For the food scientist reader, espresso is polyphasic, meaning there are multiple phases in the process of preparation: a water phase, an oil phase, and a foam phase. It is also colloidal meaning it has a stable emulsion. (the oil in water emulsion is very stable, and will not separate out.)

And yet---espresso foam falls apart very quickly. The best of them trap bubbles of aromatics pleasantly released as soon as they touch the tongue giving us first a charming appearance in the cup, a delightful texture on the palate, and the satisfying taste that is espresso: concentrated, intense, flavorful.

An abundance of carbon dioxide or CO2 is generated in the bean while roasting although much is lost as the beans cool. Grinding exposes much of what remains and that's why it's best to grind right before you brew. Adding hot water emulsifies the otherwise insoluble volatile oils of coffee, and the water additionally becomes saturated with the CO2, dissolving quite a bit of it because espresso is made with so much pressure. This pressure creates thousands of tiny bubbles, and when the bubbles settle into the cup, crema appears. Perhaps the best analogy is when a beer or cola drink is first poured out of the bottle or can into a glass and forms a head of foam.

The foam or crema in espresso happens because it is resilient and strong. A surfactant or surface acting agent compound, melanoidin, combines with proteins to cause bubbles because parts of them are actually hydrophobic (repelled by water) and turn into air that becomes what we call foam. The fats and oils also contribute to the foam and why the gravity of the weight of the oils shortens the lifespan of the foam. Another way to describe crema is that it is bubbles of CO2 covered in brewed coffee.

A good crema with an abundance of foam indicates that the coffee is fresh, making the cup appear as if it's all crema. Stale coffee is demonstrated by a thin, separate head of foam, and the coffee itself tastes watery.

Sometimes tiny flecks or mottling occur over the surface of a freshly brewed espresso and they are, in fact, tiny particles of ground coffee, which is actually a "good thing." The longer the shot, the darker the crema, and the hotter the temperature of the water, the darker the crema will be.

While it seems logical that a good looking crema indicates a great cup, the irony is that even poorly roasted coffee can still produce a great cream when very fresh and even a sloppily maintained machine might take months, or years, to produce a really bad cup.

Interestingly, robusta beans used for espresso produce a better crema, primarily because the beans are less oily and so fewer "attack" the foam. That results in a firmer crema that stays firmer longer. Alas, the actual flavor in the cup is less, too, so most espresso lovers still favor Arabica beans even though crema varies as widely as the quality of Arabica beans around the world.