4 Types of Drinks You Should NEVER Put in a Thermal Flask

Thermal flasks (vacuum bottles) are convenient for keeping drinks hot or cold for hours. However, not every beverage is suitable to be stored in them. According to experts, putting the wrong types of drinks into a thermal flask can damage the bottle, alter the drink’s quality, and even pose health risks if used long-term. Here are 4 types of drinks you should absolutely avoid putting in a thermal flask.

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1. Carbonated Drinks (Soda, Sparkling Water)

Carbonated beverages release gas over time. When stored in a tightly sealed thermal flask, pressure can build up inside the bottle. This may cause the lid to pop open suddenly, leading to spills or even minor injuries. In some cases, it can damage the inner lining of the flask. In addition, carbonation quickly dissipates, making the drink taste flat and unpleasant.

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2. Acidic Drinks (Lemon Water, Fruit Juice, Vinegar Drinks)

Drinks with high acidity, such as lemon water or citrus juice, can react with the metal lining of the thermal flask, especially if stored for long periods. This reaction may cause metal ions to leach into the drink, affecting both taste and safety. Over time, it can also corrode the inner surface of the flask, shortening its lifespan.

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3. Milk and Dairy-Based Beverages

Milk, soy milk, and other dairy-based drinks spoil easily when kept warm for too long. A thermal flask maintains temperature but does not prevent bacterial growth. Storing milk in a warm environment for several hours creates ideal conditions for bacteria to multiply, increasing the risk of food poisoning and digestive problems. Additionally, dairy residue is difficult to clean and may leave lingering odors.

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4. Herbal Teas or Traditional Medicinal Drinks

Many herbal or medicinal drinks contain complex organic compounds. When kept hot in a sealed thermal flask for extended periods, these compounds can degrade or react with the flask’s inner material. This may reduce the intended health benefits and, in some cases, produce unwanted byproducts that are not ideal for consumption.

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How to Use a Thermal Flask Safely

Thermal flasks are best used for plain water, warm tea (not too acidic), or coffee consumed within a reasonable time frame. Always clean the flask thoroughly after use and avoid storing drinks overnight, especially those that can spoil or react with metal.

Using a thermal flask correctly not only protects your health but also helps the bottle last much longer.