What is under-extracted coffee? Why it tastes sour and how to fix it

What does under-extracted coffee taste like?

It tastes sour first. Counter Culture Coffee describes it as "sour and astringent." Perfect Daily Grind adds salty notes and a short finish that disappears quickly.

Sour is not the same as bright. A well-extracted light roast can have lively acidity, like citrus or berries, balanced by sweetness. Under-extracted coffee has the tartness without that sweetness, so it tastes sharp and hollow.

Why does under-extraction happen?

It happens because the acids dissolve early and the balancing compounds dissolve later. Wikipedia's summary puts it this way: acids come out early, while sugars and bitter substances come out later. Stop too soon and the cup is mostly acid.

Anything that slows or shortens extraction can cause it:

• A grind that is too coarse for the brew method
• A brew time that is too short
• Water that is too cool
• Uneven wetting, so some grounds barely take part

Water temperature is often overlooked. Perfect Daily Grind recommends 195-205°F (about 91-96°C) for brewing.

How do you fix sour coffee?

Grind finer first, then taste again. Perfect Daily Grind's advice for sour coffee is a finer grind or a slower brew. A finer grind extracts faster and, in pour-over or espresso, slows the flow too.

For espresso, there is a second lever. Perfect Daily Grind suggests increasing the yield, the amount of liquid in the cup, as an alternative to grinding finer. Change only one thing at a time, so you know which change worked.

If you use a brew guide, start from its grind setting and move finer in small steps. Our guide to dialing in pour-over walks through the process for filter coffee.

Could it be under-extracted even with a fine grind?

Yes, if the grind is uneven or the water finds a shortcut. In espresso, channeling sends water through weak spots in the puck. Perfect Daily Grind explains that the denser parts of the puck are then under-extracted while the channels over-extract.

That shot tastes sour and bitter together. Grinding even finer can make it worse, because a denser puck channels more easily. Better distribution and a level tamp are the usual fix.

A grinder with a wide spread of particle sizes can do something similar in any method. The large particles under-extract while the fines over-extract.

Does your grinder make under-extraction harder to fix?

It can, mostly in espresso. If the right setting sits between two clicks, one click is sour and the next is bitter. Baratza says this about its own Encore: the perfect espresso shot "may lie in between two grind settings."

Baratza also notes that a few dozen microns of burr movement can change an espresso shot by several seconds. That is why espresso grinders use small steps or stepless adjustment. For filter coffee, most burr grinders adjust finely enough.

If you keep landing on sour shots and the next click is too far, look for a grinder with finer steps. The grinder finder can narrow the list by brew method.