Sour coffee is the mirror image of bitter coffee, and it confuses people more, because most of us didn't grow up expecting coffee to taste sour at all.

First, a distinction. Good coffee can be bright. Lil Red, for example, has a lively, fruit like quality that some people love and others take a week to warm to. That's acidity, and it's a good thing in the right amount. It tastes like the sharpness in a ripe apple.

Sour is different. Sour is sharp, thin, unripe and unpleasant. It tastes like the coffee stopped halfway. Often there's a hollow feeling in the cup, like something is missing. That something is sweetness and body, and the reason it's missing is under extraction.

The short answer

When hot water meets ground coffee, it pulls out acids first, then sugars, then the heavier bitter compounds. Under extraction means the water stopped after the acids and never got to the sugars. The result is sharp and thin.

The fix is always some version of "pull more out." Here are the 4 levers.

Cause 1: Your grind is too coarse

Coarse grounds have less surface area, so water extracts from them slowly. If your coffee tastes sour and the grounds look like rough gravel, this is the likely cause.

Fix: grind finer. Go one step at a time. A small change in grind makes a big change in taste.

Cause 2: You're brewing too fast

A pour over that runs through in 90 seconds, a plunger pressed after 2 minutes, an espresso shot that gushes out in 15 seconds. All of these are under extracted because the water didn't have enough contact time.

Fix: slow it down. Plunger, 4 minutes. Pour over, 2.5 to 3.5 minutes. If the water is running through too quickly, a finer grind will slow it and fix both problems at once.

Cause 3: Your water is too cool

This one catches a lot of people. Water that boiled 5 minutes ago and has been sitting in the kettle is often below 85°C. That's too cool to extract properly, especially with lighter roasts.

Fix: brew within a minute of the kettle boiling. The sweet spot is 92 to 96°C.

Cause 4: Not enough coffee

This is a subtle one. Using too little coffee for the water can produce a cup that's both weak and sour, because the small amount of coffee gets rushed through the extraction stages. It's more common than people think, especially with a rough tablespoon measure.

Fix: use about 15 g of coffee per 250 g of water. That's roughly 2 heaped tablespoons per mug.

Sour and bitter at the same time?

It happens. Usually it means the grind is uneven, with fine dust that over extracts and large chunks that under extract, all in the same brew. Cheap blade grinders do this. If your coffee tastes both sharp and harsh, a burr grinder or a good roaster's pre ground option will usually fix it.

A quick way to diagnose

Change one thing at a time. Start with water temperature, because it's free and takes 10 seconds to check. Then grind finer. Then check your dose. Brew, taste, adjust.

If you're brewing a lighter roast like Lil Red or Edelweiss and finding it sour, try grinding a touch finer than you would for Big Dog. Lighter roasts are denser and need a little more help from the grinder.

What does your coffee taste like right now: bright, or sour?