Let the bottle dry before you cap it

You unscrew the lid and there it is — a smell that is not water, sitting somewhere in the bottle you have washed several times.

It is not residue and it is not dirt. It is something growing. Reusable bottles are a good home for mould, and the thing that lets it settle in is not how badly you wash. It is what happens after.

Wash it, then leave it upside down with the lid off until it is completely dry. Never put a bottle away sealed with water still in it.

A capped bottle with a few drops in the bottom is a small dark damp container held at room temperature. That is the condition mould is waiting for, and it does not need much time.

People who never meet this problem, asked why, say that they wash the thing regularly — soap, or the dishwasher, without much ceremony. That is a real answer and it is not the same answer as this page’s. Washing removes what is there; drying removes the conditions. If you are already washing it often and still getting the smell, the drying is the part that is missing.

If it already smells

You need to break the growth once, and then the habit above keeps it from coming back. Which method depends on what the bottle is made of, so start there rather than with the cupboard.

The one people recommended to each other is a denture cleaning tablet — the plain unflavoured kind — dropped into hot water and left to sit. It is made to lift stain and biofilm off something that goes in a mouth, which is exactly the job.

People do disagree about this one, and the disagreement is worth having before you try it. Some found the tablets left a lingering minty or chemical taste in the bottle afterwards, bad enough that they stopped using them. Others had no such trouble at all. What seems to separate the two is the flavoured versus unflavoured question above, and how thoroughly the bottle gets rinsed afterwards — so rinse it more than feels necessary, and if the taste arrives anyway, one of the other methods below will do the same job.

If the problem is that you cannot get a brush inside, there is a free version: dry uncooked rice, or coarse salt, with warm water and dish soap. Cap it and shake hard. The grains do what the brush would, and they get into the curve at the bottom where the bristles never reach.

Baking soda in hot water works on smell rather than on stain. One person used three or four tablespoons, shook it now and then, and left it a few hours. Diluted white vinegar is the other household standby, and people followed it with a soapy wash.

For a strong organic smell specifically — the rancid one that a protein shake leaves behind — people used rubbing alcohol, shaken inside the bottle and then rinsed thoroughly.

What your bottle is made of

This is the part that saves a bottle rather than ruining one.

Stainless steel: no bleach, at all. And no scouring — scratches in the inside surface give residue somewhere to sit, so the harder you scrub, the worse the next smell will be. Soft cloth, soap, soak.

Anything a baby uses: no bleach on bottles, teats, or anything else that reaches a baby’s food.

Hard plastic carbonation bottles: keep the aggressive stuff away from these — they are made for pressure, not for solvents, and people reported damaging them.

Ordinary plastic is the more forgiving one, with two limits: heat and sealing. If it is not marked dishwasher-safe, do not assume the top rack is fine, because thin plastic warps and can leach. And nothing plastic goes near an oven, which somebody had to be told.

The real work is usually in the parts you do not take off. The gasket in the lid, the screw ring, the inside of a straw — those hold the moisture longest and get washed the least, and a bottle can be immaculate everywhere except the ring you put your mouth on. Take the lid apart when you do a proper clean, and let those pieces dry separately, uncapped, along with everything else.

And there is a point where none of this is the answer. If there is visible black growth inside a straw or down in a gasket you cannot get out of the lid, and it comes back within days of cleaning, the part has been colonised somewhere you cannot reach. Replacement lids and straws are sold separately for most bottles and cost a fraction of the bottle. Scrubbing harder at a part you cannot open is the one move on this page that will not work.