Nearly every bad cup of home coffee is one of two problems, and they’re opposites. Once you can tell them apart you can fix almost anything.
Brewing dissolves many compounds at once, and the cup changes as contact time, grind, temperature and agitation change. “Too much extraction” and “too little extraction” are useful diagnoses, but they are not a chemistry stopwatch and they are not the only reasons coffee can taste bitter or sharp.
Bitter, harsh, drying
If the cup is drying and harsh, the brew may be over-extracted. A dark roast, a very hot holding plate, mineral-heavy water or a grinder producing lots of fines can land in the same neighbourhood, so treat this as a first test rather than a verdict.
Fix it by giving the water less to work with, or less time to do it:
- Grind coarser. This is the big one.
- Brew for less time.
- Use slightly cooler water if the roast is dark.
- Keep the coffee-to-water ratio steady while you test.
Sour, thin, sharp
Sour and thin often points to under-extraction: the brew did not dissolve enough to balance its acidity. Some coffees are deliberately bright and fruit-forward, though, so ask whether the cup is lively or simply hollow before fixing it.
People almost always read this as “too strong” and add water, which makes it worse. Do the opposite:
- Grind finer.
- Brew for longer.
- Use hotter water, especially for a light roast.
- Make sure all the grounds actually got wet.
Change one thing at a time
The temptation is to change everything at once and hope. Then when it works you don’t know why, and when it doesn’t you know even less. Grind is the most powerful lever, so start there, move it one notch, and drink the result before touching anything else.
If it’s somehow both
Bitter andsour at the same time usually means your grounds aren’t all the same size — the small ones are giving everything up at once and going bitter while the big ones never really get started and stay sour. You are drinking both problems out of the same cup.
That’s a grinder problem rather than a technique problem, and it’s the reason we bang on about buying the grinder first.
Before you blame yourself
Check the coffee isn’t stale — nothing you do to a six-month-old bag will rescue it, and it goes off faster than you think. Check your ratio is roughly right, which is about two tablespoons per cup. And taste your tap water on its own. If it isn’t nice to drink, it won’t make nice coffee, which we wrote about in 2013.