The fineness of a coffee bean’s grind is very important in making espresso. The art and balance of pulling a shot of espresso has five elements that can be manipulated to control the final pour.
1) The amount of grounds
2) The fineness of the grind
3) The tamp or firmness that the grounds are packed together
4) The amount of water forced through the grounds
5) The pressure with which the water is forced through (what the machine is capable of)
In making espresso, the two most common problems are overextraction and underextraction. In fine tuning espresso extraction, the grind and tamp can be the two easiest things to change.
In overextraction, when the espresso tastes bitter or dirty, too much is being extracted from the grounds. This is usually because either the grind is too fine or the grounds are too tightly packed (trapping the water and keeping it in contact for too long). This can usually be fixed with either a coarser grind or a lighter tamp.
In underextraction, when the espresso tastes sharp or metallic, too little is being extracted from the grounds. This is usually because the grind is too coarse or the grounds are too loosely packed (allowing the water to pass by it too quickly). This can usually be fixed with either a finer grind or a firmer tamp.
