You can reduce the bitterness of coffee, but maybe not to a level that you'll like.
(One friend said he couldn't stand bell peppers, explaining that if they tasted 10x as strong, I probably wouldn't like them, either. I've since asked people if there's a particular flavor they don't like, and heard a variety of surprising responses.)
Judging by the coffee at many cafes, lots of people must not mind bitter coffee or covering it up with sugar and cream. I think good coffee should be good black.
It's possible to buy tasty 100% dark chocolate from some artisan chocolate makers. They have to start with suitable beans and use well-tuned processes. Sloppy won't cut it.
Similarly, it is possible to make coffee that's not (very) bitter. It requires selecting suitable beans and applying a good process.
I don't know how to find suitable beans in general, but try 100% Kona coffee. (If it doesn't say “100%” then it’s heavily diluted.) I've found some other excellent beans from Mexico and Colombia. I think all of these were medium roast.
Brewing technique is critical. Try using an AeroPress because it gives lots of control over the brewing variables, it limits extraction by brewing with a portion of the water (dilute it after brewing), and immersion methods are more forgiving.
This page on Angels’ Cup has a helpful introduction to brewing variables and experimentation. To summarize from that and other sources:
Over-extraction during brewing will cause bitter results. You can adjust that via: coarser grind, or shorter brewing time, or cooler brewing water [Angels’ Cup says hotter but other sources say cooler], or less agitation (stirring), or adjust the coffee:water ratio. These recommendations assume you're close, but any variable can be off in either direction.
All these adjustments can make the brew more sour, or too strong or weak, or otherwise unbalanced. There's a careful balance to achieve. They say temperature affects the sour notes more than the bitter notes.
(Heating the coffee afterwards should not make it more bitter, or not much, since it's no longer in contact with the grinds.)
Experiment! Start with adjusting the grind size. Finer grind makes for more bitter brew. Coarser grind makes for more sour brew. In between should be a happy trade-off.
Brewing is quite sensitive to grind size. If you don't get an even grind, you'll have some large particles and some small particles, which makes for some bitter notes and some sour notes together.
Therefore you'll need a good burr grinder. (I took recommendations from this Sweethome review.)
This NY Times article adds:
… water extracts flavor from smaller coffee grounds faster than bigger ones. An inconsistent grind means sour taste from the small grains, and a bitter one from the big, all at the same time.
… the colder the bean [when grinding], the more uniform particles it produced, and the more even the flavor.
… put the beans in the freezer — just keep them packed air tight to avoid staleness and moisture.
(However, the Baratza grinder is not designed for grinding frozen beans.)
Some tips to wrap up:
- Clean the grinder often (monthly?) and clean the brewing equipment daily so they don't accumulate rancid oils.
- Try decaf. The decaffeination process reduces bitterness.
- Try medium and light roasts.
- Start with the AeroPress manufacturer's recipe, but don't stir 10 times. Put in the grinds, pour in the hot water up to #2, dab it with the stir stick a little to help it all get wet, wait 10 seconds, stir gently 2-3 times, then press.
- Use a thermometer to get the water temperature right.
- Use a scale to measure out the coffee.
- Start with grind size #14 if you're using the Baratza Encore grinder. With 40 grind size steps, adjusting by ±1 step makes a noticeable taste difference.
- Don't bother with inverted (upside down) methods--those tend to produce more bitter coffee, and they risk a mess with hot water.
- A paper filter produces "smoother" tasting coffee than a bamboo filter does.
- Don't bother pre-wetting the filter, at least not until you've experimented with more important variables. Pre-wetting will mainly let your water cool down.