Troubleshooting: a diagnostic guide

A reference for working backwards from a bad cup, in rough order of how often each cause is responsible.

Sour, sharp, thin, salty

Under-extraction. The most common single problem.

Grind finer. The first and most effective change.

Brew longer. Extend contact time.

Use hotter water. Particularly relevant for light roasts, where ninety-six degrees or even boiling is appropriate.

Increase agitation. Stir more, or pour with more turbulence.

Check your water. Very soft or distilled water extracts poorly.

Check the coffee. A very light roast or a very fresh coffee is harder to extract. Rest a fresh coffee a few more days.

Bitter, harsh, drying, ashy

Over-extraction, or a dark roast, or both.

Grind coarser.

Brew shorter.

Use cooler water, particularly for darker roasts.

Reduce agitation.

Check for stale equipment. Rancid oils in a grinder or brewer produce a bitter, stale harshness that no adjustment fixes.

Sour and bitter at the same time

Uneven extraction. Some grounds over-extracted, others under.

Check the grinder. A wide particle distribution is the usual cause. Blade grinders always produce this.

Level the bed before brewing.

Improve the pour. Aggressive or off-centre pouring creates channels.

Check for channelling in espresso, using a bottomless portafilter.

Wet everything evenly in immersion methods. Dry clumps extract nothing while the rest over-extracts.

Flat, dull, papery, lifeless

Not an extraction problem. Extraction faults taste sour or bitter; flatness is different.

Stale coffee. Check the roast date. Check the bloom — no swelling means old coffee.

Hard water. High alkalinity buffers the acidity that makes coffee interesting. This is the most under-diagnosed cause of dull coffee.

Test by brewing with bottled low-mineral spring water. If the difference is dramatic, it is your water.

Baked roast. A roasting fault producing exactly this. If it is flat regardless of brewing and the coffee is fresh, suspect the roast.

Past crop green coffee. Check the harvest year if stated.

Weak and watery

Distinguish carefully.

Weak but pleasant, tasting like diluted good coffee: strength problem. Use more coffee, or a tighter ratio.

Weak and sour: extraction problem. See above.

Thin in texture: may be the filter, may be a very light roast, may be over-dilution.

Muddy, sludgy, gritty

Too many fines, from a poor grinder or too fine a setting.

French press technique. Use the skim-and-settle method rather than stirring and plunging hard.

Filter problem. A metal filter passes more sediment than paper by design.

Brew draining too slowly

Grind too fine.

Too many fines clogging the filter, which is a grinder problem rather than a setting problem.

Filter folded against the brewer wall, blocking flow.

Bed too deep for the brewer at that grind.

Brew draining too fast

Grind too coarse.

Water running down the filter walls rather than through the bed. Avoid pouring at the very edge.

Channelling, visible as holes in the bed afterward.

Espresso specific

Gushing, pale, fast. Too coarse, or dose too low, or channelling.

Dripping, dark, very slow. Too fine, or dose too high for the basket.

Spraying or spurting from a bottomless portafilter. Channelling. Improve distribution and tamping.

Wet, soupy puck. Dose too low for the basket, leaving headroom.

Inconsistent shot to shot. Grinder retention, inconsistent puck prep, or an unstable machine temperature.

Everything tastes stale regardless of coffee

Clean your equipment.

Grinder burrs and chamber. Brewer. Metal filters. Carafe. Espresso group head and baskets.

Rancid coffee oils are the single most common cause of persistent off flavour that survives every recipe change.

Everything tastes different from last week

The coffee has aged. Coffee becomes more soluble as it degasses. Adjust grind slightly finer as a bag ages.

Humidity has changed, which affects grinding and extraction, particularly for espresso.

A new bag. Every coffee needs re-dialling.

Scale in the kettle or machine, lowering the brew temperature gradually over months.

The general method

Change one variable at a time.

Start with grind, which has the largest effect.

If grind adjustment in both directions does not improve it, the problem is not extraction — look at the coffee, the water or the cleanliness of the equipment.

And write down what you changed and what happened, because otherwise you will do the same experiment again next month.