Bitterness that lingers and dries your mouth usually means over-extraction. The water pulled out the harsh compounds that come last. But dark roasts taste bitter by nature, and a dirty grinder or machine can add bitterness too, so check those before chasing the grind.
The quickest fix: grind coarser
Coarser grounds extract less in the same time. Move one or two steps coarser and brew again with everything else the same. On espresso the shot should run a little faster, and on pour-over the drawdown should finish a little sooner.
Other things to try
- Use less water for the same dose. Stop an espresso shot earlier (for example 1:2 instead of 1:2.5), or use a shorter ratio on filter, like 1:15.
- Use cooler water. Dark roasts extract easily, so try around 85 to 90 °C.
- Shorten the brew. Use fewer or faster pours on filter, or a shorter steep in a French press.
- Clean the gear. Old coffee oils in the grinder, basket or brewer taste bitter and stale.
Harsh but also sour?
That points to uneven extraction, like channelling in espresso or a pour-over bed that drained unevenly. Fix the evenness first with better puck prep, even pours and a gentle swirl, then adjust the grind.
Keep the setting that works
Once a coffee tastes right, note the setting along with the grinder and brew method. The next bag of the same coffee can then start from what worked.
Find your grinder's typical settings in the grind size chart.
Notchbook is a free app that remembers the grind setting that worked for each coffee, grinder, brew method and filter paper, and keeps a journal of every brew. Get it free on Google Play. The iPhone app is coming soon. More about Notchbook.