Put simply, grinding coffee exposes the inner structure and increases the available surface area for extraction. 

 

As a general rule, the finer the grind, the quicker the extraction, resulting in a shorter brew time. This is why cafetière, which uses a coarser grind, is a slower brewing time than espresso, which uses a finer grind and is much quicker.  

 

Once ground, coffee will naturally start to oxidise, meaning that it’s potency and quality will start to decrease. You’ll find that you’ll get the best results from freshly ground coffee, with a much richer flavour.