How Can a Bottle of Soda Pop Go Flat If It Has Never Been Opened?

My first reaction was no, not if there isn’t a slow leak somewhere in the bottle’s seal. But after extensive research, which consisted of dialing the 800 Consumer Information number on a Coca-Cola label, I find that it is not only possible, it’s quite common.

After prompting the nice woman who answered the phone to enter the appropriate words into her computer, I eventually learned that plastic pop bottles (they’re made of polyethylene terephthalate or PET) are slightly permeable to carbon dioxide gas and that over time, enough gas can diffuse out through the walls to diminish the effervescence.

That’s partly why, again to my surprise, many plastic soda bottles bear “drink by” dates on their caps. Glass bottles, of course, aren’t porous at all.

Classic Coke in plastic bottles, the woman said, has a recommended shelf life of nine months for optimum flavor and quality, whereas Diet Coke’s recommended shelf life is only three months. Why? “Try plugging ‘aspartame’ into your computer,” I suggested, whereupon after a few blind alleys we both discovered that the artificial sweetener aspartame is somewhat unstable and loses its sweetness over time.

By now we were having lots of fun with her computer, so I probed some more about what might affect the beverage’s quality. Freezing, the computer informed us, can lower the fizziness.

That one was a challenge for me to figure out, but this is what I think may happen: When the bottle freezes, the expanding ice can bulge out the bottle, and when it thaws the bottle may retain its expanded shape. That makes more gas space into which more carbon dioxide can escape from the liquid, lowering its effervescence level.

The moral of the story is always to check the “drink by” dates on your plastic pop bottles. A visit to my supermarket showed that Coke and Pepsi products are dated, but many other brands aren’t, except in the form of unintelligible codes. Store them all in a cool place, heat deteriorates the flavor, and chill thoroughly before opening.

And, yes, if your sister-in-law’s purveyors aren’t careful about how they handle the soda during distribution, or if it has been on their shelves, or hers, for years, it’s quite possible that when opened it will be as flat as her budget.