Why does my shaker bottle smell? Residue, warmth, and the plastic that keeps it

What the smell is

Whey is a milk protein, and the shake is usually milk or water with it. Left warm in a closed bottle, the residue does what milk does: it sours, and the proteins break down into compounds that smell of sour milk and, with some powders, of sulfur. Time and warmth are the two inputs. A bottle rinsed in ten minutes has almost no residue and no time; a bottle closed in a gym bag on a warm day has both, and by the time it's opened the smell has had six hours.

The dishwasher is the easy half of the fix, when the maker rates the parts for it — and several steel bottles do not: are shaker bottles dishwasher safe.

Where it hides

• The gasket channel. The groove the silicone ring sits in is the one place a rinse never reaches. Pull the gasket and the smell is usually there.
• The whisk hub. A wire whisk's plastic center and the wire coils trap residue; a plastic agitator's underside does the same.
• The spout and the cap hinge. A flip cap's hinge and the inside of the drinking spout stay wet and closed.
• The bottle wall itself. This is the one that doesn't wash out. Polypropylene is slightly porous and over weeks of warm residue takes the smell into the material.

Why polypropylene keeps it and Tritan doesn't

The makers make the claim themselves. BlenderBottle describes its Strada as "durable, stain- and odor-resistant Eastman Tritan plastic", which is a statement about the material not absorbing what it holds; its Classic is a BPA- and phthalate-free plastic without that claim. PROMiXX describes its PRO as "stain & odor resistant". Ice Shaker says its steel bottle "will not absorb odors". Steel can't absorb it; the gasket still can, so a steel bottle's smell is always in the lid.

What lifts it, and what doesn't

Works on a surface: baking soda and warm water overnight; then, separately, diluted white vinegar. Both address the residue's acidity from opposite sides, which is why one usually gets it and the other gets what's left. A brush in the gasket channel and the spout. Drying open.

Doesn't work: more dish soap (the residue isn't grease); leaving it to "air out" closed; bleach in polypropylene (it damages the plastic and adds a smell of its own). If two soaks don't lift it, it's in the wall.

Prevents it: the hour rule, drying with the cap off and the gasket out, and owning two bottles so the used one never waits until tomorrow.

The bottles built not to hold it