Espresso crema is a brown, foamy layer on top of the brewed coffee. Crema is a mix of gas bubbles, and emulsified fats, which are the signature of a good espresso. The perfect crema is a reddish-brown foam, with tiny bubbles. If the bubbles are too big, the shot was incorrectly extracted. If the colour is too light is an under-extracted shot, and if it’s too dark, the shot is over-extracted.
Espresso brewing produces a syrupy beverage, extracting solid and dissolved components, along with coffee fats. The crema is produced by emulsifying the oils from the ground coffee into a colloid, and injecting the CO2 from the coffee beans into the colloidal suspension. Crema has a short lifespan, because the gas bubbles will eventually escape the colloid, and because many of the extracted fats are volatile.
A perfect espresso contains at least 10% in volume crema. This is at the moment of extraction, and the longer espresso sits in the cup the more crema dissipates.
Old coffee beans do not produce much crema, because the loss of CO2 and coffee oils. The the older the coffee beans are the more CO2 and oils they lose.
Darker roasts tend to produce more crema, but they age faster and the loss of coffee oils and carbon dioxide is more rapid.