This Is How Long Coffee Is Good For In A Thermos

Whether you work at an office that doesn’t have coffee shops nearby or you don’t want to put up with the high prices that your local cafe commands, you can always rely on homemade coffee and a thermos. This gives you more control over your coffee’s quality, price, and taste, but is it practical?

Your coffee is good for 6 hours in a low-quality thermos and can retain its taste and temperature for up to 12 hours in a premium flask. The key is temperature retention, and if your flask keeps your coffee indistinguishably hot even hours later, it is good enough to consume.

The rest of this article explores how you can make sure your coffee lasts in a thermos even if you’re taking it from one state to another. By the end of this post, you’ll be able to make your coffee at home and take it to work, gym, or a picnic in a different town, city, or neighborhood without a problem.

Can You Get Sick From Drinking Old Coffee?

We must separate “bad coffee” from “expired coffee.” If you’re not interested in taste, then coffee brewed and poured into a glass bottle is good enough for microwaving and consumption days later. However, many coffee connoisseurs would be horrified at the idea because the taste of said coffee would be completely different from what one expects from a freshly brewed cup.

You can get sick from drinking coffee made with expired or rancid grounds, but a well-stored serving of coffee, despite going bad due to temperature change, will not make you sick. It will just not taste good, and if you’re in dire need of a caffeine boost, you can chug it and keep moving forward.

Now that you know that expired coffee has more to do with the actual beans and whether there’s mold or other impurities in the grounds, you can separate it from “bad coffee,” which is simply well-made coffee that doesn’t taste as good because it has lost its temperature and has overoxidized. Microwaving such coffee does not bring back its original taste though it can fool an uneducated palette.

With that said, I emphasize the quality of the thermos when you’re transporting your drink. Your coffee is as good as the thermos it is in. As long as the flash doesn’t lose heart, your coffee is good. Theoretically, if a flask retains temperature for a week, your coffee should be good for the same period, but this isn’t practical. In the real world, flasks lose heat, and you can’t expect your thermos to keep your coffee hot and tasty forever.

Is Coffee Good After 12 Hours?

Since coffee is only good as long as it retains its temperature in a vacuum and a thermos gradually loses heat, the obvious question for most people is how long can the best thermos retain heat. With some flasks losing temperature in just four hours, is there a thermos that can retain heat for over 12 hours?

Coffee is good after 12 hours only if it is stored in an 18-hour thermos immediately after brewing. If your coffee has been sitting in a coffee mug, it not only loses taste but may have accumulated impurities from the environment. If it was covered, you can drink it, but it won’t taste good.

You can realistically expect coffee to last in a high-quality thermos for up to 12 hours. Any further is a claim you must be highly suspicious of since the leading manufacturers can’t push past the heat-retention mark of 18 hours (which means coffee starts going bad after 12). If you check your thermos for its effectiveness and find that it has retained heat for 18 hours, you can feel free to store your surplus coffee in it for later consumption.

How to Check Your Thermos’s Effectiveness for Coffee?

Since most households have an insulated flask, it might be unnecessary to purchase a new one if they already have one that’s good enough. Here is how you can check how long the coffee in your respective thermos can last.

Check the Manual or Manufacturer’s Website

If the thermos or flash at your home has an identifiable make and model, you can simply check its product page. Usually, the “keeps hot things hot for” section is pretty accurate and covers how long the liquid in the flask will remain hot.

That said, you must realize that it assumes the falk will be full. If you only fill it with coffee halfway, the air that gets trapped in will start absorbing some of the heat. More importantly, the temperature loss becomes significant 6 hours before the “keeps hot things hot” hour-mark. 

For example, if a manufacturer’s website suggests that your thermostat will keep hot liquids hot for 18 hours, your coffee will be technically speaking hot at 18 hours, but it will have lost enough heat to “go bad.” But if you subtract 6 hours from the 18-hour deadline, you can derive that said coffee will remain good for 12 hours, after which it will start losing significant heat and consequently its taste.

Do a Manual Temperature Check

If the thermos at your home is generic or the label has faded away, and you’ve thrown away the packaging, you will need to manually check how long the coffee you pour in it can last.

  1. Pour a hot liquid into your flask. I recommend you experiment with your regular coffee. Make sure the thermos is filled to the brim.
  2. Dip a thermometer in and note the temperature.
  3. Close the lid tightly.
  4. Open after six hours and use the thermometer to note the temperature.
  5. If the thermos has lost more heat in six hours than your coffee in the open air does in 6 minutes, the flask isn’t good enough to retain heat.

In other words, if you dip a thermometer in a hot cup of freshly brewed coffee and then do it again 6 minutes later, you can see the acceptable temperature drop threshold. If the temperature drops below that in your thermos or flask, it will not keep your coffee good for long as it has too wide an air gap.

Execute a Manual Taste Check

If the experiment above sounds too complicated, you can always try a simple taste check. Pouring hot coffee into your thermos, filling it up to the brim, and shutting the lid is all that’s required for this much simpler “experiment.” Simply open your thermos at the desired hour and take a sip. If it tastes good, the coffee is good enough for consumption. If it tastes unusually bitter, you might need to upgrade your thermos.

What Is the Best Thermos for Coffee?

With the specifics of coffee temperature and oxidation covered, we can now decide the best thermos for your coffee. Ideally, said thermos should have an 18-hour heat retention period, be portable, and have a decent look, so you do not look odd carrying it into professional environments.

FLSK Original Drinking Bottle is ideal for coffee because it doesn’t just keep hot coffee hot for 18 hours but can keep your cold brew and iced coffee cool for 24 hours. Above all, it doesn’t look like a bulky flask but instead has a discrete opaque bottle look.

Final Thoughts

As long as your coffee stays hot and doesn’t come in contact with air, it is good to drink no matter how long it has been since it was first brewed. It is advisable to add milk and sugar before storing your drink in the thermos. Finally, you should fill the flask to the brim and drink coffee in one sitting once you open it hours later.

Tim S.

Tim loves roasting, brewing, and experimenting with coffee. After years of perfecting this craft, working as a barista, and owning a small coffee service in college, he has decided to share his knowledge with the world.

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