I have a stainless steel Nissan thermos and also just bought a lovely Technivorm coffee maker that comes with a stainless steel thermos. Neither has an opening big enough for me to get my hand into to clean. So my question is....
how does one clean a stainless steel thermos? In particular I'm concerned about the residual build up of old coffee that will adversely affect the flavor of freshly brewed coffee.
get a long-handled scrub brush - i bought one specifically to clean the green tea stains out of my stainless travel mug.
A bottle brush is your friend. With a little bit of hot soapy water it should take care of most problems.
If you get an accumulation of crud that the bottle brush won't remove, try dumping in a tablespoon or two of uncooked rice, a fair amount of dish soap, and a little hot water. Put the lid on and shake hard until your arm muscles give out. The rice will abrade off most anything.
I use hot water, a bottle brush. If the buildup is bad, dissolve a few Tbls of baking soda and let stand for a few minutes and then scrub as usual.
Urnex 'Cafiza' is the best coffee stain/residue cleaning product that I have ever encountered.
It is actually the same product recommended for blind cleaning of manual espresso machines and can be obtained from any internet site that specializes in such equipment. I originally bought it for use in our two year old Vibiemme and it keeps all parts clean and fresh from any old coffee oils; but have since found it has amazing cleaning abilities.
After decades of use, our stainless steel lined thermos' which transport coffee on every family road trip, had taken on a dark brown tint that no amount of swishing with detergent or even abrasive cleaners like comet could dislodge. I tried the Urnex product and after two overnight soakings, the interior is like brand new and coffee tastes better as well.
Since that discovery, I use it once a week or so to soak out our stainless steel bottom Bunn coffee pots at the office which literally bakes on a daily basis and the stuff works magnificently. No scrubbing needed. Just pour in a tablespoon, add hot water, swirl to dissolve and let it soak overnight. Rinse well, until the water stops being soapy, do a final hot water rinse and it's amazingly clean.
I tried the same product on a very well stained Riedel decanter that has been used for decanting vintage port for nearly a decade and had a decidedly red film that no amount of scrubbing (rice, soft brushes, etc.) could dislodge. Within minutes of soaking and swirling, the crystal was crystal clear again.
Expensive at $12. per container, but at a tablespoon per use (espresso machine only requires a teaspoon), my original 16 oz container is still half full after a year or so. Sorry I couldn't make the following into a direct link but go out to the 1st line coffee website and look for the cleaning products. It really works