fifty times more soluble in water than nitrogen is, and more than twenty-five times more soluble than oxygen.

• Pressure: The effect of pressure is just what you’d expect: the higher the gas pressure above the liquid, the more gas will be pressed into the liquid. The way it works is that at higher pressures there are more gas molecules flitting about per cubic inch in the space above the liquid, and more of them will therefore be diving each second into the liquid.

• Temperature: The effect of temperature is probably just the opposite of what you’d expect: the higher the temperature, the less gas will dissolve. Saying it the other way, the colder a liquid is, the more gas it can hold. The reason for this is a little more involved than we want to get into right now, so we’ll save it for later. But one example: At room temperature, water can hold only about half as much carbon dioxide as it can at refrigerator temperature.

Our conclusions, then, are that in order to keep as much carbon dioxide dissolved in the soda as possible, we must keep the gas pressure high and the temperature low. Temperature is no problem; we’ll just make sure it’s good and cold before we open the bottle, and then we’ll put the leftovers back in the refrigerator as soon as possible.

But pressure is quite another matter. At the bottling plant, the carbon dioxide molecules were forced into the soda like a crowd of claustrophobes into an elevator. The instant we open the bottle, a frantic exodus takes place, and virtually all the carbon dioxide pressure goes off in one big whoosh! Once that happens, your soda is inevitably going to flatten; it’s just a matter of time.

But is there really nothing we can do about that? Can’t we restore the pressure somehow, that we may live to belch another day?

Enter the gadget hucksters. Just screw their gizmo onto the bottle, they say, pump the piston a few times, and there you are. Next time you open the bottle, you’ll be treated to the biggest, most satisfying whoosh! you ever heard. And you’re supposed to think that your soda is factory fresh.

But guess what? There isn’t one more molecule of carbon dioxide in there than if you had simply screwed the cap on tight. You’d get the same big whoosh! if there were nothing but plain water and air in the bottle. The gizmo is nothing but an expensive stopper.

What you’ve pumped into the bottle is air, not carbon dioxide. Sure, there’s a little carbon dioxide in the air, but it’s only about one out of every three thousand molecules. The escape of a gas from a liquid can be decreased only by putting more of that particular gas into the space above the liquid. The amount of carbon dioxide that will stay dissolved in the soda depends only on how many collisions take place between carbon dioxide molecules and the surface of the liquid. If you had pumped in carbon dioxide gas, that would be another story; but nitrogen and oxygen are simply irrelevant. Bottom line: Keep it capped and keep it cold. It’s especially important to keep the bottle tightly sealed while it is out of the refrigerator, because that’s mainly when the carbon dioxide is emerging, due to the higher temperature. So pour what you want, cap the bottle, and put it right back in the fridge.

But don’t get your hopes up too high. You can slow down the exodus of carbon dioxide, but you can’t stop it.

And oh, yes. Whatever you do, never shake the bottle. That only speeds up the emergence of the gas.