Thermos is incredibly useful for storing hot and cold foods and liquids and maintaining their temperature for hours thanks to their vacuum seal, but can you safely put one in the fridge, and what will happen if you do?
A thermos can be put in a fridge with no ill effects or risk of damage because the fridge temperatures are not low enough to freeze the contents and thereby risk damaging the vacuum seal or the Thermos itself. Still, it will cool the contents much slower due to the insulation.
It’s essential to understand why you can put the Thermos in the fridge and not in the freezer, and this all has to do with the way Thermos are designed and some natural thermodynamics, so let’s jump straight in!
How Does The Thermos Vacuum Seal Work
The magic with Thermos is the vacuum seal, which creates an insulated air-proof barrier between the liquid in the Thermos and the outside air. No air between the inner container and the outer shell prevents heat or cold from moving between the two sections.
With regular cups and mugs, the heat or cold from the food or liquid will naturally equalize with the air temperature around it. So hot air from hot food will flow toward the colder surrounding air, which cools your food down.
With cold liquids and warmer air, the opposite applies as the liquid will absorb heat from the outside air, making your drink warmer – this is why ice cold beer on a sweltering day gets warm so quickly because the temperature difference between the icy beer and the scorching air is so significant!
But, if you put that same ice-cold beer in a thermos, the vacuum seal prevents the temperature transfer between the inner and outer, so your same ice-cold beer will stay ice-cold for much longer as the hot air cannot penetrate the Thermos and the icy beer!
What Happens When You Put Thermos In The Fridge With Hot Contents
When you put a Thermos in a fridge with hot contents, it will take them much longer to cool down than if it was in a regular non-insulated container. This is because the insulation prevents the heat from the hot food or liquid from escaping, and it bounces the heat back into the hot contents, thereby keeping it warmer for longer.
The contents will eventually cool as the colder air in the fridge will start to cool the Thermos, and the contents will also begin to lose heat, but this will take a lot longer than hot food in a regular container as the insulation.
This means that if you put a thermos with hot soup or a stew in the fridge, it will still be warm-hot within an hour and still be warm after a few hours, whereas a regular container would cool that food off within an hour.
What Will Happen If You Put A Thermos In The Fridge With Cold Contents
The same principle applies to having cold liquids in your Thermos and then putting that in the fridge. The insulation won’t allow cold air from the refrigerator to penetrate through the vacuum seal, so the contents will remain at the temperature they started when placed in the fridge.
Over time, maybe five to eight hours, the effect of the colder air in the fridge will start to cool the contents down a little, but not to the same degree as it would for a non-insulated container. If the liquid is ice-cold, it will stay ice-cold for a good few hours but eventually start warming up.
Why Can’t You Put Thermos In The Freezer
The primary reason why you should never leave a thermos in the freezer for a long time is that the effect of freezing liquid will cause that liquid to expand and, in a contained environment like Thermos, the pressure of an expanding fluid in a closed container would rupture the vacuum seal and cause the insulation to fail.
In some cases, especially if the Thermos was full, the liquid could expand to such a degree that it could crack the Thermos itself! Remember, ice and water caused the formation of some of the most remarkable geological structures on earth by cracking and breaking solid rock, so frozen water expanding in your Thermos will have no trouble cracking it!
So, you can leave a thermos in the freezer for a short time, but not for too long, as the frigid air in the freezer will start to cool the outer shell of the Thermos and gradually work its way to the seal and then it will begin to cool and eventually freeze the contents.
Using Your Fridge To Cool Liquids In A Thermos
Suppose you want to take an icy drink with you, whether to a game or gym, but you have to mix the drink at room temperature. By putting the liquid in the Thermos and then putting the Thermos in the fridge, you can use the fridge’s cooling ability to get your drink nice and cold!
You do this by leaving the lid off the Thermos, which will allow the cold fridge air to penetrate the liquid and cool it down! Then, once it’s at the right temperature, you can close the Thermos, and off you go!
Remember that to do this, you must leave the Thermos lid off, or it will then seal the Thermos and activate the insulation, so by the time you leave, your drink will still be at room temperature or maybe slightly colder, but not that crisp-cold temperature you want!
Putting your Thermos in the fridge will not affect the insulation or the vacuum seal. Still, it also means that it will take longer to cool whatever is in the Thermos because these flasks are designed to keep the contents warm or cold for a very long time, so in most cases, you don’t need to put the Thermos in the fridge.
But, if you want to store your Thermos in the fridge, especially if it has cold contents, then you can do so without risk and, depending on how long you leave it, you may find your ice-cold drink is that little bit colder when you take it out!