It is the quality and formation of the 'crema' that reflects the quality of the espresso beverage. A good crema undoubtably means a good espresso.

The 'crema' traps and holds the finest coffee oils and aromas that in other brewing processes are lost to the atmosphere. The crema should form as a thick, golden-brown, marbled foam across the whole surface of the espresso cup and cling to the sides.

In addition to the quality of the crema, the appearance of the flow or pour of the espresso from the spout of the espresso machine also signals quality. Initially, you should see a thick, almost viscous, dark, honey coloured flow that gently lightens as the extraction proceeds. The flow should form a continuous, flickering stream, similar to the tail of a mouse, up to the point of shut-off 20 or so seconds later.

It is important to look out for the signs of under or over extraction in the espresso process and take corrective action accordingly.

Over-extraction is signified by a slow, broken trickle with a dark brown (burnt) crema. This means that the water and coffee were in contact for too long. This occurs if the coffee grind is too fine, effectively blocking the filter, or by using too much coffee in the filter basket. The final beverage will taste burnt as the coffee is scorched in the espresso basket.

If the brewing cycle is too long then the taste can be impaired as harsh compunds are taken from the coffee during this over extraction. The appearance of white marks or streaks appearing towards the end of the pour is an indication of the brew cycle being too long.

Under-extraction is signalled by a rapid, bubbling flow from the spout and a thin, broken crema. This can be a result of using a coffee grind that is too coarse, by not using sufficient coffee or from tamping too lightly. All result in the hot water passing through the coffee too fast so it is unable to extract the desired coffee solubles and oils. The result is a thin, insipid sour brew of espresso.

Under-extraction can also be a result of the water temperature being too low.

By carefully watching out for these symptoms the espresso machine operator or barista can take corrective action to ensure delicious espresso drinks are always available.