Why Your Coffee Tastes Wrong
Find what your coffee tastes like in the table, make the one change listed, brew it again. Most problems are one of two things, and the fix is usually the grind.
Nearly every bad cup is under-extracted or over-extracted, and the two taste nothing alike.
Under-extracted coffee is sour, sharp and thin, because acids dissolve first. Over-extracted coffee is bitter and drying, because the harsh compounds dissolve last. Once you can place a cup on that line, the fix is obvious.
Change one thing at a time. Two changes at once teaches you nothing about either.
Find the taste
| It tastes | The cause | Change this first | Then this |
|---|---|---|---|
| Sour, sharp, lemony | Under-extracted | Grind finer | Hotter water, closer to 96°C |
| Bitter, harsh, drying | Over-extracted | Grind coarser | Shorter contact time |
| Weak and watery | Not enough coffee | More coffee, try 1:15 | Check the grind is not too coarse |
| Strong but unpleasant | Too much coffee, or over-extracted | Less coffee, try 1:17 | Grind coarser if it is also bitter |
| Flat, dull, like cardboard | Stale beans or stale water | Fresher beans, under a month old | Fresh cold water, never reboiled |
| Muddy or silty | Fines in the cup | Plunge the press slowly, stop early | A better grinder, burrs not blades |
| Salty | Severe over-extraction | Grind much coarser | Shorter brew, check the water |
| Ashy or burnt | Over-roasted beans, or a hot plate | A lighter roast | Decant rather than leaving it on heat |
| Papery | Unrinsed paper filter | Rinse the filter with hot water first | Discard the rinse water |
| Oddly flavoured | Dirty equipment | Scrub out the coffee oils | Descale if the water is hard |
Grind is the first lever in most rows because it changes extraction faster than anything else you control.
Sour and bitter at the same time
This confuses people, and it has a specific cause: an uneven grind.
If your grinder produces dust and boulders in the same batch, the dust over-extracts into bitterness while the boulders under-extract into sourness. You drink both. No adjustment to time, temperature or ratio fixes it, because the two populations need opposite treatment.
That is the clearest sign of a blade grinder, which is a propeller rather than a mill. It is also what a very worn burr set does. If a cup tastes sour and bitter at once and the grind looks visibly inconsistent, the grinder is the problem.
Water is half the cup
Brewed coffee is about 98 percent water, and most people never think about it.
Hard water, heavy in calcium and magnesium, makes coffee taste flat and chalky, and it scales up a machine until the temperature drops. Very soft or distilled water is worse in a different way: minerals are what carry flavour compounds out of the grounds, so distilled water brews a thin, hollow cup.
Filtered tap water is the practical answer almost everywhere. If your tap water tastes of chlorine, your coffee will too.
Temperature matters nearly as much. Between 92 and 96°C is the working range. Water straight off a rolling boil scalds the grounds and pulls bitterness; water below 90 cannot extract properly and tastes sour however fine you grind. Boil it and wait thirty seconds.
How old are the beans, really
Look for a roast date, not a best before date. A best before date tells you the bag is sealed, not that the coffee is good.
Coffee is at its best from about three days after roasting, once the carbon dioxide has settled enough to brew evenly, to around three or four weeks. After that the aromatics fade steadily. It will not hurt you at three months, it will just taste of very little.
Store the beans whole, in an airtight container, at room temperature and out of light. Not in the fridge, where they pick up moisture and the smell of everything else. The freezer works for long storage if the beans are sealed properly and you take out only what you need, but for a bag you will finish in a fortnight it is not worth it.
A fifteen-minute diagnosis
When you have no idea what is wrong, work through this in order. Each step rules something out.
-
Check the roast date
If there is no roast date on the bag, or it is more than two months ago, stop here. Buy fresher coffee and start again. Nothing else you do will matter.
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Weigh everything once
Use a scale for both the coffee and the water. Start at 1:16 for filter. People who think they know their scoop are usually out by a third.
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Fix the water temperature
Boil, then wait thirty seconds before pouring. If you have a thermometer, aim for 94°C.
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Taste and place it
Sour or bitter? That is the only question at this point. Use the table above and make the one change it names.
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Change one thing and repeat
Brew again with that single change. If it moved in the right direction but not far enough, go one more step the same way.
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Clean the equipment
If none of that helped, the problem is probably rancid coffee oil in the brewer or the grinder. Scrub everything, run a descale if your water is hard, and start again.
Questions
My coffee is bitter so should I use less coffee?
Usually not. Less coffee makes it weaker, not less bitter, and often makes bitterness worse because a smaller bed over-extracts. Bitter means over-extracted: grind coarser or shorten the brew.
Why does the same coffee taste different each morning?
The usual culprits are an inconsistent dose, water temperature drifting, and the beans ageing across the bag. Weighing the coffee removes the biggest of the three.
Does a dirty machine really change the taste?
Yes, and more than people expect. Coffee oils go rancid and coat everything they touch. A brewer that has never been cleaned adds a stale, slightly fatty note to every cup regardless of the beans.
Is burnt-tasting coffee the roaster or me?
Both are possible. A very dark roast tastes ashy no matter how you brew it. If a lighter roast also tastes burnt, the problem is water that is too hot or coffee sitting on a hot plate.
Can old coffee make me ill?
Stale coffee is unpleasant rather than unsafe, as long as it has been kept dry. Coffee that has been damp is a different matter: discard it.