A travel mug that smells after washing is almost never the body. It is the lid, and specifically the parts of it nobody removes.
Where the Residue Lives
A silicone gasket sits in a groove around the underside of the lid, and there is a gap between the gasket and the channel it sits in.
Milk, sugar and coffee oils work into that gap. A dishwasher does not reach it, because dishwashers clean by water striking a surface and this surface is enclosed.
Over months that becomes a reservoir of residue, and with milk involved it grows visible mould.
Cleaning It Properly
- Peel the gasket out of its channel, gently, from a corner
- Wash both the gasket and the groove with hot soapy water
- Use a small brush or a cotton bud in the channel
- Do the same for any slider or flip mechanism, which has its own cavities
- Dry both completely before refitting
- Seat the gasket evenly, the right way round, with no twists
Where a lid is designed with a removable gasket, of the kind on models listed at https://copco-usa.com/, that is a component intended to come out rather than be sealed in.
Milk Makes It Urgent
A mug used for black coffee tolerates neglect. One used for milky drinks does not.
Milk residue in a warm sealed cavity is a genuine hygiene problem rather than an aesthetic one, and a mug left in a bag over a weekend with milk in it needs a thorough strip-down rather than a rinse.
Rinse It Immediately
The habit that prevents most of this. Thirty seconds under a tap when you finish, even without soap.
Residue that dries in is considerably harder to remove than residue rinsed while wet.
Do Not Leave It Sealed
Storing a mug with the lid on traps moisture, which is exactly what the residue needs.
Lid off, both parts dry, stored separately.
Stains Are Not Dirt
Coffee and tea stain plastic permanently. A bicarbonate paste lifts some of it, and it is cosmetic provided the mug has been washed properly.
Narrow Openings Need a Brush
A cloth wrapped around a hand does not reach the bottom of a tall mug, and a straw needs a straw brush.
Dishwasher Is Not Enough
For any lid with a gasket or a mechanism.
Check It Monthly
Peel the gasket and look.
Two Lids Helps
Where a spare lid is available, having one in rotation means a mug is usable while the other is properly stripped and dried.