Why is my AeroPress coffee bitter?

Bitter AeroPress is nearly always over-extraction: water pulling too much out of the grounds, so the heavy, bitter compounds that come out last take over the cup. With an AeroPress the three usual causes are water that is too hot, a grind that is too fine, or a steep that ran long. The good news is the AeroPress is forgiving, so a small change in any of these usually cleans it right up.

Fix it: change one thing at a time

1. Cool the water. Off-the-boil water (around 80 to 90°C, 176 to 194°F) is plenty for the AeroPress and a common bitterness fix, especially for darker roasts. If you pour straight from a rolling boil, let it sit a minute first.
2. Grind a little coarser. A grind that is too fine packs tightly and over-extracts. Go a notch coarser, roughly medium, and see if the harshness drops. See the grind-size-guide.
3. Shorten the steep. Long steeps and lots of stirring pull out more. Try a shorter total brew (around a minute to ninety seconds before pressing) and press gently. Forcing the plunger fast does not help; an easy, steady push is better.

This is one end of the sour vs bitter axis. If you overcorrect and the cup turns sharp and sour, that is under-extraction: grind finer, brew warmer, or steep a touch longer.

Other things that creep in

• Ratio: too little water for the coffee tastes muddy and bitter. A common AeroPress starting point is around 15 to 17 g of coffee; loosen the ratio with a bit more water if it tastes heavy.
• The beans: a very dark or stale roast tastes bitter no matter how carefully you brew. Check the roast-date; smoky or ashy flavors point to the roast, not your technique. A lighter roast may simply taste sweeter to you.
• Inverted vs standard: the inverted method gives you more control over steep time, which helps if dripping during setup is dragging your brew long.

Adjust the brew first. The four levers, grind, ratio, temperature, and time, live in the-four-dials, and the full technique is in aeropress-method.