If you have a can or two of unopened soda sitting in the back of the cupboard for months, how long does it take for it to go flat? I bet you’re wondering if that soda will still have any fizz when you open it or if it will have gone flat.
Unopened soda will eventually go flat as the carbon dioxide escapes the liquid and tiny gaps in the container. Faulty seals or dropping and shaking the bottle will make this process faster. Most sodas’ shelf-life is estimated at 6-9 months in a sealed bottle before the drink goes flat.
It’s worth noting that drinks in glass bottles or cans last longer than drinks in plastic bottles due to less carbon dioxide loss.
When you buy several bottles of soda on special, you wonder how long you can keep them before they went flat. After all, few people like the bland taste of a soda that’s lost its gas. I found several reasons your soda goes flat, even when you haven’t opened it.
Does Unopened Soda Go Flat?
Soda gets its bubbles from adding high amounts of carbon dioxide, which is dissolved into the liquid. This carbon dioxide creates a weak acid (carbonic acid, which is why it’s not good for teeth) and helps preserve your drink for longer.
If you have a Soda Stream or similar at home, you know from making your fizzy sodas that it’s easy to inject the gas into the drink from your canister. But it doesn’t take long for the soda to go flat if you keep it in the fridge, even in a sealed container.
Your soda will eventually go flat because the carbon dioxide – which gives your soda its fizz – is a gas that will slowly escape if there is a leak or if the container is made from a gas-permeable material.
But my bottle is sealed, you think! So how is the gas escaping? Well, you might have a faulty seal on your soda cap, but the regular plastic “PET” bottles most sodas come in are slightly gas permeable. While it will still take months for gas to escape, eventually enough will dissipate that your soda will lose its bubbles.
Most sodas are very tightly sealed, but sometimes the twist top is faulty, or the soda has been dropped, which agitates the carbon dioxide and makes the cap less securely closed.
The carbon dioxide is dissolved into the soda liquid and remains dissolved thanks to the combination of low temperatures and high pressure. If the pressure is released or the temperature is too warm, the gas will escape the soda faster.
When you pop the cap on a can of soda, all the carbon dioxide sitting at the top of the container gets released—giving you that crisp, satisfying hiss.
Why Does The Fizz Leave Soda?
Initially, the carbon dioxide is dissolved into your soda at the bottling factory. In bottles, there is an air pocket at the top, where some carbon dioxide dissolves (this helps prevent exploding bottles from being overpressurized).
Even when gas escapes into this space, there’s more carbon dioxide pressurized in the liquid than in the outside air.
If you remember your chemistry classes from school, you’ll know that gases want to be in equilibrium. The extra carbon dioxide in your soda is under pressure to move to where there’s a lower concentration of the same gas.
The gas in your bottle of soda will slowly bubble out until the gas left in your bottle is in equilibrium with the gas concentration in the air.
How Long Before Unopened Soda Goes Flat?
Most sodas have a use-by shelf life of about six to nine months. After that, the loss of fizz can be noticeable, and your soda will likely have gone flat.
By looking at the best-before date, you can know how long manufacturers expect the gas to remain in the bottle. You usually have a rather long shelf-life on sodas, but the manufacturers assume that by the use-by date, enough carbonation will be lost to start affecting the quality.
As long as the soda hasn’t spoiled, flat soda is OK to drink as I mention in a previous article.
If a bottle or can is tightly sealed, and there are no other cracks, gas can still escape—it can do this because no item is perfectly packed. However, the better the seal’s quality, the more gas-impermeable the container is, and the longer this will take.
Some things will make a drink go flat faster, even when the bottle is unopened. Avoid storing sodas in a warm or hot area, and don’t drop or shake them. Both of these will make your soda go flat sooner.
Does Shaking An Unopened Soda Make It Flat?
When you shake a closed bottle of soda, the carbon dioxide inside nucleates – this is where it begins to form small bubbles along the sides and top of the bottle. When you open it, you get a fountain of spray. This explosion is because the change in pressure has released all the carbon dioxide bubbles.
Dissolved gas reforms as bubbles after being shaken, so it is easier for the gas to slowly permeate the walls of the plastic bottle, allowing it to go flat faster, even when unopened.
It’s best to avoid dropping or shaking your soda bottles if you want them to stay carbonated for longer. However, if you drop a bottle accidentally, it might help to cool it down to let some carbon dioxide dissolve into the liquid.
Learn how to calm a shaken soda.
How Long Can You Keep Unopened Soda?
Most unopened soda has a shelf life of about nine months, but this is about the gas contents of the drink. After 6-9 months, most sodas will have lost some of their fizz, making them less tasty to drink.
The U.S. Department of Agriculture clarifies that these drinks are safe to consume as they are not perishable goods. The USDA suggests you can safely drink unopened sodas several months after their listed expiry dates.
They say diet sodas should be consumed three months after the expiry date, while regular soft drinks have a longer time frame of nine months after expiry. While the soft drink won’t taste as nice once all the carbon dioxide has gone, it’s unlikely to harm you.
It’s not advisable to freeze sodas because they can explode.
How to Keep Your Unopened Soda from Going Flat
Most of us would like to store our soda bottles to avoid them going flat too quickly. You can even keep them beyond their expiration date if the conditions are right.
Here are some tips to follow to keep your drinks fizzy:
- Glass bottles keep the carbon in for longer than plastic. If you plan to store soda for a considerable time, buy glass bottles instead. Plastic is more gas-permeable and changes the taste of your drink thanks to the acetaldehyde in the lining.
- If glass bottles are too pricy, rather buy your soda in cans. Again, they are generally a better gas seal than plastic bottles.
- Store your unopened drinks in a cool, dark place with little temperature variation. Heat will make the dissolved carbon in your drinks nucleate bubbles and escape, while cold helps keep the gas dissolved.
- Avoid dropping or shaking your bottles or cans of soda. Not only does shaking cause bubble formation, but it’s also easy to damage the container and allow more gas to escape.
Hopefully, you now know whether soda goes flat if you don’t open it. While it takes a fairly long time for the carbon dioxide in carbonated soda to escape, it will eventually be lost to the outside air. While this loss of carbonation won’t make your drink go off, it does lose its flavor and fizz.
Expect most unopened soft drinks to go flat in around nine months, though it can be faster or slower, depending on the container, seal, and storage.