Sight

The most evident and characteristic expression of an espresso is the cream, that in chemistry is called foam and has to be observed in order to understand if the espresso is correctly prepared: it has a subtle texture, a brown colour and light tiger stripes without the presence of air bubble or white spots (which indicate an espresso incorrectly prepared).
 
A good, consistent and durable cream indicates a good quality espresso, prepared perfectly. Its dark colour is due to the sugar caramelization that happens during the roasting process.