The coffee is ground to a size in which the extraction process takes 23-28 seconds. It is important to only adjust the grind and not the pressure one tamps with to control the flow rate. In addition to particle size, the humidity plays a dramatic role on extraction time. Since coffee is hydroscopic, it absorbs moisture from the air causing a tighter pack and longer extraction time. Thus, the grind setting must be changed slightly throughout the day as the barista perceives changes in extraction time.
Coffee is freshest immediately after it is ground.
After grinding the volatile oils that were previously protected inside the bean are exposed to the air which oxidizes and stales the coffee. This effect occurs immediately after grinding so it is important to tamp and extract the espresso as quickly as possible. The grinder should be activated for 15-20 seconds every time a shot is desired so that only freshly ground coffee is used. Instead of two pulls on the doser, the barista should pull several times until the entire basket is filled with ground coffee. Coffee Research Institute.