You can have the best coffee in the world and top-class equipment, but if the grinder is not dialled in, the cup will disappoint. Grinding is the bridge between bean and water, and understanding microns and particle size is what separates amateurs from professionals.

What is extraction, really?

When hot water passes through ground coffee, it dissolves its compounds. Ideally we want to extract around 18% to 22% of the soluble material in the bean. Fruit acids come out first, then the pleasant sugars and ripe compounds, and only at the very end the heavy, bitter ones. The key is to stop extraction at exactly the moment when acidity, sweetness and bitterness are in perfect balance.

Depending on how the grinder is set, there are two traps you can fall into:

Trap 1: under-extraction (the grind is too coarse)

If the coffee is ground too coarse, water rushes through without resistance, like water through gravel. It only picks up the surface acids and never dissolves the heavier sugars and oils.

• How to spot it: the coffee runs too fast, looks pale, the body is thin and watery, and the taste is aggressively sour, salty and short.
• The fix: set the grinder finer to slow the flow and increase the contact surface.

Trap 2: over-extraction (the grind is too fine)

If the coffee is ground too fine, the particles compact like wet sand on a beach. The water lingers too long and starts dissolving unwanted compounds, plant fibres and heavy bitterness.

• How to spot it: the coffee barely trickles or drips, the crema is dark with pale blotches, and the taste is dry, astringent and hollow, leaving an unpleasant bitterness in the throat reminiscent of ash or charcoal.
• The fix: set the grinder coarser to open up the path for the water and shorten the contact time.

The golden rule for dialling in

When you change something on the grinder, change only one thing at a time. If you are adjusting the grind, keep the same dose in the portafilter and the same tamping pressure. Watch the shot time and, most importantly, trust your palate — it is the best refractometer you own.