A thermos that smells of yesterday is the most common complaint we hear, and it is almost never a manufacturing fault. It is nearly always the lid.
Why the smell starts
Coffee and tea leave oils behind. Those oils collect in three places: the seal ring under the lid, the threads at the neck, and the pour spout. The body of the flask is smooth steel or glass and rinses clean easily — the smell lives in the parts most people never take apart.
The second cause is storing a flask closed while it is still damp. A sealed, humid, warm space is where odour actually develops.
Daily cleaning
- Empty it as soon as you are done. Coffee left overnight is the start of every problem.
- Rinse with hot water immediately — hot water lifts oil, cold water does not.
- Wash with a drop of dish soap and a long, soft bottle brush.
- Take the lid apart. Most push-button lids unscrew into two or three pieces and there is a removable silicone ring. Wash each part separately.
- Leave it open to dry. Never store a flask with the lid on.
Deep cleaning, once a month
For stains and stubborn smell:
- Two tablespoons of baking soda, fill with hot water, leave overnight, rinse well.
- Or half water and half white vinegar for an hour, then wash normally.
Both are safe on stainless steel and glass liners.
What will damage it
- The dishwasher. Heat and pressure can affect the vacuum seal in the wall. Almost all quality flasks are handwash only, and this is why.
- Bleach and chlorine. They pit stainless steel and cause permanent spotting.
- Steel wool or hard scourers. Scratches give oils somewhere to hold and make the next smell worse.
- Boiling water into a cold glass liner. Sudden temperature change can crack glass. Warm it gradually.
The habit that matters most
Store the flask open. That one thing prevents most odour problems permanently. Stand it upside down in the cupboard with the lid beside it and there is nothing for smell to develop in.
If a flask has already lost its insulation — a drink that is lukewarm after two hours — that is the vacuum seal, and it cannot be repaired. You will find replacements across our thermos collection.