The Process Of Grinding Coffee, From A Scientific Perspective – While grinding is the turning point of almost any extraction process, today’s grinders are so automated that it makes us forget whether the coffee has been appropriately ground.
On the one hand, rubbing the coffee is just a pre-brewing process; on the other, it determines how the flavours are extracted from the beans.
But due to the specific structure of coffee beans, choosing problems related to the grinding process (uniformity, grain size, dynamics, ..) becomes unnecessarily complicated.
Some will see it simply as suitable for the concoction; others will place certain expectations on further studying the process.
Why grind coffee?
If a roasted coffee bean is placed in water, hot enough, and stirred long enough, the ingredients that make up the flavour will eventually be extracted from the bean. Doing coffee extraction that way would require too much time.
However, if we cut the coffee beans in half before brewing, the water will come into contact with the two new surfaces of the beans. Four new surfaces will be created if these pieces are again cut in half.
By dividing a coffee bean like that, from the whole bean to finely ground (about 0.38mm), we will have more than 20,000 pieces and 128 times more surface area per unit than coffee beans.
That’s why coffee needs to be ground into coarse (coarse) and outstanding (fines ) beans before being used, making extracting the coffee from them more accessible.
In short, the smaller the particle size, the higher the amount of solute that will be removed from the grain.
|Number of seeds/gram||Split ratio||Contact area
More specifically, the separation of coffee beans dramatically increases the contact area, allowing the release of CO2 and the absorption of hot water.
At the same time, it shortens the distance from the centre of each bean to the surface, thereby significantly reducing the space (and time) of coffee flavouring materials travelling out to dissolve into the coffee extract.
More contact area also increases the number of fats, oils and ultrafine particles that help form the colloidal system in the coffee extract – or crema, in the case of espresso in particular. All the benefits of this grain separation are encapsulated in one grinding process.
Grinding – Grinding can be called in many different ways, including crushing, rubbing, cutting, tearing, .. ( crushing, rubbing, grating, cutting, pulling, milling..) of gradually reducing the size of the coffee beans.
No process will produce complete uniformity in the size of the particles produced. Therefore, the biggest problem in the grinding/grinding process is creating a uniform particle size distribution within a specific range.
Particle size and uniformity
A coffee bean, after roasting & grinding, can be roughly described in terms of size (x), similar to a sieve for grading coffee beans – only it is not a definite number.
Because the process of grinding coffee beans always creates a lot of fragments of different sizes and shapes. The difference in size around a specific range (x) is called the particle size distribution (PSD for short).
As a result, the coffee industry typically specifies bean sizes as x10, x50 and x90, respectively, 10%, 50% and 90% of all beans of the same size that will be found in a batch. Where x50 is called the average particle size.
If the primary goal of brewing is to achieve a high-quality cup, then the biggest concern is ensuring that the PSD is uniform. To do this, each grain after grinding should have the same surface area to volume ratio, which should be the same size.
Because larger flakes release less solute than smaller flakes, and differently shaped flakes interact with different water molecules, causing each unit to release a certain amount of solute. Inconsistency during extraction.
Setting a certain percentage for grain size uniformity (%PSD) means it is almost impossible to guarantee that the beans are evenly distributed with all fragments.
Some will be very large; others will be tiny. Of course, smaller & more seems better. Because extraction is faster with fine particles (we can easily abuse this property).
However, finer grinding may promote faster extraction but does not guarantee faster brewing. This is because grind size can strongly affect flow rates. Similarly, it is easier for water to pass through a layer of rock than a layer of sand.
Fine grain; Risks & benefits
Too many “microscopic” particles will entail two related risks: First, they can clog the crevices between larger particles, significantly increase the penetration time of water or even stagnate the extraction process.
Second, these microscopic particles can pass through the filter to follow the extract, create sediment in the cup, and affect taste perception. Such particles are called Fine – and are one of the main problems when grinding espresso for espresso.
Although there is no universal standard for the size of a fine, however, according to Britta Folmer (The Craft and Science of Coffee) particles smaller than 0.1 mm (100 micrometers) are considered fine.
It is almost impossible for us to prevent the presence of fine beans in the coffee grinding process; partly, they depend on the equipment; on the other hand, when the coffee beans break or crack, they will release (slough off) the beans fine under 0.1 mm.
This repeatedly happens until the beans reach the required fineness. Therefore, after grinding, the fine can increase to about 10 – 50% by weight, depending on the previous bean size.
In most cases, the distribution around the average particle size should be as narrow as possible, which means that all the particles will have similar sizes.
Espresso may be the only exception. Here, a certain proportion of fine particles is considered necessary to achieve pressure resistance, to create a body character and a fine crema in the extraction, despite the short extraction time.
So that too fine particles do not enter the extract and form sediment that can be felt on the user’s tongue. The Craft and Science of Coffee have a minimum particle size of about 0.05 mm (or 50 micrometres).
Factors affecting the grinding process
The individual properties of coffee beans significantly affect the grinding process results.
For example, people never grind coffee beans that are still hot (warm) right after roasting; when the beans are too soft, they will be broken and flattened under the impact of the grinder’s friction.
Therefore, it is best to grind coffee beans after they have cooled, becoming more complex and brittle.
Understanding the differences in grain properties allows us to make adjustments during the grinding process and thus achieve the desired particle size. These differences include:
As pointed out, the moisture content and moisture state in the roasted coffee beans are decisive factors affecting the grinding results.
However, when talking about humidity, we need to know that it includes the remaining moisture from the green coffee (after roasting is about 1 to 2%) and a part of re-moisture (the humidity in the air is reabsorbed by the coffee beans) consumption) after roasting.
Since moisture reabsorption occurs from the outside to the inside, it will first moisten the grain surface before entering the grain structure.
This is why there should be a break between roasting and grinding to ensure moisture can be evenly redistributed inside the beans by diffusion.
After roasting, the minimum resting time before grinding coffee is 6 to 12 hours. If the coffee bean has a moisture content greater than 6% it becomes more elastic and difficult to grind even after a long rest period (Baggenstoss et al., 2008).
The degree of roasting, which the colour of the roast can indicate, in general, the darker the roast, the easier it is to grind.
The longer the roasting process takes, the more dehydrated, brittle, and fragile the beans become. Therefore, the darker the roast ( dark roasts ), mainly when obtained in a short time, the more favourable the grinding process.
However, it will lead to a relatively wide PSD ratio and many Fines. For coffee that is lightly roasted with higher moisture content, the grinding results are better with a high degree of homogeneity, compensating for the greater energy consumption of the equipment.
On the other hand, the slower the coffee beans are roasted, the more uniform the grain texture and the larger the size and density of pores. Therefore, when grinding, they quickly give a more homogeneous result.
Some studies have also shown that the effects of oils (lipids) in coffee may also have been pushed to the surface, especially after dark roasting and accelerating the grinding of the beans (but the effect was not significant).
It also takes time for the oil to move back inside and ensure a more consistent grind.
This entirely depends on the natural origin of the coffee. More specifically, the porosity or hardness of the beans before grinding depends significantly on the characteristics of the coffee variety, where it is grown & processed.
With the same degree of roasting, the new crop (first-fruiting plants) produces fewer fine beans than perennials (coffee beans from other Arabica and Robusta varieties in different degrees of maturity) grain size distribution).
Because the coffee tree grows above sea level, the grain structure is denser, denser and more complex than plants grown at lower altitudes.
The friction created during the grinding process generates a large amount of heat, which means that in an operating grinder, the coffee temperature can reach 100°C (M Petracco, 2005).
Usually, the commonly given visual explanation for this is that the metal parts of the blender expand at a higher temperature, increasing the distance between the burrs and allowing particles to be larger than the set.
This is not entirely true, however, as the expected expansion of the metal for such a small temperature change would be negligible.
Instead, what is happening is that an increase in temperature is making the coffee more pliable and less brittle, changing the way it is fragmented & ground into particles (E Uman et al., 2016).
According to Illy’s Espresso Coffee: The Science of Quality ( M Petracco, 2005), Coffee oils are very viscous at room temperature, but they start to become more liquid above 40°C.
When this happens, the oil can easily flow from the beans through the micro-cracks and coat the outer surface with a sticky layer; the coffee powder becomes “smudged” like porous soil (clumps) for the extraction of these particles is not uniform and is a risk for channelling in espresso.
High temperatures during grinding can also speed up degassing and oxidation, even in the short time the coffee has to go through the grinder.
The coffee powder inside the grinding chamber can reach temperatures as high as 80 – 100°C (M Petracco, 2005), resulting in a great loss of aroma.
Grind coffee for different brewing techniques
The different coffee extraction methods can be distinguished (or characterized) by Time of contact between water and coffee (e.g. 30 seconds for Espresso versus 3 minutes for Pour over ); Water pressure (9 bar for Espresso), and finally, water temperature, (e.g. 94 o C for drip and 12 o C for Cold brew.
In addition to time, pressure, and temperature, all the different extraction methods require grinding the coffee to a specific (relative) grain size.
Thereby, it ensures a specific water flow (i.e. the time the water is in contact with the coffee) and the desired coffee extraction. The longer the contact time between water and coffee, the coarser the coffee should be ground.
Excellent particles can lead to over-extraction, increasing the bitterness of the coffee. However, when extraction time is concise, like with espresso, we need finer & smaller particles to ensure that the necessary flavours are still dissolved.
With Espresso, each extraction takes 20 to 30 seconds per cup (30-45ml), so the coffee must be ground finer. In each espresso grind, one gram of coffee contains about 500,000 ground coffee beans – 20 times finer than regular fine ground coffee.
On the other hand, the peculiarity of Espresso is that it is a multi-phase extraction system (it is a solution, emulsion, suspension and foam), so the control of the fineness of the granules becomes much more rigorous & complex to ensure the preserve characteristic flavour properties.
In general, grinding coffee for espresso is highly dependent on the specific circumstances of the beans and how it is combined with the brewer & barista’s skill.
There is almost only one brewing method that requires the particle size to be reduced to a finer level than that required for espresso – Turkish coffee , a beverage popular among the Moslem cultures of Arabia. Arabic and Indonesian.
Is there a common standard for grain size?
Finally, despite a great deal of work on the grain size aspect of grinding coffee, it must be admitted that there is no universal grind size or a standardized language to talk about grind size other than the terms fine, medium, coarse, boulders – fine, medium, or coarse, etc. are generally very subjective.
Furthermore, different industrial roasters or grinders come with different terminology. A blender set to 14 may not produce the same size as another with 14 scales.
There are scientific ways to measure particle size, but unlike thermometers or electronic scales, these instruments are not straightforward and are impractical for in-store brewing, let alone at home.
In the late 1940s, the United States Department of Commerce established a simple laboratory test procedure to measure the size of coffee beans after grinding.
The device consists of four sieves with different sieve hole sizes mounted on the vibrating system. The sieves are stacked; the coarsest at the top and the smallest at the bottom are #30, #20, #16 and #14, respectively.
These decks were first used in production by WS Tyler Co. You can learn more at SCA’s The Coffee Brewing Handbook.
- The Coffee Brewing Handbook, by Specialty Coffee Association Of America; Chapter 5. Grind
- The Craft And Science Of Coffee, by Britta Folmer; Chapter 13. The GrinddParticles and Particularities
- Craft Coffee, by Jessica Easto; Chapter 1. Brewing Basics, Grind Size and Contact Time.