Why Your Filter Coffee Tastes Bitter (And Five Fixes)

A lot of people think bitterness is just what filter coffee tastes like, and that milk and sugar are there to cover it. It isn't. Good decoction is bittersweet, like dark chocolate. If yours is harsh or ashy, one of five things has gone wrong.

In short: take the kettle off the boil for 20 seconds before pouring, stop pressing the disc down, and check the grind is like fine rava. If it's still bitter, look at the chicory ratio, the roast level and the roast date. Change one thing at a time.

First, work out which bitter you have

Two different faults both get called bitter, and they need opposite fixes.

• Harsh, ashy, burnt. It sits at the back of your throat. That's over-extraction or a very dark roast.
• Sharp, sour and thin, bitter at the end. Puckery and watery. That's under-extraction, and more coffee makes it worse.

Taste the decoction on its own before the milk goes in. Milk hides sourness, so the finished cup won't tell you much.

Fix 1: Take the kettle off the boil

This is the most common cause and the easiest fix. Boiling water scalds the grounds and pulls out the harshest compounds first. You want about 90 to 96°C. Take the kettle off the heat, count to twenty, then pour.

People who make this change usually say the coffee got sweeter. Same thing.

Fix 2: Stop pressing the disc down

The pressing disc keeps the bed level. It isn't a tamper. Press hard and the bed packs tight, water sits on the grounds for 25 minutes instead of 12, and everything bitter comes out.

Set the disc on, give it a gentle quarter-turn and leave it. If it still takes more than 20 minutes, the grind is too fine.

Fix 3: Check the grind, both ways

• Too fine (espresso grind, or loose "coffee powder") chokes the flow. Harsh and muddy.
• Too coarse (pour-over or French press grind) lets water rush through. Thin, sour, bitter finish.

Aim for fine rava: sandy with a little powder. When you buy ground coffee, ask for the South Indian Filter grind by name. "Fine" usually means espresso, which gives you a 20-minute drip and a bitter cup.

Our grind size chart lines up every brewer, and the right grind for a South Indian filter goes deeper on this one.

Fix 4: Look at the chicory and the roast

Chicory is bitter by nature. At 80:20 it adds body and a caramel edge. At 70:30, or 50:50 in some cheap powders, bitterness takes over. If the pack doesn't state the ratio, assume the worst.

Very dark roasts taste of carbon and smoke rather than coffee. They're common in filter blends because they're cheap and hide poor beans. A medium roast in the same filter tastes noticeably sweeter.

Try a 100% coffee, medium roast in your filter once, with no chicory. It's a different drink. Velvet Mocha is the one we built for the dabara, and Toasted Almond is sweet enough that most people cut their sugar within a week.

Fix 5: Check the roast date

Stale coffee goes flat first, then papery and bitter as the oils oxidise. It happens slowly, so most people just add more sugar. Ground coffee ages fastest of all.

Look for a roast date, not a best-before date. Best-before is usually a year out and says nothing about flavour. If a bag won't say when it was roasted, that's your answer. More in does coffee go bad?

Two more causes

• A dirty filter. Old coffee oil in the holes goes rancid and bitter. Rinse with hot water after every use, no soap, and soak it monthly with a spoon of baking soda.
• Your water. Very hard or heavily chlorinated water tastes bitter. If your kettle scales up fast, try one batch with filtered water.

The order to try things in

1. Take the kettle off the boil for 20 seconds. Free, and it fixes about a third of cases.
2. Stop pressing the disc. Also free.
3. Taste the decoction alone, then move the grind the right way.
4. Buy a bag with a roast date and a sensible chicory ratio.
5. Clean the filter properly.

Change one thing at a time, or you won't know which one fixed it. The full method is in our guide to filter coffee at home, and the brew guide is the printable version.