While dose, yield, and brew time are the three main parameters that baristas should focus on when dialling in espresso, there are a number of other important variables. One of these is the rate at which water flows throughout extraction – known as the “flow rate”.
Changing the water flow rate is becoming more and more popular among baristas, as it provides them with a different kind of control. By altering the amount of water passing through the coffee puck at any given time, the barista can bring out some of the more nuanced and subtle flavours in the resulting espresso.
To further discuss flow rate and its effect on coffee profiles, I spoke with award-winning Mexican barista and owner of PalReal, Fabrizio Sención, as well as Lukasz Bertoli and Carlo Ciciliot, the co-founders of Emo Design Studio.
Flow Rate And Flow Profiling
The flow rate of an espresso machine is a measurement of how much water passes through the grouphead while the pump is active. Most commercial machines have a flow rate of between 250 and 500 grams per 30 seconds (g/30s), but the ideal range is between 200 and 280 g/30s.
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It is advised that you calculate the flow rate of each grouphead a number of times in order to achieve accurate results. To do so, remove your portafilter, get some scales, and grab a receptacle for the water. Engage the pump for a set period of time. Once it’s finished, you divide the total weight of the water by that time to figure out your flow rate in grams per second (g/s). For example, 300 grams of water over 30 seconds means your grouphead has a flow rate of 10g/s.
Understanding your machine’s flow rate will help you to fine-tune how the water and coffee interact during extraction. Greater control over how the water passes through coffee will enable you to pinpoint specific levels of acidity, sweetness, bitterness, and body to create a unique espresso shot. This process is known as flow profiling, or flow control.
Mexican barista Fabrizio Sención co-founded several cafés in Guadalajara, and came seventh in the 2010 World Barista Championships. He explains why flow profiling is so important: “Flow control allows you to target totally new extractions and discover the full complexity of the cup. You can refine existing recipes and potentially find undiscovered ones.
“It benefits how we approach and extract espresso, and helps us to ensure that all the grounds are evenly saturated with water.”
The Relationship Between Flow and Pressure
The pressure your espresso machine produces will dictate your flow rate. To understand how pressure affects flow in your machine, you need to understand what causes resistance to pressure.
There are a number of things that limit pressure in an espresso machine. Alongside the diameter of the pipes themselves, you also have flow restrictors, which keep the pressure within the machine as constant as possible.
Beyond this, there is also the coffee puck’s resistance to water. The grounds themselves will affect how much water can pass through and at what speed. However, the puck’s resistance also varies throughout extraction. As pressure and flow rate increase, the coffee takes on more water and provides less resistance.
Generally, as the pressure increases, so does the flow rate. However, a sharp increase in pressure may compact the grounds, which will lead to a slower flow rate and affect extraction.
The pressure should generally be kept stable throughout to ensure that the flow rate does not fluctuate and affect the flavour of the espresso. The easiest way to achieve this is by using a machine with integrated pressure controls. The Dalla Corte ZERO, for example, comes equipped with a Digital Flow Regulation (DFR) system.
“With Digital Flow Regulation, you can control the amount of water coming out of the grouphead over the puck of coffee over a set period of time. This maintains constant and stable pressure,” Fabrizio explains.
How To Change Your Flow Rate
There are a number of different ways to control your flow rate. You can manually adjust the pressure by changing your machine’s flow restrictors, or by reducing the pump pressure.
However, manually tweaking a machine’s flow rate can be quite complicated, especially for new baristas. Using an espresso machine with dedicated controls makes flow profiling much easier.
Lukasz Bertoli and Carlo Ciciliot are the co-founders of Emo Design Studio, an industrial design consultancy based in Italy. They have developed a number of different products for the coffee industry, including the Dalla Corte ZERO.
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They explain why it’s useful for baristas to have access to a dedicated flow control system: “On the ZERO, the flow rate can be changed from 2 to 10g/s through the dedicated touch screen, which is clearly visible on each single grouphead. Once the flow rate has been set, it remains constant throughout extraction.
“You can also use the control panel to change the machine’s general settings. This is useful for setting the coffee’s extraction time, the flow, the water temperature during extraction, the weight in the cup, and much more.”
How Does Flow Rate Affect Extraction?
Flow rate has a significant impact on every single phase of espresso extraction, including pre-infusion. Pre-infusion occurs when water first hits the coffee puck and it starts releasing CO2.
From the beginning of pre-infusion until the extraction chamber completely fills up with water, the pressure remains at zero bar. This doesn’t change depending on the pressure coming in from the machine or the water flow that has been set; pressure only increases once the chamber is saturated with water.
Fabrizio explains how controlling the flow rate allows baristas to achieve the desired flavour profiles during pre-infusion: “Pre-infusion for the first 12 to 15 seconds with a restricted flow rate allows coffee to extract more easily, even with light roasted beans or a very fine grind size.
“Controlling the flow rate means you can grind as fine as you want with medium to light roasts without sourness.”
Lukasz and Carlo explain that the ZERO is equipped with two different flow rate settings.
“The ZERO has two major flow control modes – fixed water flow and freestyle water flow. With the fixed water flow, the flow rate can be adjusted on the touchscreen, and fixed throughout extraction.” This helps a barista when they’re making the same beverage repeatedly, as they can fix the flow rate for a period of time.
Lukasz and Carlo add: “With the freestyle water flow setting, each lever on the ZERO has four separate steps. Each of these steps is set to a specific flow rate – 4, 6, 8, and 10g/s – giving the barista full control during extraction.” This can be useful at the end of the shot, as the flow rate of the coffee naturally starts to increase when the coffee puck is wet and its resistance is low. The ability to manually reduce the flow rate provides the barista with more control.
Both the fixed and freestyle modes on the ZERO provide the barista with greater control. Lukasz and Carlo say that these features “give the baristas the power to explore specialty coffee aromas”.
“The barista can use different coffees and set the best flow profile on every single grouphead, as well as changing flow profiles with the same coffee to obtain different results.”
Useful Tips When Using Flow Profiling
You can also use flow profiling to inform your dose or your grind for an espresso. If the dose is too small, or the grind too coarse, then there will not be enough pressure to create the desired flow rate, causing it to under-extract. Similarly, there will be too much pressure if the dose is too big or the grind is too fine, resulting in over-extracted espresso.
If you want to further adjust the flow rate, you can also change the size of your portafilter baskets. Some machines, such as the ZERO, come with a filter kit designed to help you reach an optimal flow rate.
You can also adjust your flow rate to suit different roast levels. To tone down acidity and sweetness in light roasts, pre-infusion should last between five and eight seconds at a flow rate of 2 or 3g/s. For medium roasts, a five-second pre-infusion at 5 or 6g/s will extract more oils and increase the body, and decreasing the flow rate towards the end of the brew time will reduce bitterness to create a pleasant aftertaste. Finally, you can even minimise the bitterness of dark roasts by lowering the flow rate from 10g/s to 4g/s after pre-infusion. This will bring out more sweetness in the coffee.
Ultimately, it is down to the barista to experiment and find the perfect flow rate for each individual coffee. Fabrizio explains that it’s important to allow the barista to experiment: “I like to give baristas the freedom to find complexity and balance of the shots. Behind the bar, I advise fixing your flow to suit the coffee you are using. If you want to push the boundaries of your coffee and play with it, then go with freestyle extraction.”
Next time you find yourself experimenting with espresso, consider using flow profiling. Not only can it improve the flavour and balance of your shots, it could also lead to you unlocking hidden or undiscovered flavours in your coffee. This allows you to create a whole new experience for the customer.
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