I have a carafe of coffee brewed today that is still mostly full, and I would like to serve it tomorrow.
How can I reheat it without negatively impacting the flavor?
It's not going to be nice to drink a day later, no matter what. I'd use it in baking a chocolate cake or something like that instead, if you can't bear to throw it out.
The problem is not just the reheating, which will further cook the coffee and affect flavour, but that it's been losing aroma and oxidizing for a day first. If you're serving it to anyone you like even a little bit, serve them fresh coffee!!
I've recently been experimenting with this, and the best I've come up with is mixing it with hot milk.
I make 2-cup stove top espresso machine each day, 1 for the morning and 1 for the afternoon. I used to drink the afternoon cold as I couldnt find any other way to drink it and it still taste nice.
But now I put about 50ml of milk in the cup, heat it in the microwave for 30 seconds to get it nice and hot, then add the cold coffee (as its espresso its probably 50 - 100ml). It helps that I don't like super hot coffee anyway - luke warm and tasty.
Overnight however I'm unsure how much the flavour would deteriorate.