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 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.