Espresso must be topped by a foamy, reddish-brown, consolidated surface layer called crema. This metastable foam with a specific lifetime of ≤ 40 min is a marker of quality for expresso.
What is the Crema?
It has been suggested that triacylglyceride, diacylglyceride, fatty acids, glycolipids, glycoproteins might be involved in formation and/or stabilization of the foam.
How is the Crema Formed?
How Should the Crema Look Like?
stable, reddish-brown, pervaded by wispy, lighter stripes (tiger-skin effect)
chemical composition in ppm includes
– too much coffee powder
– too hot water temperature
– too long brewing time
– too firmly tamped filter cake
light, thin, unstable
– too coarse grind
– too little coffee powder
– too low water temperature
– too short brewing time
– too old beans
How Does the Crema Affect Taste?
After enjoying a cup of espresso with a good crema, one can actually still smell and taste it 20 – 30 min later.
Why is the Crema Important?
The crema prevents too rapid cooling, holds aroma volatiles back, and protects against hydrolysis.
In this article we are referring to Coffea arabica. From the over 60 Coffea species, also Coffea canephora (Coffea robusta) is used commercially.
 Ernesto Illy, Luciano Navarini, Neglected Food Bubbles: The Espresso Coffee Foam, Food Biophys. 2011, 6, 335–348. https://doi.org/10.1007/s11483-011-9220-5
 Klaus Roth, Espresso – A Feast for the Senses, ChemistryViews 2010. https://doi.org/10.1002/chemv.201000016