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How do I keep coffee hot?

Optimally, brew a fresh batch whenever you want coffee. To keep coffee hot for shorter periods of time, or for travel, use a thermally insulated container; an enclosed container will reduce the loss of the aromatics that constitute an essential part of the overall flavor (but see here). Insulated containers with glass internals, though somewhat delicate, have the least effect upon the coffee taste, followed by good quality stainless steel. Coffee with significant residual sediment, such as coffee brewed in a French press, fares less well when kept hot for extended periods; the sediment continues to extract, making the coffee bitter.
Optimal flavor is obtained by holding the coffee at high temperatures, at least 170 degrees Fahrenheit.
Direct heat—e.g., via a warming plate—should not be applied to brewed coffee, at least not for more than few minutes; continued heating will make the coffee bitter. This issue is primarily a concern for autodrip coffee makers; choose a model that dispenses into an insulated carafe over one that uses a warming plate.
Reheating coffee in the microwave is controversial; the key issue may be the uneven heating microwaves are known for. There are those who theorize that parts of the coffee that overheat may taste unpleasant, thereby spoiling the cup as a whole.

Source: www.thecoffeefaq.com