What's Actually Happening: Under-Extraction
Coffee extraction is the process of water dissolving compounds from your ground coffee. The compounds extract in a specific order: acids first, then sweetness and body, then bitterness last.
When coffee is under-extracted — meaning water hasn't dissolved enough from the grounds — you get mostly the acidic, sour early compounds without the sweetness and balance that come later. The result is a sharp, unpleasant sourness that makes your lips pucker.
This is different from natural acidity, which even excellent coffee has. Natural acidity is bright and pleasant — like a citrus note. Under-extraction sourness is unpleasant and hollow, with no sweetness to balance it.
Three Fixes — In Order of Impact
Fix 1: Grind Finer
This is the most powerful lever you have. A finer grind creates more surface area, which means water can extract more compounds in the same amount of time. If your coffee tastes sour, try grinding one step finer and taste again. This single change solves the problem 70% of the time.
One step at a time — don't jump from coarse to fine all at once. Small increments let you find the sweet spot.
Fix 2: Use Hotter Water
Water temperature directly controls extraction speed. If you're brewing at 85°C or below, try pushing to 93–96°C. Hotter water extracts more quickly and more completely, especially important for medium to dark roasts.
If you don't have a thermometer, boil your kettle and wait 30 seconds before pouring (rather than 60–90 seconds as you might have been doing). This raises the brew temperature significantly.
Fix 3: Brew Longer
More contact time between water and coffee = more extraction. If you're using an immersion brewer (French press, AeroPress, cold brew), try adding 30–60 seconds to your steep time. For pour-over, try slower pours to increase contact time.
This is the least reliable fix on its own, but combined with a finer grind it can make a noticeable difference.
One More Cause: Stale or Low-Quality Coffee
Very old or very low-quality coffee can also taste sour because it lacks the flavour compounds that create sweetness and body. If you've tried all three fixes above and coffee still tastes sour, try switching to fresher, better-quality beans. Coffee is at its best between 7 and 28 days after roasting.