Never Just Rinse Your Thermos with Water! Japanese Experts Reveal the Right Way to Clean It—Failure to Clean Properly Could Lead to Ingesting Mold and Heavy Metals

Thermos bottles have become indispensable in many people's daily lives. Thanks to their ability to maintain beverage temperatures for long periods—whether it's water, coffee, or tea—many people use them every day. However, if used over a long period without proper cleaning, not only do tea stains, coffee residue, and unpleasant odors accumulate, but bacteria can also multiply. Worse, incorrect cleaning methods may scratch the inner coating, reducing insulation performance and potentially causing users to ingest harmful bacteria.

Zojirushi, a Japanese thermos brand, warns against using inappropriate cleaning tools or methods and recommends a 4-step daily cleaning routine to help consumers maintain hygiene and extend product lifespan.

### Why Does the Inside of a Water Bottle Feel Slimy? Doctors Explain the Truth About 'Biofilm'

According to Zojirushi, thermos bottles are best suited for hot water, tea, and coffee. However, drinks like miso soup (high in salt), carbonated beverages, milk, and fruit juice should not be stored for long periods. These beverages may affect the container's material, causing corrosion or rust on the metal interior, damaging components, and ultimately reducing thermal insulation performance.

On the other hand, family physician Dr. Chen Chongxian warns on his Facebook page that the water we drink is not completely sterile. Every time we drink, bacteria from our mouths transfer to the bottle's interior. If the bottle remains damp for extended periods, these bacteria can adhere and multiply, forming a slimy layer known as 'biofilm.' Children's bottles are especially prone to food particles entering during meals, providing nutrients for bacteria and accelerating microbial growth. If this biofilm isn't removed regularly, hidden pathogens could negatively affect gut health and overall well-being.

### The Correct Way to Clean a Thermos: 4 Steps

Tan Dun-ci, known as the 'Mother of Non-Toxic Living,' also warned on the TV program 'Wish You Health' that if the inside or mouth of your thermos feels 'slimy' when touched, it's a sign that bacterial biofilm has formed. Immediate deep cleaning with a brush is necessary.

Tan also emphasized that the 'silicone sealing ring' on the lid is a 'high-risk zone' for mold and dirt buildup. Due to its crevices that easily trap moisture and dry slowly, it quickly becomes a breeding ground for bacteria and mold if not regularly removed and cleaned.

Regarding cleaning tools, Zojirushi strongly advises against using steel wool, rough scrubbers, or brushes with abrasive particles. These can scratch the inner coating, making it easier for dirt to adhere and odors to linger. Additionally, unless the product explicitly states it's dishwasher-safe, Zojirushi does not recommend using a dishwasher. High-temperature water and strong water pressure may cause the outer coating to peel, damage components, or allow water to seep inside, affecting the insulation structure.

Zojirushi recommends the following 4-step daily cleaning routine:

1. Rinse the bottle with water immediately after drinking. This prevents tea stains, coffee pigments, and odors from setting in. 2. Disassemble and separately clean all removable parts, such as the lid and sealing ring, to prevent dirt buildup in crevices. 3. Use a neutral detergent with a soft sponge or a long-handled bottle brush to gently clean the interior and all parts. 4. Rinse thoroughly with plenty of water, ensure no detergent residue remains, then invert and air-dry. Leaving moisture inside can lead to odor development.

### Should You Avoid Baking Soda? Natural Cleaning Methods Recommended by Experts

Zojirushi also cautions against the popular use of natural cleaning agents like baking soda and citric acid. While baking soda effectively removes tea stains, coffee residue, and odors, its mild abrasive properties mean frequent daily use may damage the inner surface over time, so it's not recommended for regular cleaning.

If rust or persistent odors appear, citric acid can be effective. Zojirushi suggests dissolving about one teaspoon of citric acid in 500ml of warm water, pouring it into the bottle, and soaking for approximately three hours. Then rinse thoroughly with plenty of water to prevent cleaner residue and minimize material damage.

Zojirushi also strictly prohibits the use of chlorine-based bleach, as it may corrode the metal interior and spring components inside the lid, leading to rust or holes and shortening the product's lifespan. For deep cleaning, oxygen-based bleach is recommended—following product instructions, soak once a month to remove stubborn tea stains, mold, and odors.

Proper cleaning not only ensures hygienic drinking but also prevents damage from incorrect maintenance. Zojirushi emphasizes that regular disassembly, drying, and periodic deep cleaning will keep your thermos in excellent condition for years.

Source: Zojirushi Responsible Editor: Lin Li