Why does my coffee taste bitter?

This is the most common cause of bitter coffee. When the grind is too fine, water passes through too slowly and extracts bitter compounds from the coffee grounds. For espresso, this shows up as a shot that takes longer than 35 seconds and tastes harsh rather than complex.

The same applies to filter coffee — too fine a grind causes over-extraction even in a drip machine, producing a bitter, astringent cup.

Brewing with water that's too hot scorches the coffee and produces harsh, bitter flavours. Boiling water (100°C) is too hot for coffee — it over-extracts the bitter compounds before the sweeter, more desirable flavours have a chance to develop.

The ideal brewing temperature is 92–96°C. This applies to espresso, filter, and pour-over equally.

Coffee contains oils that go rancid quickly. Old coffee residue builds up in the group head, portafilter basket, shower screen, and drip tray, and this rancid oil contaminates every cup you make. A machine that hasn't been cleaned in weeks will make bitter coffee regardless of your beans, grind, or technique.

This is one of the most overlooked causes of bitter coffee at home. Coffee shops clean their group heads after every single shot — most home users clean theirs monthly at best.

Coffee beans go stale faster than most people realise. Once roasted, beans are at their best within 2–6 weeks. After that, the volatile aromatic compounds degrade, leaving behind the bitter, woody, flat compounds. Pre-ground coffee goes stale even faster — within hours of grinding.

Supermarket coffee that's been sitting in a warehouse for months will taste bitter no matter how well you brew it.

Dark roasted coffee is inherently more bitter than medium or light roasts. During dark roasting, the sugars in the coffee caramelise and eventually carbonise, producing bitter compounds. If you're using a "Italian roast" or "espresso roast" (which are often very dark), the bitterness may simply be the roast profile rather than a brewing problem.

Very oily, dark-roasted beans also clog grinders in bean-to-cup machines, leading to inconsistent extraction and bitter results.

Using too much coffee relative to water increases extraction and produces bitter, intense results. This is different from over-extraction by grind — it's about the ratio. A double espresso should use 18–20g of coffee to produce 36–40g of liquid (a 1:2 ratio). Using 25g for the same output creates a harsh, unbalanced shot.

For filter coffee, the SCA recommends 60g per litre. Going much higher produces bitter, over-concentrated coffee.

Limescale inside the heating elements of your machine affects water temperature, flow rate, and ultimately coffee flavour. A heavily scaled machine runs hotter than calibrated, produces inconsistent water temperatures, and can make coffee taste metallic or bitter. In the UK's hard water areas, limescale builds up quickly.

Some beans suit some brewing methods better than others. Light roast single-origin beans brewed as espresso often taste sour and bitter because espresso's high pressure and concentration amplifies acidity and astringency. The same beans through a V60 or filter machine produce a cleaner, more nuanced cup.

Conversely, medium-dark blends designed for espresso often taste over-extracted and muddy when brewed as filter coffee at the wrong temperature.