The Role Of Pressure In Coffee Extraction – For brewing in general and Espresso in particular, Pressure is one of the main drivers of the extraction process.
It is not just an independent variable but a result of the equilibrium between the force acting on the layer of coffee powder (water flow) and the resistance (resistance) of the coffee powder layer against the penetration of water.
Therefore, when studying Pressure, we are not only talking about the Pressure from the pump but, more complicated, the interaction between other variables such as flow, particle size & distribution, time, and temperature with Pressure, as well as properties of coffee powder during the preparation process.
The most familiar method of making coffee using high pressure is Espresso. The water is pressurized to about nine times the pressure of air at sea level (ie 9 bar) before being compressed through a layer of coffee powder, creating a larger amount of solute than it would have been without the pressure.
From the first application of Pressure in dispensing
At the beginning of the 20th century, Southern European coffee culture emerged in Italy – the birthplace of the first generation of Espresso machines, leading to the emergence of different coffee cultures.
La Pavoni Ideale (1905) was the first espresso machine to use Pressure to speed up coffee extraction, allowing a fresh cup of coffee to be prepared in front of each customer’s eyes.
Contrasted with the bulk pre-made coffee pot commonly used in food stalls at this time. Since the extraction time is still long (about 50 seconds) and the Pressure used is around 1.5-3 bar, “Early Espresso” tastes more like a concentrated version of “Drip coffee”.
In 1948, Achille Gaggia introduced a lever espresso machine that used a hand-operated spring piston to force water through the coffee under a pressure of about 9 bar.
This was the first time the extraction carried essential oils and a large amount of CO2 from the coffee into the cup to create a golden, iridescent foam – what we now call Crema.
Since then, Crema and Pressure have become the exclusive standards for Espresso and paved the way for further research into the nature of Crema foam and the Effect of Pressure on the extraction process.
Pressure in different dispensing methods
Today each extraction method has its pressure dynamics and resistance properties except for its typical coffee powder. As a result, each technique is characterized by its dispensing pressure range.
To make Espresso, hot water is pressurized by a pump (allowing a higher pressure to be created, while for a Moka kettle, lower Pressure is created by boiling steam. Recently, Aeropress is also used.
And became more common to produce a cup similar to Espresso with arbitrary Pressure from the hands of the bartender. On the other hand, with Drip – or Pour over, almost no pressure is created when water flows through the coffee powder; it is merely gravity.
Aero Press, starts as a steeped extracted coffee, however with a piston added at the end to press the coffee through the filter paper making it have elements of both filtration and steeping methods. ( drip filter & immersion )
The type of pump and its performance are essential for espresso brewing. These characteristics depend on the pump specifications (determined by the manufacturer’s capacity curve).
Simply put, the water flow rate of a pump is inversely proportional to the Pressure supplied. A given pump can provide the highest Pressure at the lowest flow rate. As the water flow rate increases, the pump will create a lower pressure and vice versa.
The Moka pot (or Moka pot) is the most popular home brewing method in Italy, invented by Alfonso Bialetti in 1933. This is another pressurized tool for making coffee.
However, the Pressure obtained from the steam chamber of the Moka pot is significantly lower than that of the Espresso concoction and does not yield Crema like this method of preparation.
Although the design and operation are relatively simple, the thermodynamic mechanism of Moka is very complicated compared to other coffee brewing methods.
Several variables affect the extraction process and are not easy to control, which can quickly lead to over-extraction. To avoid this, the key lies in finishing the extraction at the right time.
Besides the forces acting on the coffee powder, the properties of the coffee layer itself are even more critical as they determine the permeability of water and the way water passes through the grain structure.
Particle permeability that is too low can increase extraction pressure, resulting in lower overall flow rates and longer extraction times, which can eventually lead to over-extraction.
The Role of Pressure in Espresso Extraction
The size of the coffee beans after grinding is one of the main parameters affecting the permeability and extraction efficiency of coffee, so it is not surprising that the particle size is the main factor to be adjusted. or easy to adjust) for a barista.
Different particle size distributions will be obtained depending on the type (and quality) of the grinder. So it’s not just the average grind size but also the bean shape and the uniformity in the grain size that determine how the water flows through the coffee.
Each brewing method requires its optimal grind size. In general, coarsely ground coffee can show a lower flavour profile than a finer ground version of the same coffee.
In the case of Espresso, the ground granules increase the surface in contact with the water, allowing for more efficient extraction while also increasing the resistance to Pressure resulting in the coffee’s viscosity.
The Espresso is higher, which ultimately enhances the flavour intensity.
However, when the coffee is ground too fine, it can interfere with the distribution and permeation of water through the coffee powder, resulting in over-extraction.
On the other hand, because the space between the particles is smaller, it increases the pressure on the coffee layer and reduces the overall flow rate of the water.
Most of these effects are explained in terms of the correlation between particle size – and coffee extraction. Still, in parallel, Pressure is the primary driver responsible for the change in the taste of coffee after coffee.
We should see more coffee grinding for Espresso so as not to miss the necessary technical requirements.
Permeability of coffee about Pressure
On first contact, water causes wetting of the entire coffee powder mass, followed by dissolution of low-molecular-weight and solubility compounds and volatile flavour compounds.
Immediately after, the coffee beans will swell due to the presence of polysaccharides in the grain structure, these molecular chains are insoluble in water, but when exposed to hot water, they re-arrange geometrically, causing the coffee swells and impedes water flow (or counter pressure).
This whole swelling process causes a gradual decrease in the porosity of the coffee, a reduction of overall flow and an increase in Pressure on the coffee powder.
Therefore, depending on the barista, the coffee must be ground fine enough to allow effective diffusion of solutes from the coffee but not too acceptable to prevent, clog or prolong extraction.
On the other hand, an excessive amount of fine coffee will not allow it to swell sufficiently during wetting, causing excessive compaction and eventually leading to the deposition of sediment (solids) in the espresso cup.
The permeability of the coffee powder will determine the actual operating pressure, corresponding flow rate and brewing time. Too low coffee permeability can increase extraction pressure, cause overall flow rates to be low, prolong extraction time, and ultimately lead to over
Crema formation dynamics and aromatics extraction
For Espresso, the most critical aspect of Pressure is Crema formation.
The Pressure causes most of the CO2 in the ground coffee to enter the extract, which is then released slowly, pulling some solids and oils with it to form a dense and stable foam on the top of the cup.
All methods of extracting coffee without Pressure cannot produce anything similar to Crema. So Pressure is undoubtedly essential for Crema formation with the Espresso method.
Like CO2, flavouring compounds will easily escape from the coffee in the case of high-pressure brewing and show a higher total content than in the high-pressure extraction method.
However, some non-volatile compounds are less sensitive to Pressure. For these compounds, the importance of Pressure is mainly related to maintaining a uniform extraction and a steady flow rate.
Depending on the permeability of the coffee powder, Pressure helps keep the proper flow time to extract substances from the equilibrium phase.
Again, over-extraction can occur if the flow rate becomes too short or too long. Here pressure is a key factor that optimizes the flow rate to deliver the desired flavor profile
The final extraction driving force of Pressure acting on coffee powder is lipid extraction (coffee oil). The hollow structure of the coffee bean cell acts like a sponge, helping to keep oil and other dissolved substances inside.
Enough pressure is required to force the oil droplets to travel to the surface of the seed, where they are dragged with the water to end up in the beaker.
For Espresso, the oil content is higher than other preparation methods and has higher durability due to the smaller extraction volume.
Therefore, lipids have a more significant impact and help the taste sensation in the mouth become more pronounced, especially when compared to coffee in the pour-over or immersion method.
As a barista, it is essential to have insight into the interplay of all the variables in the bartending process. Particle size and shape, particle size distribution, water quantity, water temperature, pressure and flow time, etc.
These variables influence the extraction process, and many will affect the extraction process’s mutual influence.
For example, increasing the fineness of the grind will often increase the efficiency of the extraction and prolong the flow time, which multiplies the total number of extractions.
Therefore, making minor adjustments and tasting often is best practice, as is the objective measurement when possible.
More broadly, environmental changes, such as temperature and humidity in the coffee shop, can also affect extraction parameters. A barista working in a shop may have to make some adjustments. variables during a working day – The Craft And Science Of Coffee
- The Craft And Science Of Coffee, by Britta Folmer | Chapter 15: The Brewed Extracting for Excellence / Which Key Variables Are Available to Modulate In-cup Coffee Flavor? / Extraction Pressure