Crema goes so fast.  You turn around it’s past.  Make no mistake; crema is absolutely crucial to coffee.  Crema holds all the volatile compounds that constitute the breathtaking aroma of espresso.  Crema is where your nose gets most of its information about your coffee.  If your nose is well informed, your taste buds will gladly come along for the ride. 

Let’s consider espresso first.  Brewed espresso is an enigma of physics.  It is all at once a solution, a suspension and an emulsion.  Espresso has body, more viscosity than regular coffee, provided by trace amounts of fats and starches.  It contains sugars, polysaccharides, acids, caffeine, proteins, lipids, oils, colloidals in suspension, bits of fibrous plant matter, and carbon dioxide.

Coffee beans literally exude carbon dioxide as a byproduct of the roasting process. The conical burr coffee grinder, slices beans into flaky grit.  The flakes increase the surface area tremendously, rupturing cellular walls and allowing even more carbon dioxide to escape into the atmosphere.  When hot water is forced through the finely ground coffee, much of this carbon dioxide is liberated and is dissolved into the water.

Protein strands, available from the ruptured cellular material and denatured by the hot water, become surfactants and stabilize the tiny bubbles.  The polysaccharides and lipids increase the viscosity of the fluid, resisting the pull of gravity and slowing the drainage.  As the fluids drain off, the bubbles dry out and when the surfactants can no longer support them the walls collapse.  The breaking down continues until the crema foam dries out or disappears completely.

Crema is foam of carbon dioxide and coffee.