Pulling a Shot of Espresso

The espresso brewing process is the forcing of high pressure heated water through a bed of very finely ground coffee packed into a small metal filter.  The extraction process occurs in approximately 25 second.  The single shot espresso is about 3/4 on an ounce of liquid containing about five times the flavoring compounds that a shot of regular coffee would have. 

The espresso machine is a highly specialized one containing a boiler to produce steam for steaming milk and a pump or hand lever to deliver high pressure 130 psi hot water to the protafilter where the extraction takes place. The water travels through a heat exchanger contained inside the boiler to produce the 195 degree water needed to brew the shot properly.  Because of their cost most espresso machines are found only in coffee houses and restaurants. 

To pull a shot of espresso the first and most important step is to properly grind the espresso beans to the right particle size.  The espresso grinder is a more precise design of the burr grinder that will allow precise control of the burr spacing so that very fine adjustments can be made to the particle size.  When ground espresso is pinched between the fingers the grinds will feel like a course powder and will slightly pack together like flower. 

Approximately 7 to 8 grams of ground coffee is placed into the portafilter and tamped using a tamper that fits the portafilter.  This is where the experience of the barista is important.  Properly ground and tamped espresso will provide just the right amount of resistance to the flow of water during the extraction process to produce a shot of espresso in about 25 seconds.  Given that the tamp is done right if the coffee is ground too course the 1 ounce of water will pass through too quickly and under extraction will occur.  And if the espresso is ground too fine the water will pass through too slowly and over extraction results. 

The same results will occur if the tamp does not pack the ground coffee tight enough or packs it too tightly.  There is a balance between the grind size and the tamp that will yield a perfect extraction.  Uniformity of the tamp is also important.  Even if the ground particle size is right an uneven tamp can result in a blowout where the water punches a hole in the ground coffee and short circuits straight through without extracting the flavorings.

If the espresso grind is right and the tamp is uniform and packs the coffee right the finished drink will be about 3/4 of an ounce of espresso with a thick reddish brown layer of crema floating on top.

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