Vacuum insulated thermos flasks are great at keeping drinks hot or cold for hours at a time and can even keep ice for 24+ hours. They are commonly built from stainless steel although some thermos flasks are still made with glass with a stainless steel exterior.
They are designed to keep drinks hot or cold on the go but what about the fridge? Can you put a thermos flask in the refrigerator or will it somehow damage your thermos and cause it to break or stop working?
I couldn't find any instructions from Thermos about it so I did some research to see if there are any problems with putting your thermos in the fridge or any risks associated with it. Here's what I found:
You It is safe to put a thermos flask in the refrigerator. They are designed to handle both cold and hot temperatures and the fridge poses little to no risk to your thermos. The vacuum insulation means the fridge will cool your drinks/food very slowly, but it'll keep cold drinks cold for longer.
However, you cannot put a thermos flask in the freezer. When liquids freeze they expand in all directions and this can put pressure on the metal or the glass of your thermos and cause it to break. If the vacuum insulation fills up with air your Thermos will no longer work to keep things cold or hot for long periods of time.
But there shouldn't be any issues putting your thermos flask in the fridge.
The reason people think you can't put it in the refrigerator is because Thermos (as well as other brands) tell you not to put them in the freezer. However, they don't really tell you why you shouldn't put them in the freezer.
But the fridge and freezer are very different and putting your thermos in the fridge is absolutely ok.
What Does Thermos Say About Putting Their Flasks In The Fridge?
I couldn't find very much information from Thermos about whether or not you can put their insulated flasks in the fridge.
Thermos' Care Intructions page makes no reference to either the fridge or the freezer and simply warns against scalding from hot drinks, whether or not you can put your Thermos in the dishwasher and if they can be microwaved.
This means I needed to look to other brands to try and work out if this is ok and if it isn't why not?
Given that thermos flasks are designed to be used outside, and sometimes in extremely cold temperatures, I would assume that yes the fridge would be fine. It's not going to freeze your drinks and cause them to expand and it'll cool them down slowly over time so there is going to be no rapid change in temperature.
It won't cool your drink down very quickly which we'll talk about next, but it shouldn't damage your bottle.
Will a Drink In Your Thermos Get Cold in The Fridge?
When you ask people online if you can cool down a drink in the fridge inside a thermos most people tend to respond with “why bother?”
Given the thermoses are designed to keep drinks cold then wouldn't it make sense to keep them in the fridge so they stay colder for longer?
Thermoses are able to keep drinks cold for so long because they are double walled with a layer of vacuum installation in between the inner and outer walls. Heat struggles to pass through a vacuum and so this stops most heat transfer.
When used with hot drinks this traps the heat inside your thermos and when used with cold drinks this stops the heat from getting into your thermos.
However, when you're putting a thermos in the fridge you want the cold to get into the thermos but the vacuum installation will stop the fridge from cooling down your beverage.
It doesn't work perfectly and so eventually the fridge will cool down your drink it'll just take a really long time.
If you want to speed up the process then you can take the lid off your thermos and the fridge will cool down your drink much faster.
Why Can't You Put A Thermos In The Freezer?
Thermos don't state in the care instructions that you shouldn't put a thermos in the freezer however lots of other brands say that you should never put an insulated flask in the freezer.
I discovered this article by Contigo (a competing brand who make similar products) that explained it simply.
When liquids freeze, they expand and take up more space. As the liquid freezes and expands, it can cause your water bottle or travel mug to become deformed and misshapen.
Even worse, the expanding liquid might break the welds and seals holding your water bottle together. If you place a stainless steel water bottle in a freezer, you might unintentionally and up damaging the vacuum seal, causing it to lose its insulating properties and not keep your water cold for as long. Ironic, right?
When you place your thermos inside the freezer eventually your drink will begin to freeze as it cools down. As it starts to freeze the liquid inside will expand in all directions.
This can put a lot of pressure on the walls of your thermos.
This is enough pressure to completely break a glass thermos but it's also enough pressure to bend the metal in a stainless steel thermos.
It is possible that the liquid will break the vacuum seal as it expands. When this happens the vacuum compartment will fill up with air and it will no longer insulate properly.
Why Is It Safe To Put a Hydro Flask In The Fridge?
Well it's not safe to put a thermos in the freezer because of expanding liquids it is safe to put it in the fridge. The fridge isn't going to freeze your drink so you don't have the same expansion and pressure issue.
Services are made from either stainless steel all glass and they are designed to handle cold temperatures very well. It can easily handle the cold, but not freezing, temperature of a fridge just as easily as it can handle the cold temperatures of the snow.
The vacuum insulation will work to slow down the cooling effects of the fridge and it will take a very long time for warm or room temperature drinks to completely cool down.
If you put an iced drink in a thermos and put it in the fridge the ice will last longer than if it was just left at room temperature. This is because there is less heat energy that your thermos is exposed to.
For a lot of people a thermos will keep drinks or food cold for long enough that they don't need to worry about putting them in the fridge. But if you want to store something in your thermos for a longer period of time then putting it in the fridge is actually a very good idea.
Just don't expect your fridge to quickly cool down your warm drinks, it's better at keeping cold drinks cold for longer.