Particle size distribution explained
Jenna Gottlieb takes a deep dive into the complex world of particle size distribution with the help of 2020 UK Brewers Cup Champion, Matteo D’Ottavio and technical manager at European Coffee Brewing Centre, Stig Hiller.
In 2021, Matteo D’Ottavio took to the stage to present his 15-minute routine at the World Brewers Cup. Having been crowned UK champion the year before, he was keen to impress the judges with his deep understanding of sensory science.
For the first part of his routine, he focused on how the coffee’s grind size distribution could affect its characteristics in the cup. By analysing these variables, he said it allowed him to achieve an accurate correlation between distribution and flavour.
“The particle size distribution is important because it means we can understand more about the composition of our ground coffee,” Matteo explains. “It can be determined by the type of grinder we use, the burr set, and the speed of grinding.”
To achieve optimum extraction and create a wider range of flavours, he told the judges he had mixed two distributions: 10g of coffee with a distribution peak of 600 microns and 3g of coffee with a distribution peak of 1,000 microns.
Optimum extraction can be defined as the equal extraction of a coffee’s soluble components during brewing.
The opposite of this is if some parts of the ground coffee are extracted more than other parts. This can happen when the grind is not optimally matched to the duration and temperature of the brewing process, which leads to bland or unpleasant tasting coffee.
“It’s important because particles extract differently,” says Stig Hiller, technical manager at the European Coffee Brewing Centre. “Bigger particles need more time with water, whereas smaller particles need less time. Brewing coffee is about controlling the extraction and achieving a balanced brew.”
Typically, a higher concentration of fine particles results in bitter, over-extracted coffee because they are easier to extract. A higher concentration of boulder, or coarse, parts, on the other hand, can result in under-extracted coffee.
How to measure particle size distribution
Along with water temperature, particle size distribution is one of the most important variables when it comes to brewing coffee. Therefore, according to Matteo, an understanding of how it affects a coffee’s characteristics should be high up on the list for all coffee professionals.
“Working as a barista, it’s really important to understand the impact of the particle size distribution on coffee,” he explains. “Not only for producing delicious cups of coffee, but also to educate the customers.
“Everyday at least a couple of customers ask me for information about grind size and for help on how to brew a good cup of coffee at home. So I think it is really important for coffee professionals to learn about the impact of particle size distribution.”
However, accurately measuring coffee particles without equipment is hard. Although the fragments may appear uniform to the naked eye, they are often different sizes and weights.
One of the ways to obtain an accurate reading is to use what’s known as a coffee particle analyser – a small machine that uses photo analysis to capture the particles from different angles in a single picture. The software measures the particles based on their surface area and translates the data into graphs.
Stig says that for those who learn to use the data, this sort of technology offers great potential.
“They are useful for professionals if they have the training to read them,” he explains. “We are testing some equipment from one producer who uses digital photo analysis of particle size. They analyse the particles and present reports. It’s an indicator for choosing the optimal brewing method, and in terms of quality and taste in coffee, it’s exciting.”
That said, relying on data alone is limited.
“Particle size distribution graphs are very useful to achieve control and consistency on brewing,” Matteo says. “But I also have to say that just curves and numbers are not enough to determine the quality of a cup of coffee – we need to use sensory analysis, too.”
When the technology is used in conjunction with cupping, it can help coffee professionals get closer to creating a complex and balanced cup of coffee.
The importance of high-quality grinders
To obtain uniform particle size distribution, a high-quality grinder is essential.
Blade grinders and cheap burr grinders tend to hack the beans into different-sized fragments, inevitably resulting in an unbalanced cup.
This can also happen when the burrs become misaligned or “dull”. Dull burrs are when the sharp edges start to crack and smash the beans rather than slicing them.
While there are a wide range of designs and price points, one of the most essential things to note is the difference between flat and conical burrs.
Flat burr sets have two compatible shapes that lie “flat” against each other and are often horizontally aligned with angled teeth. Conical burr sets comprise a ring and cone shape that fit in each other.
Some suggest that flat burrs offer higher consistency and precision thanks to their improved “parallelism”. However, because they require more power than conical burrs, they tend to heat up more quickly. Therefore, they might not be the best option for high-volume coffee shops or hotels.
Matteo also stresses that the most common brew method should be taken into account. “The quality of the grinder matters,” he says.
“Otherwise, the coffee could end up being very unbalanced with a terrible taste. However, some grinders are better for espresso and some are better for filter.”