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.