Getting the coffee smell out of stainless steel

A flask that smells of yesterday's coffee is the second most common complaint about insulated drinkware, after lids. It is also the one with the most wasted effort behind it, because most people attack the part of the vessel that is innocent.

Stainless steel is non-porous. Properly washed, it holds no smell at all, that is most of the reason the material won this category. What holds smell is every soft part of the assembly: the silicone gasket in the lid, the sliding hatch and the recess under it, a straw and its port. Those parts are all removable, and once you know that, the problem becomes an hour of soaking rather than a fortnight of scrubbing.

Why silicone holds it and steel does not

Coffee carries oils. Oils are what make coffee taste of coffee, and they are also what stains a cafetiere brown. Silicone is a polymer with enough molecular space in it to take those oils up, slowly, and give them back, slowly. Rinsing does nothing to an oil that is not on the surface but in it.

Steel has nowhere to put an oil. What you can get on steel is a film, a residue on the surface, especially at the shoulder where liquid dries, and a film comes off with hot water and detergent in twenty seconds. If the smell survives a proper wash of the body, the body was not the source.

The method that works

This is an hour long and two minutes of effort. Bicarbonate of soda is doing the work, and it works because it is mildly alkaline: it saponifies the oils rather than masking them.

• Take the lid apart. Lift the silicone ring out of its channel with a fingernail. Pull off the sliding hatch if there is one, and pull the straw and its port out if there is one. You should end up with three or four small parts.
• Make a soak: a tablespoon of bicarbonate of soda in a cup of warm water. Warm, not boiling, heat is not the active ingredient and boiling water hardens silicone over time.
• Put every soft part in it and leave it an hour. Silicone gives oils up slowly and the time is the method. Twenty minutes gets you a fraction of the result.
• While that soaks, scrub the lid's gasket channel and the recess under the hatch with a small brush. This is where the film lives on the hard parts and where a rinse never reaches.
• Wash the body with hot water and washing-up liquid, paying attention to the shoulder just under the rim where drips dry.
• Rinse everything, then dry it in pieces. Reassembling a wet lid traps moisture against silicone, which is how a smell that took an hour to remove comes back in a week.
• Refit the ring flat and even all the way round. Run a fingernail round the channel afterwards: you are feeling for a step, and a step means it is proud on one side and the lid will seep.

Things that get recommended and should not be

Bleach. It will remove the smell and it will pit stainless steel if it is left, especially at the weld line. It also leaves its own smell that takes longer to shift than the coffee did.

A dishwasher on its own. It gets the hard parts clean and does almost nothing for absorbed oil in silicone, because the cycle is short and the detergent is designed for surfaces. It is a fine follow-up and a poor treatment.

Neat vinegar for hours. Vinegar deals with mineral scale, which is a different problem with a different smell, flat and mineral rather than rancid and roasted. It has little effect on oils, and long acid soaks are not kind to gaskets.

Rice and salt shaken in the body. A folk remedy for narrow vessels that scours a film off hard-to-reach steel. It works on the body, which is rarely the problem, and it does nothing for the lid, which usually is.

Which pieces have removable parts

The method above depends on what comes apart. Across this range that is most lids, and it is worth knowing before you buy if smell has been an issue for you in the past.

Why it smells of yesterday and not of this morning

Fresh coffee smells of coffee. What people describe as the flask smell is not coffee at all: it is oxidised coffee oil, which is a different and sourer thing, closer to old cooking oil than to a roast. That distinction is useful because it tells you what to attack. Oxidation needs oxygen and time, so the smell develops in a vessel left closed and damp overnight, not in one rinsed and left open.

Milk changes the problem entirely. Milk proteins bake onto steel at drinking temperature and go rancid in the seams, and a flask that has carried milky coffee needs washing the same day rather than the next. Every one of the worst cases we have read about in reviews of this category involves milk, a warm car and a weekend.

Tea leaves a stain rather than a smell: a brown film of tannin on steel that scrubs off with bicarbonate and does not affect taste much. Fruit and squash leave a smell in silicone specifically, and it is the most stubborn of all because the compounds are small and sit deep.

A cleaning schedule that prevents rather than cures

Nothing in that table takes longer than five minutes except waiting, and the waiting is unattended. It is a great deal less work than the fortnight of experiments that a neglected flask eventually forces.

Storage, which causes more smells than drinking does

A closed insulated vessel is a small, dark, damp, sealed container at room temperature. That is a description of an incubator. Almost every dramatic case in this category, the ones with visible growth rather than a smell, comes from a flask that was closed while damp and opened a week later.

The fix costs nothing: store it with the lid off, or resting on rather than closed. If the lid must go on, make sure everything is bone dry first, which in practice means leaving it in pieces overnight rather than towelling it and reassembling.

Cupboards matter less than people think, and airflow matters more. A flask upside down on a draining rack dries. A flask standing upright with the lid resting on top holds a cup of humid air all night.

What about the steel itself?

Stainless steel does hold two things: a film, and metallic taste from damage. The film comes off with detergent. The taste is rarer and worth understanding, because it is usually blamed on cheap steel when it is actually the result of what went in.

Acid attacks the passive layer that makes stainless steel stainless. Long soaks in vinegar, undiluted citrus juice or bleach can pit the surface, especially at the weld line near the base, and a pitted surface holds residue and eventually gives a metallic edge to whatever it carries. That is why the method on this page uses a mild alkali rather than an acid, and why the folk remedies involving neat vinegar overnight are worth skipping.

If a vessel has developed a persistent metallic taste and the lid parts are new, the interior has likely been damaged and no cleaning will bring it back. That is an honest and unsatisfying answer, and it is better than selling you a routine that cannot work.

When you have a set of pieces rather than one

Anybody who owns three or four of these ends up with a drawer of lids and a question about which goes with which. Two habits save a surprising amount of irritation. Keep each lid with its own vessel rather than in a communal pile, because gaskets are cut for a specific rim and a nearly-fitting lid is the most reliable way to make a good vessel leak. And run the monthly soak on all the lids at once, in one bowl, which turns a chore repeated four times into a chore done once.

Prevention, which is easier than any of the above

Two habits remove the problem almost entirely. The first is rinsing within an hour rather than when the day is over, an oil that has not dried has not gone anywhere yet. The second is storing the flask with the lid off, or at least with the lid resting on rather than closed. A sealed damp vessel is a small, warm, dark place, and the smell that develops in one is not always coffee.

If you drink strongly flavoured tea or anything with milk in it, add a monthly bicarbonate soak to the routine before you notice a problem. It takes an hour of waiting and prevents the fortnight of experimenting that brought most people to this page.

When it is over

Silicone is a consumable. A ring that has taken espresso twice a day for two years has absorbed all it can and lost some of its spring, and no soak brings either back. At that point the ring is asking to be replaced rather than cleaned.

That is the awkward end of this guide, and honesty is better than a workaround: we could not find replacement gaskets sold separately for this range. If yours has reached that stage, ask the seller through the retail partner's message system on your order, they can answer for their own stock in a way we cannot from here.