Texture
The way in which your beans are ground is one of the most essential details to achieving a good espresso. You can buy your coffee ready ground however the best option is to invest in a grinder and do it yourself.
Not only will you be able to grind to the exact texture you require but will get the benefit of fresher coffee! It is the texture of the grind, how coarse or fine it feels, that either allows the water to flow through the coffee quicker or slower.
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Coarse grind
If your coffee is ground too coarsly then the water will flow through the gaps between the coarse particles too quickly (normally less than 20 seconds) and will not have time to extract that rich coffee flavour. You will need to adjust your grind finer and try again.
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Fine grind
If your coffee is ground too finely then the water will struggle to find a path through and will flow slowly (normally more than 30 seconds) and will extract the more bitter flavours from your coffee. You will need to adjust your grind coarser and try again.
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If you are buying your coffee already ground it is essential you choose the one that is a fine grind specifically designed for espresso machines.