Making espresso is sometimes considered an art, with countless variables to play around with. Different techniques and a large number of different results that can be obtained with the same coffee.
Traditionally, espresso is brewed at a pressure of 9 bar, which is maintained throughout the brewing process. This means that as soon as you insert the portafilter into the machine and press "start", the pump turns on and pushes the coffee at full force throughout the extraction.
This ensures that the coffee is brewed evenly: the same pressure is applied to the coffee throughout the entire process. Uniformity is one of the most important indicators of good extraction, because it means that all of the coffee is participating in the entire brewing process, and generally results in a balanced and well-extracted cup.
However, a consistent pressure is not the only factor for a consistent extraction. A pressure of 9 bar is a tremendous force - nine times the pressure of our atmosphere - and this violent water jet can cause its own problems. If the coffee puck has not been properly prepared, air pockets, cracks or varying densities can develop in the coffee bed. Water can pass right through these weak spots, leading one to what is known as channeling. The water that flows through a crack does not come into contact with all of the coffee, resulting in uneven extraction despite the even pressure.
One solution that baristas have been using since machines began allowing more and more precise control over extraction and brewing profiles is what is known as preinfusion. This starts the brewing process at a lower pressure, which is slowly increased as the extraction progresses until the full pressure of 9 bar is reached.
This gentle start helps to ensure that all the coffee is evenly saturated before the pressure is increased. In this way, the water slowly penetrates the coffee, eliminating irregularities in the puck and creating a homogeneous, well-saturated coffee bed through which the water then flows evenly.
To achieve this, some sophisticated tools are required. Some machines have a paddle or lever that allows the barista to manually control the pressure in the group head during the brewing process. Machines that are designed for high volume and where it is too time consuming to control each shot may have electronically programmed preinfusion profiles where the machine automatically controls the pressure. If you only have an on/off button, you can achieve a type of preinfusion by simply peeling off the pump for a moment after the first few seconds of extraction.
A well executed preinfusion can help improve the uniformity of an espresso, which not only results in a more balanced coffee flavor, but also allows for a higher extraction overall. This allows us to extract more flavor from the coffee and better showcase its natural characteristics without over-extracting or unbalancing the coffee.
Want to dive deeper into how to make espresso? We offer barista courses for professionals and amateur baristas, where you can learn about the full range of espresso. Find out more here.
