Is your soda fresh? If that sounds like a strange question, flip a can of soda and look at the bottom. You will notice an expiry date (see below), and you can use that to decide if your soda is fresh!
Expiry dates can also be found on the plastic bottles of soda (usually near the neck area) - why do these dates exist? And say we wanted to keep a stash of soft drinks for a period of time but would like them to be as fizzy as possible when we opened them; would you choose the aluminium soda can or the plastic PET (polyethylene terephthalate) bottle?
We might want to choose the soda can because of permeability. Permeability is the penetration of gas, vapor or liquid through a solid. According to a Slate article, PET has a higher permeability than aluminum. Hence, CO2 can pass through the PET container more rapidly than through a soda can. In fact, as much as 15% of its CO2 could escape over a period of 12 weeks (and hence the expiry dates)!
Of course this is if the only criteria is fizziness. The different materials may also affect the taste. Very tiny bits of the soda can's metal may still dissolve in the soda, or very small amounts of acetaldehyde from the plastic bottle may make its way into the drink in the plastic bottle. Either way, the taste is affected, even if the amounts are minute.
Our best bet may be glass bottles, which neither dissolve nor are permeable. But, of course, we don't see glass bottled soda as much anymore.
Second Helpings
Not all have it, but more and more brands of soda come with expiry dates. And now that you know why it matters, paying more attention to them guarantees you will always have fresh soda!
photos: in order - vook/depositphotos; personal; depositphotos/scanrail; scukrov