During the roasting process, coffee beans undergo major physical and organoleptic changes – augment and develop a great range of aromas. During this transformation, the lipids (oils and fats) are brought to the surface of the bean: they are an important part of the coffee powder and the main elements of the espresso crema. Yet, they have a "fault" - they oxidise rapidly.
During the first extraction, lipids come immediately in contact with brewing group internal parts (shower, filter and filter holder) and start to deposit. Moreover, they are soon carbonised by the high temperature they are exposed to. These residues give the drink an unpleasant, persisting bitter taste – which is typical of the burnt lipids – and a rancid taste caused by the oxidation of oils, which takes place within a short period of time (12-18 hours).