An emulsion achieved when hot water passes through the layer of ground coffee, characterised by the presence of essential oils, water, carbon dioxide (found in the roasted coffee beans), caffeine, protein and sugars.
It is an essential feature of espresso, which is the only extraction method that allows it to form. The formation of a proper crema, which should be reddish-hazelnut in colour, perhaps with a tiger-skin effect, is the product of the perfect balance between the operating pressure of the coffee machine and the quantity, quality and grind size of
the ground coffee.

Examining the crema can provide information on the sort of coffee you are about to drink and the quality of its extraction.