The water inside the portafilter machine is heated and pressed through finely grained coffee powder supported by a pump generating the required pressure. The coffee for this type of lever-operated machine is always ground in a separate grinder.
The brewing of an espresso or coffee is done at a boiler pressure of 9 bar. Exactly 7 grams of ground coffee are filled into the portafilter. The water should not take longer than 30 seconds to pass the filter. The 30 seconds are divided in 5 seconds for pre-infusion (ground coffee is wetted) and 25 seconds for the pouring of the coffee. To prevent the water from passing the ground coffee too fast, the coffee is lightly tampered beforehand (tampering with higher pressure can extend the pre-brewing/infusion phase). After the wetting is accomplished, 30 ml of rich and creamy espresso with a light foam pours for the next 25 seconds into the cup. If the procedure ends too quickly this is an indicator for too coarse a coffee powder – if it takes longer or no coffee pours out of the portafilter at all, then the powder is grinded too finely.
The exact duration of the extraction and a consistent pressure of approx. 9 bar are the keys for a perfect espresso. The short but intense contact made by the 88°C to 92°C hot water with the ground coffee at a pressure of approx. 9 bar releases most of the aromas, but only few of the bitter substances and little caffeine (only around 120 mg/cup) from the ground coffee.
You can recognize a perfect espresso by its consistent and rich crema.