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Flow rate: a hot topic

If you give the same coffee to ten baristas, you end up with ten different tasting drinks. Years ago, this inconsistency was romantically understood as the “art of making espresso”.

Written by Andre Eiermann
Posted in Research on September 17, 2020

The coffee extraction process is extremely complex because many variables affect the chemical composition and the sensory profile of the final brew. Until now the coffee extractions are mainly optimised using a trial and error approach combined with sensory evaluation, according to Corrochano (2015). The addition of pressure makes understanding flow rate even more complex compared to filter coffee brewing says Ellero (2019). At two extremes, the flow of water through a bed of coffee in an espresso filter basket can deliver a good shot, or a cup of disappointment. After reading through the latest scientific research papers on flow rate, it is widely acknowledged that we don’t fully understand flow rate.

The quality of the cup depends heavily on the barista’s skills and experience to select the appropriate set of extraction variables, e.g. temperature, pressure, brewing recipe, grinding (particle size distribution) or bed preparation and tamping. Moroney et al. (2019) highlight that if you give the same coffee to ten baristas, you end up with ten different tasting drinks. Years ago, this inconsistency was romantically understood as the “art of making espresso”, but greater knowledge and industry pressure is shifting some of the ‘art’ to science. The Specialty Coffee Association has set a global standard for the espresso extraction. The definition of an espresso according to the 2020 World Barista Championship Rules and Regulations is as follows:

  1. Espresso shot volume: 30mL +/- 5mL
  2. Brewing temperature: 90.5 to 96°C
  3. Brewing pressure: 8.5 to 9.5 bars
  4. Shot time: 20 - 30 seconds (recommended, but not mandatory)

Understanding the coffee extraction process is of great importance for the standardisation of the espresso preparation. This is why the analysis of extraction kinetics for chemical and physical compounds has gained relevance in recent years. Corrochano (2015) emphasises that understanding flow rate is one important part of this research trend, as flow rate is a variable that can really influence the beverage properties such as taste balance, flavours and tactile.

So, what is flow rate? By definition, flow rate is:

Volumetric flow rate (Q)=(volume of fluid)/(unit of time)

The flow rate of an espresso machine is a measurement of how much water passes through the group head while the pump is active. The flowrate can be measured in different ways: The simplest way is to place a cup on a scale under the group head with no portafilter engaged. According to Simonelli Group Product Manager Lauro Fioretti, most commercial machines have a flow rate of between 250 and 500 grams per 30 seconds (g/30s).

Another way is to place a scale under the cup while extracting a shot. You can then calculate the average flow rate by knowing the total shot time and the weight in the cup. New mobile phone applications will now allow you to see the live flow rate per second into the cup during the extraction.

Figure 1: Measuring flow rate with an external scale.

For the World Barista Championships, the Victoria Arduino Black Eagle is calibrated using a Scace 2, with a so-called ‘water flow regulator’, positioned downstream of the thermometer probe. This instrument simulates the flow resistance of the coffee cake . The machine is then set at 52 grams of water in an elapsed time of 25 seconds +/- 3 seconds according to WCE (2011) .

In a traditional espresso machine, the flow rate is limited by the hydraulic resistance of the coffee bed, which depends on three factors according to Corrochano (2015): a) the geometric characteristics of the brewing chamber b) the fluid viscosity and c) the coffee bed permeability, determined by the porosity of the bed and the particle size distribution of the ground coffee. It is also becoming more and more clear that the particle size distribution significantly affects the extraction kinetics with smaller particles leading to a higher extracted amount of several components, e.g. caffeine and trigonelline (Kuhn et al., 2017).

What we understand so far about flow rate is that it is variable during an espresso extraction. Research shows that the flow rate of water to produce a traditional espresso is not constant. At the beginning, the flow reaches a maximum value almost instantaneously (Figure 2). This corresponds to the low resistance hydraulic circuit before the extraction chamber being filled. Then, flow rate decreases over the first few seconds as the coffee bed resists the flow of water, until a minimum value is reached. Afterwards the flow rate increases again. Depending on the brew recipe, the flow rate is then stable and might even slightly decrease, as observed by Corrochano (2015). Petracco et al. (1993) observed that the flow rate increased when a higher pressure is applied until a point in which either the flow rate would not increase or would even decrease. This observation can be explained by the consolidation of the bed.

Figure 2: Flow rate is variable over time during the non-steady state.

Flow rate fixes the extraction time, defined as the time required to produce a given drink volume, and water residence time, defined as the time spent by the water inside the bed. Experiments have proven that a higher flow rate results in a faster shot time, while a slower flow rate will result in a longer shot time, while keeping the other parameters unchanged. Interestingly, for a given volume of an espresso recipe, a lower flow rate increases the extraction time and that results in an increased extraction yield, according to Corrochano (2015). In addition, the flow rate has an impact on the final beverage temperature: a higher flow rate results in a higher average temperature while a slower flow rate results in a lower average temperature of the beverage, as observed by Corrochano (2015).

A recent study by Ellero (2019) explored the so-called fines migration. Fines are defined in this article as particles with a size of 30 μm. A computer model showed, that these cellular fragments migrate within the basket due to the machine pressure and eventually even percolate through the porous structure. When they finally arrive at the bottom of the filter basket, they turn into an obstacle to the flow of the down-streaming fluid particles which exit the filter basket. This again reduces the flow rate. So, we know that during an espresso extraction, water is in contact with different sections of the coffee bed, for different periods of time, and this variation influences the extraction kinetics in ways we cannot precisely control or predict.

More precise machines with new control features and the latest analytical sensor technologies give you a real-time view on the extraction. This will empower baristas to experiment with flow rate and to create new flavour profiles, to finetune the taste balance and to increase the tactile qualities. Certain machines even allow you to change and adjust the flow rate on-the-go during the extraction.

In my own experiment, I studied the effect of water flow on my espresso quality. To do so, I conducted a total of 20 extractions with a 1:2.2 brew-ratio into double espresso cups. I kept all extraction parameters unchanged (temperature, dose-in, dose-out, shot-time, grind setting). For each flow rate setting I conducted five repetitions. My sensory evaluation shows, that a different flow rate has an impact on my cup experience. Unfortunately, I am not able to say why, because the espresso extraction is so complex, and the reasons are manifold.

Figure 3: Different espresso sensory profiles, as a result of a different water flow profile.

Other baristas are now experimenting with other new machine features and are also working with new barista tools trying to influence the flow rate in order to optimize the cup profile of their beverage. These experiments range from pressure and flow profiling to new distribution tools and a new control over the particle size distribution of the coffee grounds.

Specialty coffee is growing around the world. While the initial drivers were mainly specialty coffee shops, recently chained coffee shop operators and commercial coffee roasters have been upgrading their offers to follow the specialty coffee movement. Therefore, there is a larger focus on product quality and product consistency. There is more demand for scientific research to achieve these objectives. This is putting pressure on coffee machine manufacturers to engineer high precision machines that are easy to use and deliver a consistent, high quality output in a reproducible manner.

Science will continue to help us to better understand the complex coffee extraction in the future to reproduce higher quality coffee beverages. This will permit us to further democratise specialty coffee, allowing us to reliably serve the best flavours in the cup. It will add a dose of science, where cup quality is concerned, and allow the true art of espresso to shine through service and design.

Before you start working on the flow rate it is nevertheless important to master first your basic barista skills like grinding, dosing, distribution and tamping. Without solid basic barista skills, the differences in the cup will be caused by the inconsistency in your puck preparation. Experimenting with flow rate will then give you the opportunity to maximize the flavour potential of each coffee. But it will likely be many more years, until we can scientifically predict how a cup is going to taste by setting the machine and the flow rate.

Summary & Key Take-Aways

  • Flow rate is variable over time and influences the beverage properties,
  • Higher flow rate = shorter shot time / Lower flow rate = longer shot time,
  • Higher flow rate = higher beverage temperature / lower flow rate = lower beverage temperature,
  • Lower flow rate = higher extraction time  higher extraction yield,
  • Flow rate increases when a higher pressure is applied until a point in which either the flow rate would not increase or would even decrease,
  • The migration of fines might hinder the flow through the coffee bed,
  • Computer modelling, studying the uniformity of the extraction and grinding technology will help us in the future to better understand the espresso extraction.

References

Moroney KM, O’Connell K, Meikle-Janney, P, O’Brien SBG, Walker GM, Lee WT (2019), Analysing extraction uniformity from porous coffee beds using mathematical modelling and computational fluid dynamics approaches.

Corrochano B (2005), The University of Birmingham, Advancing the Engineering Understanding of Coffee Extraction.

Ellero M, Navarini L (2019), Mesoscopic modelling and simulation of espresso coffee extraction.

WCE (2011), World Coffee Event Procedure for Measurement of Brewing Water Temperature in Espresso Coffee Machines (2017).

Kuhn M, Lang S, Bezold F, Minceva M, Briesen, H 2017, Time-resolved extraction of caffeine and trigonelline from finely-ground espresso coffee with varying particle sizes and tamping pressures.

Moroney KM, O’Connell K, Meikle-Janney, P, O’Brien SBG, Walker GM, Lee WT (2019), Analysing extraction uniformity from porous coffee beds using mathematical modelling and computational fluid dynamics approaches.

Petracco, M, and Liverani, F S (1993), Dynamics of fluid percolation through a bed of particles subject to physico/chemical evolution, and its mathematical modelization. In ASIC (Ed.), 15th International Conference on Coffee Science (pp. 702–711).

WCE (2011), World Coffee Event Procedure for Measurement of Brewing Water Temperature in Espresso Coffee Machines (2017).

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Comments (2)

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trying to be consistently inconsistent, very nice

iordanis CHRYSAFIDIS ·
a year ago

Thank you for this great article, Andre! Amazing work on the current literature!

Evangelos Koulougousidis ·
7 months ago
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