Can You Put A Thermos In The Freezer Or Fridge?

Carrying a thermos or a flask, is an excellent way to stay hydrated when you're outdoors. If you've recently purchased a thermos, you must be wondering whether you can keep it chilled in the freezer or fridge. Will it affect the contents? Are thermoses worth the purchase? We have researched answers for you.

You can keep your thermos inside your freezer or refrigerator since it's been designed to withstand extreme temperatures. However, because of its insulated body, the interior will not increase or decrease in temperature--or if it does, it will do so very gradually. 

Having a thermos has a lot of benefits, especially if you love traversing outdoors. So, you may want to know how you can best utilize them in various situations. If you want to learn more about them, keep reading below!

Man pour tea and coffee in the cup from thermos, Can You Put A Thermos In The Freezer Or Fridge?

How do thermoses and flasks work?

Thermoses or flasks have a double-walled body that prevents heat from escaping. These two walls do not have air inside it, creating a vacuum, which means heat won't have anywhere to escape to.

This is why your coffee or soup should remain hot for long periods. There's nothing for heat to conduct because it's surrounded by a vacuum, and the mouth has a seal that prevents heat from escaping as well.

Woman drinking coffee from her thermos on the beach

In the same way, your drink can remain cold for long periods because heat can't enter and make the liquid lukewarm. The vacuum within the walls is impenetrable from heat.

Most thermoses even have a silver coating to reflect infrared radiation away, keeping heat trapped whether it is inside or outside so the contents in the thermos won't be easily compromised.

Why is retaining the cold temperature easier than the hot temperature?

You might have noticed that your coffee or tea seems to be getting lukewarm faster in the thermos compared to when you only keep cold liquids inside. Science explains that the heat transfer between two objects becomes faster the greater the difference in temperature is between them.

Also, heat escapes quickly every time you open your thermos for a sip of coffee, effectively cooling it. So, when you close the lid shut, it retains the temperature that it was left with.

Perception also plays a part in how hot or cold the drink feels. We are naturally more aware of heat than we are of the cold, so we can easily spot the differences in temperature even though the thermos performs the same way whether the contents are cold or hot.

How long can drinks remain hot in a thermos?

Temperature retention time depends on the quality of your thermos flask. High-quality thermoses can retain heat for up to 24 hours, while contents inside a mid-range flask can remain hot for up to 12 hours.

Hiker pouring a hot drink from a metal thermos bottle into a steel mug

Cheaper thermoses retain heat for 6-8 hours, which can still be ideal if you don't frequent the outdoors too much.

Flasks can retain even a boiling temperature, but make sure you have a steel thermos and not a glass-lined one. The sudden change in temperature will cause the glass to shatter and cause an accident. Before filling your steel thermos with steaming or boiling content, warm it up first.

Wash it down with boiling water and let it sit for around 10 minutes before putting your food or drink inside so its temperature won't drop by 10 degrees.

As much as possible, avoid opening and closing your thermos intermittently since it can make cold or hot contents lukewarm.

Read: "Can You Reheat Day-Old Coffee?"

How long can drinks remain cold in a thermos?

Drinking water can remain cold for 24 hours inside a high-quality thermos or vacuum flask. 

If you want it to remain cold for longer, avoid opening and closing the bottle often throughout the day. However, you won't really notice the change much, especially at room temperature.

How long can cold milk last in a thermos?

If you're trying to keep milk cold, make sure it doesn't spoil by keeping it at a constant temperature of under 40F. Anywhere above, it should be consumed immediately, provided it didn't sit out longer than 1-2 hours under a high temperature.

Milk that has been sitting at a high temperature (between 40F and 140F) can cause bacteria to grow. Dairy, even when placed in a vacuum-sealed environment, can still host bacteria under the right conditions.

To make your cold milk last longer, pour it into the thermos at a temperature of 32F-36F.

How long can fresh juice last in the thermos?

Fresh juice can remain drinkable and refreshing for 24 hours. However, you need to make sure that the juice doesn't have contact with the air for long periods--otherwise, it will go bad faster.

Pour the drink into the thermos once it hits the temperature of 35F. The temperature is ideal for lengthening the chilled state of your juice. The best way to enjoy your juice in the thermos is to consume it faster. The more you open and close the thermos, the more likely it is to spoil quickly.

Do not store your juice in the thermos for more than 72 hours. If you do, do not consume it--it may have already grown some dangerous bacteria that can be risky to your health. Besides, you won't be able to consume it anymore anyway if it is obviously spoiled--it would look moldy and taste rancid.

Benefits of Owning a Thermos

Pouring mug of coffee from a hot thermos

If you love the outdoors--whether going on a leisurely walk, traveling to different places or going on a hike--having a high-quality thermos will serve you well by providing a refreshing drink to hydrate or fill you up.

It's also useful for emergency situations. Vacuum-sealed thermoses and flasks can keep water fresh and uncontaminated for an indefinite amount of time, and you can also use them to store soups or food that you can carry for 24 hours.

To ensure that it doesn't break when you need it, get a food-grade and high-quality thermos that is highly durable and puncture-resistant.

Read: "Do Glass Containers Keep Food Warm?"

Light and Portable

Vacuum flasks and thermoses are light and portable, so you can carry them around while you jog or go on a hike. Some flasks have a slim body, so you can have more room to store other items. Thermoses won't weigh you down while you travel, so you're free to run or walk without worry.

Check out this portable 10 ounce Thermos on Amazon.


This is the most sought-after component of thermoses and vacuum flasks. The insulation of these bottles will prevent heat from transferring outside if you have hot coffee, or inside if you have cold water.

This quality ensures that your food remains appetizing, and your drinks remain refreshing.

Check out this 8-ounce durable Stanley flask on Amazon.

Resistant to Rust and Stains

Thermos and cup on wooden table in forest near lake

Stainless thermoses and flasks are rust and stain-resistant, which means they can keep the contents safe. However, you still need to clean your flask regularly, even if you only put water inside. This is because thermoses don't prevent mold growth, especially since it has a damp and dark environment.

Even though it has a stainless steel outer body, make sure you dry it out thoroughly so bacteria won't grow while it's not being used.

Check out this stainless 40 ounce Thermos on Amazon.

Things to Consider When Getting a Thermos

Woman drinking water from reusable water bottle

Vacuum thermos and flasks are an investment, so you need to get your money's worth. Thermoses vary in size, color, and, most importantly, quality. Make sure you get one that fits your lifestyle and routine and--if you're adventurous--that it can weather any situation.

Content Capacity

A thermos is only useful if it fits your routine. If you're going to be using your flask or thermos while commuting or walking around, you'd want to get one that can hold 16-32 ounces of liquid. 

If you're planning to use it for strenuous activities such as hiking, you'd want to get a bigger one, possibly around 64 ounces. That way, you'd get a steady supply of refreshing water or hot coffee while you traverse outdoors.


Screw-on lids are more effective at sealing the container and retaining the temperature. Make sure the lids aren't loose since it can make the contents spill while you're on the move.


Make sure you purchase one that fits your budget but still has a durable quality. Some brands are more expensive than others, so familiarize yourself with the features of a high-quality thermos and get one that has all the features you want from a brand that offers them for less price.

Final Thoughts

Thermoses are highly useful, and you should get one if you're often outdoors or if you need to be hydrated while on a long commute. Make sure to keep it clean so you can safely eat and drink from it.

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