Look at any espresso machine and you'll find a grinder either attached to it or close by. At first glance it may look like any other grinder but it's not - it's special. That's because espresso requires an extraordinary grind. Traditional filter brewing machines just use gravity to push the water through the coffee grounds. Espresso machines, on the other hand, use a pump to force the water through at 9 bars of pressure (but you know that - you read about it in the first edition!).Typically a filter brew will take 4-6 minutes to extract the liquor whereas espresso is brewed in just 20-25 seconds. Control of this brew time is of paramount concern to ensure great tasting espresso and the correct crema. In fact unless a single shot is brewed within this time frame it simply isn't espresso.

There's no such thing as the 5, 10 or 15 second espresso! Without the correct brew time you don't extract the right solubles from the coffee grounds and nor do you produce the classic espresso emulsion formed by the lipid fraction and so important to the aroma, mouthfeel and appearance of the crema.

So how does the grind affect the brew time and the quality? With espresso grinds we run the paradox of requiring a fine enough grind to give the correct brew time whilst having a large enough surface area in the wad of coffee to give the extraction of the right compounds in the correct quantities.

In order to do this we require a completely different kind of particle size distribution to that found in filter coffee where the particles tend to be of a more even size (see fig. 1). In contrast, the espresso grind shown in fig 2 has two main peaks or "modes" thus allowing for the desired flow rate as well as the extraction needed for real espresso. Simple, really.

This article originally appeared in Fresh 2