Espresso machine dispensing pressure
For the most part, baristas starting with essential Espresso, know the extraction pressure is 9 bar. And to achieve these 9 bars is a journey of continuous improvement on espresso machines. But, can you imagine what the bar pressure is like? Think of tire inflation.
Most car tires specify a pressure of 32 PSI per square inch (a standard unit of force in the US); it is 65-85 PSI for bicycle tires. However, 9 bar is equivalent to 130 PSI – 4 times more than a car tire. That’s the pressure of the water compressed into the coffee powder to extract a cup of Espresso for you!
Why must the dispensing pressure be 9 bar?
You are entitled to question this because it was not until 1938 (more than 30 years after the invention of the Espresso machine) that Giovanni Achille Gaggia incorporated the lever mechanism into the boiler espresso machine to apply pressure. Output from 1.5~2 bar to 8~10 bar. And today, some espresso machines ( Slayer is a typical) allow you to tweak the pressure up to 15 bar instead of fixed at 9 bar; So why do we still see the default on many models of 9 bar?
However, you should note that the standard pressure of 9 bar is measured at the Espresso machine’s group head because this is where the coffee is under pressure from the water, in some other areas, such as pumps, pipes… The pressure gauge can exceed 9 bar, not representing the actual pressure involved in the extraction.
Back to the main point, you can imagine a good cup of Espresso does not depend entirely on pressure because many other factors play a part in shaping its quality, including brewing temperature, flow rate, coffee fineness, extraction time, extraction rate, etc. Even some elements that are not in the category of an espresso machine, such as the roasting of the beans, the freshness of the coffee, the hand. Barista, …
Therefore, if only focusing on the extraction efficiency of the machine. We will need a pressure (x), matched to a temperature (y) so that when extracted with fine coffee (z), the coffee will flow out in (t) seconds with an extraction ratio of ( e),.. and (x) in this case x = 9bar
In other words, the pressure is not equal to 9 bar by itself, there has been a lot of research on how to choose the right pressure for all other factor variables, and then it was found that at 9 bar pressure will give Best results in optimum extraction ratio – CompoundCoffee
That’s why we always put Espresso into the framework of a certain amount of coffee (18-20g) ground, then extracted at a temperature of 90~94 o C for about 25~30 seconds with a pressure of 8 ~10 bar for a ~36ml cup with 18~20% extraction rate. Each parameter is always constrained by the other, and thus 9 bar is the best fit within a standard range; You can make Espresso at 6 bar, but for it to still be “Espresso,” you have to change a few other numbers!
So the next problem, do you wonder if the 9 bar pressure will suddenly drop to the coffee in your hand or slowly rise like blowing bubbles? And which one is better? And is it possible to interfere with this process? The answer is yes, and it’s called Pressure profiling!
Typical pressure mechanism in extraction
Pressure profiling is commonly used to provide an incremental pressure during the first pre-extraction period and then a pressure-reducing phase until the end.
In its simplest form, each espresso shot is divided into several stages, with a different pressure for each step. Machines that offer this pressure configuration include the Synesso MVP Hydra and the Sanremo Opera. Others, such as the La Marzocco Strada and Decent Espresso machines, allow the barista to control pressure flexibly throughout the extraction process.
Contact pressure / First Contact
So far – in this article, we have talked about pressure as a fixed variable. Some machines can vary the contact pressure in the early stages of extraction. Preinfusion is the simplest term, which involves a period of low pressure at the start of the extraction and gradually increases as you complete the espresso shot.
Preinfusion gives the coffee powder in the portafilter – basket time to absorb water before subjected to 9 bar pressure. This helps the coffee powder wet evenly, facilitating a more even extraction. Since dry coffee grounds are hydrophobic, any dry areas left at the start of extraction will interfere with water flow, creating uneven extraction and drainage channels (or channeling).
To do this, some espresso machines are set to low pressure initially (usually 3-4 bar), lasting a few seconds, before maxing out. According to Scott Rao (The Professional Baristas Handbook, 2008), this stage, in addition to wetting the coffee, also helps the beans expand and distribute themselves more tightly, reducing Fine migration.
This phenomenon occurs when fine beans and Large insoluble protein molecules are deposited at the bottom of the filter funnel. They can form clumps and clog the filter pores ( compact layer ). This impedes the flow path – and again, causes channeling.
Preinfusion is most effective when coffee powder can become fully saturated. To start, the liquid level can be seen at the bottom of the hopper filter. The grind size and flow rate depending on how long this takes, but it can be 8-10 seconds or longer, depending on the model.
Boost / Increase in Pressure
The pressure will increase to 9 bar, and extraction will begin. Many popular blenders start from this step. If the machine does not have a Preinfusion stage, it has already reached it. With a sudden increase in pressure from 0 to 9 bar and instant extraction, these machines can still make a good shot of Espresso but will depend on many barista skills.
Pressure Drop / Ramp Down
Towards the end of each extraction, with more coffee being extracted into your cup, the clumps of coffee powder in the filter basket also begin to decompose, which can lead to channeling. Reducing pressure can minimize this and result in a higher and more even extract content.
Suppose the 9 bar pressure is fixed until the end of extraction. In that case, the flow rate also increases towards the end of this period, which means that the final volume of water is in contact with the coffee is concise—reducing pressure offsets. This helps increase the contact time of the last few grams of water, which experts at Baristahustle say can increase the refractive index per milliliter of liquid in the final part of the extraction.
Pressure Profiling and Espresso machines
As I just mentioned, it is called Pressure Profiling (it’s a valuable keyword to find more information on google). And of course, that’s not always the case, but there are many other “pressure configurations,” too.
When using the pressure profile, most baristas choose to use low-pressure preinfusion before pressurizing until full pressure is reached and then gradually reducing pressure at the end of the extraction. Incidentally, this profile is similar to the pressure profile produced by a lever espresso machine – according to baristahustle.
So we can all test our Pressure Profiling? It’s possible, but it won’t be that simple, as this is a theory, practice will be much more complex. First of all, your Espresso Machine allows for different pressure configurations (as mentioned above). At the same time, the second issue is that you have to understand the type of coffee you use and how you want the final taste of the Espresso cup.
In fact, Pressure profiling will be configured differently on each model, for example La Marzocco Strada allows Barista to adjust pressure directly during extraction. Models like Sanremo Opera or Syness Hydra allow preset pressure in 3 separate steps.
Mastering the pressure when making Espresso
If the coffee is of poor quality, even the elaborate pressure adjustment will not noticeably change. In contrast, with Specialty Coffee, the coffee beans are usually lightly roasted (Light-medium to Medium) to retain their distinctive character. Distinctive in taste. If only using primary machines, the extract will melt quickly (Under), and the flavor is incomplete. Still, if the grind is finer when extracting, the Compact layer and Over phenomenon will occur. However, the extended Preinfusion process allows for a finer grind without over-extraction with Pressure Profiling models, thus achieving better extraction even with light roasts.
There are also some notes about the pressure to achieve the effect of Espresso extraction mentioned by Rao-Scoot as follows:
- While making Espresso, the water is drained in another group, so avoid flushing while making Espresso
- While making Espresso but extracted in another group, if making Espresso in both groups, you should drain the water and mix with both hands simultaneously.
- The pump head automatically pumps water during extraction; it is best to wait for the water pump to finish before making Espresso.