The key to making espresso is pressure, and the crema, weight, and flavor of espresso all come from the effects of pressure.

But why is stress so important? How does it affect concentrated formulas? Why are nine atmospheres the most recommended pressure setting for brewing coffee?

Knowledge About Pressure

Pressure is measured using atmospheric pressure, which is calculated from the pressure created by the atmosphere, usually at sea level.

And the nine atmospheres (9 bar) at which espresso is usually made is nine times the pressure of one atmosphere. "Espresso has been around for a very long time, and nine atmospheres is the pressure that's thought to be best for espresso," Stephen said.

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In order to make it easier for everyone to understand, you can imagine the situation of tire inflation. Most car tires need to be inflated to 32 PSI (about 2.2bar), and bicycles are 65-85 PSI (about 4.4-5.8bar).

The 9 atmospheres of espresso extraction is equivalent to four times the normal car tire pressure.

Control Pressure

Imagining such high-pressure water flowing through coffee grounds seems to create a messy situation, so we have to control and apply these nine atmospheres of pressure, which is the core of espresso theory.

How can we manage pressure? You can control the coffee brewing time by filling the coffee powder.

First, let's look at the coffee powder. To avoid the mess mentioned above, we need something to stop the water from being pushed quickly through the coffee grounds.

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So the first thing to confirm is that the coffee powder must be very finely ground. Any coffee can be made into espresso, but it must be extremely finely ground and filled into the powder cake state in the brewing handle. This creates a dense, flat surface that blocks the flow of water through the grounds.

Next, let's look at the extraction time, which is a bit more complicated.

Adjust Coffee Flavor

While extraction times may vary slightly, the extraction time for a concentrated dose of 9 atmospheres is typically between 25-30 seconds. If the pressure is lower, more time is required for extraction; if the pressure is higher, the extraction time needs to be shortened.

But this part can be a little more complicated because coffee takes more time to reach the proper level of extraction, and even if you turn up the extraction pressure, a 12-second high-pressure brew will not give you an excellent drinking experience.

The actual extraction time depends on many variables, including the grind size, characteristics of the coffee itself, water temperature, extraction rate, brewer's habits, etc. And as mentioned earlier, there is not much room to change the grinding thickness.

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This means that ground quantity, extraction rate, and extraction time are the main variables in espresso recipes (unless a professional coffee machine with controlled pressure and water temperature is available).

Many people also prefer to extract by varying the pressure: start with a low-pressure presoak, then use a high-pressure extraction of 9 bar, then lower the pressure to the end. Maxwell Colonna-Dashwood advocates low-pressure extraction using 6 bar pressure. There are always new directions to be tested in coffee, and our ideas about extraction continue to evolve.

However, 9 bar pressure extraction is still the most standard-setting. In fact, controlling pressure is only available on premium coffee machines with pressure adjustment.

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Why not try adjusting other parameters like grind and water temperature? Taking light and medium roast coffee as an example, roasting will degrade the structure of the coffee bean, making the substance of the coffee bean more soluble in water. Light roasts, due to shorter roasting times, result in lower coffee solubility, which, unless paired with the correct brewing parameters, can lead to under extracted results (e.g., more acidity, almost stinky taste), which is why many The roaster will also roast an Italian-style special roast that is darker than the pour over.

To sum up this section, there are many ways to adjust the concentrate recipe, including changing the extraction time, grind, water temperature, pressure, and more. But usually, finely ground coffee is brewed with 9 bar pressure, and conversely, these conditions set the 25-30 second brew time.

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Of course, there are many key factors in making a good cup of espresso: filling method and strength, coffee roasting date, coffee powder, bean density, quality of machinery and equipment, size of brewing handle filter, water hardness, The current room temperature, etc., but these are not the subjects of this article.

Clarify The Confusion Caused By Crema

There is still one point of confusion we should pay attention to, which is the condensed crema.

"Freshly brewed espresso has a nice golden crema on the surface, and crema is created when the ground coffee is extracted at 7-9 bar," says Stephen.

Crema is made up of the oils and fats of the coffee itself, and although often defined as an indicator of a good espresso, it still depends on the properties of the bean itself. For example, washed coffee usually produces less crema, so crema is not an absolute guarantee of quality.

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That being said, crema is definitely visually pleasing, and customers will love it, but you can't get a crema if you don't have enough pressure to extract it.

The Espresso Machine Maintains Pressure

How do we use 9 bar for the entire extraction process? Usually, springs/slots, steam power, or pumps are used.

There are still many coffee shops that still use slot machines because they provide the barista with control over the extraction pressure. Operating this machine, which pulls the lever of the coffee maker and draws hot water in a vacuum, and flows through the portafilter, requires skill and experience to operate.

Steam machines, on the other hand, boil the water and force the water vapor from the process through the coffee grounds, which cannot achieve the pressure needed to make crema.

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Pump coffee machines are usually the type most commonly seen by consumers, which add machine power to the pump to maintain water pressure.

The Pressure Of Home Machines And Pour-over Coffee Machines

Since making a cup of espresso requires machine power and precise parameters, many people wonder whether the home function can maintain the pressure during the extraction process, especially on the more economical models.

However, these equipment manufacturers continue to refine their production techniques.

For example, the patent-pending Cafflano uses Pascal's principle to make the portable coffee machine Cafflano Kompressor maintain its extraction pressure, which means the machine uses fluid mechanics to operate.

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Pascal's principle points out: "The pressure acting on a closed fluid can be transmitted from the fluid to all parts of the container without change in magnitude so that the same changes will occur anywhere." This sentence can be a little confusing. From the picture below, it means that if you push the fluid from the smaller diameter cylinder to the end of the larger diameter cylinder, the force of the liquid will be amplified.

Let's go back to the example of life. For example, the hydraulic press that raises the car in the garage uses Pascal's principle to operate. This principle is also used in the Cafflano Kompressor pour-over coffee machine to ensure that the pressure during its operation can be maintained.

Cafflano also uses squeezing instead of pressing on the machine to increase the extraction pressure. Although the Kompressor machine weighs less than 200 grams, which is even much smaller than the average coffee machine, it can extract 15 grams of coffee powder. The extraction pressure of 9 bar was maintained at all times.

When you make coffee, there are many variables that can be adjusted: water quality, water temperature, bean freshness, roast level, barista habits, etc. But if you want a real espresso, you need the pressure to brew, which is usually 9 bar.

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