The World Atlas of Coffee

Firstly, whether the coffee is relatively fresh – the longer ago it was roasted, the less carbon dioxide it will contain so the less foam it will produce. And secondly, whether the cup of espresso is strong or weak. The darker in colour the foam, the stronger the liquid will be.

The darker the roast of coffee, the easier it is to extract. This is because the coffee bean becomes increasingly porous and brittle as it is roasted.

Cheap commodity coffee that has been poorly roasted or badly brewed is often incredibly bitter and lacks any sweetness. Milk, and even more so cream, does a great job of blocking some of the bitterness, and sugar makes it more palatable.

If you don’t take enough from the grounds then the cup of coffee will not only be weak, but it will often also be sour and astringent. This is called ‘underextraction’. And if we take too much from the grounds then the cup of coffee will taste bitter, harsh and ashy. This is what we call ‘overextraction’.

Water is a vital ingredient in a cup of coffee as it makes up around 90 per cent by volume of an espresso and 98.5 per cent of a cup of filter coffee.

The roasting process can be controlled to determine three key aspects of how the coffee will taste: acidity, sweetness and bitterness. It is generally agreed that the longer a coffee is roasted, the less acidity it will have in the end. Conversely, bitterness will slowly increase the longer a coffee is roasted, and will definitely increase the darker a coffee is roasted.
Sweetness is presented as a bell curve, peaking in between the highs of acidity and bitterness.

Coffee beans start with a moisture content of around 60 per cent, and should be dried to around 11–12 per cent to ensure they do not rot while waiting to be sold and shipped.