Why Does Tea Taste Funny In A Thermos Flask?

Whether you’ve been walking or working, there’s nothing like opening your thermos and pouring a steaming cup of tea. Unfortunately, tea from a thermos flask often tastes, well, flasky. Why does tea taste funny in a thermos flask?

Tea from a thermos flask will taste funny if the flask is dirty or contains residue from another beverage. Leaving the tea in the flask for too long will mean the tea is over-brewed and the milk curdled. Tea can also react with the material of the flask, making it unpleasant tasting.

A thermos flask is handy for keeping beverages warm when you’re on the go or don’t have access to a kettle. Making sure your tea always tastes delicious, even when from a thermos, takes a little effort. Let’s look at why tea from a thermos can taste funny and how you can prevent it.

A Dirty Thermos Makes Tea Taste Funny

Tea can taste funny or unpleasant when poured from a thermos because the thermos flask is not thoroughly clean, so you can taste the residue of earlier beverages.

The odd flavor will be more prominent if you use your flask for more than just tea – coffee and soup can leave stains in a thermos and lingering smells.

It’s tricky to clean a thermos, even if you wash it out with soap and water and scrub it down with a cleaning brush. A quick cleaning is not good enough, as bacteria can start developing, particularly if you’ve made a drink with milk and sugar.

A clue to the flask needing a deep cleaning is the strange taste of the tea and an odd smell when you open the flask.

To deep-clean your thermos, do the following:

  • Rinse out the old tea and give the thermos its usual wash with soapy water.
  • Remove the rubber seals and lid and set them aside in a bowl of hot water.
  • Stand the open flask in the sink.
  • Put a tablespoon of baking soda into the flask.
  • Add half a cup of distilled vinegar.
  • The vinegar and soda will react and froth up.
  • Once the foam has subsided, fill the flask with hot water and let it soak for 10 minutes.
  • Meanwhile, put the rubber seals and lid in a vinegar and baking soda mixture.
  • Scrub the flask, lid, and rubber seals with a bottle brush.
  • Rinse the flask with hot water until there is no lingering smell of vinegar or soap. The residue of either can make your tea taste vile.
  • Dry the flask as best you can.
  • Tilt the flask upside down to drain all water and allow the thermos to air dry.
  • Store the flask without the lid screwed on. Putting the flask away when it is not completely dry also causes a musky smell and funny flavored tea.

Tea Tastes Funny When Brewed Too Long

A second reason tea from a thermos tastes strange is that it has been brewed for too long.

Unless you’re a fan of strong tea, builder’s tea as it is called in the UK, you probably find tea from a thermos bitter.

One way to avoid this is not to brew the tea in the thermos flask unless it is specifically made for infusing tea. Leaving tea leaves or tea bags in the thermos makes the tea stew for hours, upsetting the balance of bitter tannins and caffeine.

Unfortunately, even if you take out the teabags before filling the flask, the tea already infused in the water can over-brew if you keep it hot for hours, resulting in unpleasant tea.

Ideally, drink the tea from the flask within an hour or two to prevent it from becoming bitter.

Another way to prevent over-brewed tea is to fill a thermos with water and take your teabags with you to brew a fresh cup.

Tea Tastes Funny If The Milk Is Sour

A third reason why thermos tea tastes weird is that you made it with milk in the flask.

People tend to make tea with boiling water, pour it into the flask, and add dairy or plant-based milk.

By the time you drink the tea, it doesn’t taste very good because the boiling water combined with the tea’s natural acidity reacts with the milk – and curdles it.

If you’ve left the tea in the thermos for hours before pouring, the milk has had ages to break down and form sour clumps. Plant-based milk is particularly prone to curdling at high temperatures.

Avoid this tea disaster by taking milk to add to the tea when you want a cup. Long-life milk is ideal.

Alternatively, make a green or herbal tea that doesn’t require milk.

Tea Tastes Funny If Your Flask Is Poor Quality

Thermos flasks were invented to keep tea hot, so it sounds unlikely that the flask itself should make the tea taste nasty. Most flasks are made with inert materials that don’t react to the tea.

However, poorly made flasks or old, overused flasks can affect the flavor of your tea.

Tea is an acidic beverage and can react with the flask’s materials, especially if the thermos is stainless steel. Poor quality stainless steel will react with tea, with metals like nickel, chromium, and iron leeching or seeping into your drink.

Metal leaching increases with lower grade stainless steel that is subjected to high temperatures for long periods and when it contains acid liquids.

If the metal is corroded, rusty, or scratched, you can also have metal leeching into the tea. The more leaching occurs, the more worn the flask is.

Always buy the best quality thermos you can, preferably one produced to hold tea, like a traditional glass-lined flask.


Tea stored in a thermos flask often tastes strange because the flask isn’t properly clean, the tea has brewed too long, the milk has curdled, or the tea reacts with the flask itself. For the best-tasting tea, carry a thermos of hot water and make your tea fresh.


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