mocca

 

The  Perfect Espresso...

 

...is 30ml (+/- 5%) served in an espresso cup and is covered by a 2-4mm fine pored crema that can have some tiger mottling. The crema should stay at least 2 minutes and close again if you open the surface with the back of a spoon.

The Perfect espresso can only be extracted if the water has the pressure of
8,5 - 9,5 bar. The exact amount has to be tested with the chosen machine and coffee.

The desired temperature that the water needs to dissolve the solubles, the CO2 and the oils, lies around 88 to 94 °Celsius. Again, depending on the origin/s and roastprofile.

The ground coffee has to be compressed to give the water a hard time to get through. The extraction time should be between 20-30 seconds, but exceptions might be made. Therefore we use a tamper and press 15-20kg.

The grinder adjustment also influences the extraction time.
So let's assume we always tamp with the same pressure, then we have to find the right size of coffee grounds to explore the best of the beans.

* But be aware! Grinding too fine enlarges the surface and water dissolves too  much ingredients, water stays too long and the coffee tends to get bitter.

** Too coarse grinds lead to coarsely porous crema and too quick water flow.
That results in a weaker taste and less aromas.

The perfect extraction means...

...the Barista was able to extract the best out of the bean, not less, not too much.

 

All he needs is ground coffee, hot water and the balance of power :-)

But single origins are diverse, so the crema can have different colors and texture.

A professional Barista diagnoses the reason for those differences.

 

Underextraction

A too light crema can occure if the water has been to cool to have enough power to extract the sollubles and oils. A lower pump pressure can be cause, too. Or a too coarse groud that leads to a short contact time of water in the coffee. Anything that prevents the water from extracting with enough power leads to “under extraction” and therefore a lighter crema that disappears quickly.

The espresso tastes rather weak and the aromas couldn’t fully develop.

 

Overextraction

A too dark crema occurs from the opposite effects. Too much pressure dissolves too much solubles. Too much temperature burns the solubles. A too fine grind means a bigger surface and more extraction of solubles, additional to an extended extraction time which also leads to overleaching of the coffee. This all leads to a burnt surface and a burnt, bitter taste.


In the pic you see from the left to the right:
underextracted, prefect extracted and overextracted espressi.

When the hot, pressurized water goes through the ground coffee, besides emulsifying the oils and disolve the solubles, it gets totally saturated with the CO2 that's still left in the coffee. That leads to tiny bubbles of CO2 which would dissapear when coming out of the filterbasket.

So a molecule called melanoidin covers the air bubble and makes its structure stronger and resilient.
(Melanoidin comes from the Maillard-Reaction that occurs when coffee is roasted and gets brown, but it must not be mistaken with caramellizing, which happens parallel. )
The melanoidin covered CO2 bubbles can't be resistant for ever due to the oils that are extracted, too.

Oil destroys foam.
So for a while the crema protects the aromas in the espresso, but when we let it rest, the solubles sink, the oils reaching the surface destroy the bubbles and the aromatic oils volatilize.
Or we drink the espresso and the bubbles burst on our tounge, releasing the oils and they evaporate up our nose which helps us detect the aromas.

So the reason that robustas have a stronger crema is partly because they have less oils then the arabicas. Someone once told me that it also has something to do with the kind of oils a species contains.
But also within the arabicas we can find different kinds of cremas, depending on how much CO2 the molecular structure of the bean can store and what kinds of oils they content.