Fixing bitter or sour coffee: a diagnostic guide

Almost every brewing problem comes down to extraction. Sour coffee is under-extracted — the brew did not pull enough of the right compounds out of the grounds. Bitter coffee is over-extracted — the brew pulled too much, including the harsh compounds. Neither is about "bad beans" most of the time.

If the brew tastes sour

Sour means thin, sharp, lemony, vinegary. The lighter notes are there but the body and sweetness are missing. Causes, in order of likelihood:

Grind too coarse. Water passed through the grounds without enough contact. Tighten one or two steps on the grinder — see the grind size guide.

Water too cool. Especially noticeable on light roasts. Bring the kettle back up to a full boil and let it rest only briefly before pouring. The water and temperature page covers targets for each method.

Brew time too short. Either the grind is too coarse or you poured too quickly. Slow the pour and aim for a longer total brew time.

Beans too fresh. Coffee within four days of roast often tastes uneven and bright. Wait a few days and try again — see bean storage.

If the brew tastes bitter

Bitter means harsh, ashy, drying on the tongue, sometimes with a hollow sweetness underneath. Causes, in order of likelihood:

Grind too fine. Water spent too long in contact with the grounds and dissolved the bitter compounds. Loosen one or two steps.

Water too hot. Especially noticeable on dark roasts. Let the kettle rest longer before pouring, or use slightly cooler water from the start.

Brew time too long. Either the grind is too fine or the dripper is choked. For pour-over, look for water pooling on top of the grounds — that is a stalled brew, almost always a too-fine grind.

Stale beans. Beans more than four to six weeks past roast often turn flat and bitter even with perfect technique. Check the roast date.

If both are present at once

Tasting both sour and bitter in the same cup means the extraction is uneven. Some grounds are over-extracted, others under. The cause is almost always the grinder — uneven particle size means simultaneous over and under extraction. A grinder upgrade is usually the answer. In the meantime, you can sometimes mask the problem with a slightly coarser grind and slower pour, but you cannot fix it.

If nothing helps

Buy beans from a different roaster. Some beans are simply roasted too dark or too light to brew well at home. A change of source often solves problems that no amount of dialling in could fix.

See also

• Grind size by brew method — the most common fix.
• Water quality, temperature and brew ratios — second-most-common fix.
• Bean storage and what "fresh" actually means — old beans never recover.