That dense froth that is created when coffee-bean oil, protein, sugars and carbon dioxide unite to perform their magic. More crema forms on Robusta espressos than on pure Arabicas. This is why blends are frequently made from both varieties. Inexpensive espresso machines, fully automated coffee machines and machines with pads produce an artificial crema by frothing the coffee through an air valve. You can recognize this through its relatively coarse bubble structure.