How Long Can Tea Be Stored In A Thermos?

If you’ve got a long day ahead at work, it’s comforting to know you have a hot cup of tea ready in your thermos. But is it safe to drink tea that’s been sitting there all night? How long can tea be stored in a thermos?

How long you can store tea in a thermos depends on the quality of the thermos and the tea’s ingredients. Best-quality flasks will keep tea without milk hot and safe to drink for 8 to 12 hours, and an average quality flask for six to eight hours. Tea with milk will start to go bad within 4 hours.

A thermos flask is handy for two reasons. First, it keeps your beverage hot and tasty. Second, it keeps your favorite cuppa from going bad and developing bacteria that can make you sick. Let’s look at how long you can keep your tea in a thermos before you should throw it out.

How Long Can Tea Be Stored In A Thermos?

How long you can keep your tea in a thermos covers two questions:

  • How long will tea stay hot in a thermos?
  • How long can tea remain safe to drink in a thermos?

 The answer depends on how well insulated the thermos is and whether or not the tea contains milk.

To understand why and how to store tea safely in a thermos, let’s first look at some facts about food safety.

Storing Tea Safely

The USDA’s safety guidelines around food and beverages note that when consumables are between 40⁰F and 140⁰F, they are in the so-called danger zone.

The danger zone is ideal for bacteria that cause food-borne illnesses  (like staphylococcus, salmonella, and campylobacter). At this temperature, bacteria levels can double in food within 20 minutes.

To keep food and beverages safely out of the danger zone and avoid the growth of bacteria, the USDA recommends the following:

  • Never leave food and beverages out of the fridge for more than two hours. In other words, you can drink a cup of cold tea if it’s been standing for two hours, not more.
  • To safely keep warm food out, keep it at 140⁰F or above. The food will start cooling over time, so this is safe for only four hours unless in an insulated container (like a thermos).
  • Keep cold food at under 40⁰F. If it is on ice or displayed, the food can only be out for two hours.

However, food and drinks can enter the danger zone more quickly if other factors apart from time and temperature are present:

  • Food: Tea contains milk and sugar, which bacteria are eager to consume.
  • Acidity: Tea is naturally acidic, especially if you’ve added lemon. Bacteria love an acid environment.
  • Oxygen: Exposure to oxygen causes bacterial growth.
  • Moisture: Tea is a liquid, making it an ideal petri dish.

To store tea safely, you will therefore need to keep it under these conditions:

  • Time: Up to four hours
  • Temperature: Over 140⁰F
  • Food: Preferably store tea without milk and sugar
  • Acidity: Preferably store tea without lemon
  • Oxygen: Keep the tea airtight in vacuum conditions
  • Moisture: Tea is always moist

A vacuum or thermos flask fulfills these conditions, making it ideal for storing your tea safely. Take care to distinguish between a thermos flask, travel mugs, and water bottles, as only a thermos will keep your tea hot enough for safety.

How Long Can You Store Milky Tea In A Thermos?

Because a thermos flask keeps your tea in safe conditions – out of the bacteria danger zone – you can keep tea with milk in a thermos for four to six hours.

Unfortunately, milk can curdle quickly in a thermos, despite the insulation. Both dairy and plant milk tend to curdle when heated and exposed to acid. Tea made with boiling water is acidic enough to sour milk. Storing milky tea in a thermos for hours gives the milk time to curdle.

As the flask cools, the milk’s temperature enters the danger zone, and bacteria can develop.

It is best to drink milky tea within an hour or two of putting it in a thermos for taste, heat, and safety.

Alternatively, take the milk with you and make your tea fresh.

How Long Can You Store Tea Without Milk In A Thermos?

Tea without milk offers a less inviting environment for bacteria, extending the safe drinking period.

An insulated mug or traveling mug will keep your tea warm for an hour. A low-quality flask will keep your tea hot, tasty and safe to drink for around six hours.

If you have a modern vacuum or thermos flask with the latest heat retention technology, you’ll be able to enjoy your tea for eight to 12 hours because these flasks can maintain a heat of 160⁰F.

After about eight hours, the temperature of the flask drops and enters the danger zone, so you should discard tea kept longer than 12 hours.

What Is The Best Thermos For Storing Tea?

Thermos flask technology has come a long way since the days of breakable glass flasks that shatter when you drop them.

Although die-hard fans still believe glass flasks keep your tea warmer for longer, flasks made from modern 18/8-grade stainless steel are the current choice.

Choose a flask made with quality stainless steel that won’t leach metals, that is rust-proof and highly durable. Avoid plastic flasks, which are not always BPA-free.

Like the old-fashioned flasks, modern flasks have a double-wall insulation layer separated by a vacuum – some thermoses boast even more layers. The vacuum is what prevents heat from leaving or entering, keeping your tea hot. Because the flasks are 100% stainless steel, there is no chance of the inner walls breaking.

A good tea flask has an insulated cup to drink from, so the tea stays hot, and you don’t burn your hands. It also has a neatly fitting lid to avoid leakage.

A feature of flasks created specifically for tea drinkers is a filter for your teabags or tea leaves that you insert or open into the flask of hot water when you want a fresh brew. This feature prevents your tea from over-brewing and becoming bitter.


Because thermos flasks can maintain a safe heat of over 140⁰F, they are safe for storing beverages like tea. The best flasks will keep your tea hot and healthy for eight to 12 hours, while cheaper flasks do the job for six hours. Store tea with milk for only four to six hours as milk can sour quickly.


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