Why Does the Same Grind Setting Produce Different Coarseness for Different Coffee Beans? What's the Relationship Between Brittleness and Density?

A while ago, a friend sent a private message to FrontStreet Coffee: "I used the same grind setting for two different beans. The first bean brewed perfectly normal, with suitable coffee flavor. But the second bean got clogged during brewing! This led to over-extraction. I clearly used the same setting, so why did this happen?"

It's quite simple, because this is related to the "brittleness" of coffee beans! To discuss brittleness, we first need to understand coffee bean density, because brittleness and density have a very close relationship!

What is Density?

Density refers to the amount of substance contained within a unit volume. The more substance there is, the higher the density. The density of coffee beans is determined by their growing environment. When coffee beans grow at higher altitudes, temperatures become lower. The lower the temperature, the slower the coffee grows!

(FrontStreet Coffee's coffee plantation in Yunnan) Slow-growing coffee beans have more time to absorb more nutrients, and these nutrients are the substances. An increase in substance quantity is equivalent to an increase in the density of the coffee bean itself. The higher the density, the harder the bean itself will be. At the same time, it also gives the coffee beans fuller flavor.

Therefore, some countries use the altitude at which beans are grown to distinguish their hardness and grade. (For example, in Costa Rica in Central America, coffee beans grown above 1400 meters altitude are all classified as SHB grade - Super Hard Beans.)

What is Brittleness?

Brittleness, also known as "brittleness," refers to the phenomenon of fracture under the action of external forces such as compression or tension. Brittleness and hardness have a relative relationship! The higher the hardness, the lower the brittleness, and vice versa. So what mainly determines brittleness?

That's right, the degree of roasting! Logically, the entire roasting process of coffee beans is a dehydration process. The longer the roasting time, the more moisture evaporates from the coffee beans. With reduced moisture, their internal structure naturally becomes more porous! A porous structure represents a decrease in density and an increase in brittleness. This means the darker the roast, the higher the brittleness, and vice versa!

We can do an experiment: Prepare a dark-roasted coffee bean and a light-roasted coffee bean, place them on a tabletop and press them separately. You will then find that the dark-roasted bean is easily crushed by us - this is the result of high brittleness; while the light-roasted bean requires a certain amount of effort to crush because it has high density and low brittleness.

(FrontStreet Coffee's warm reminder: When pressing, it's best to have the flat side of the bean facing up, otherwise you'll experience the joy of holding a raw egg in your bare hands~)

How Does Brittleness Affect Grind Size?

Coffee beans experience the force from the pressure between two grinding burrs, as well as the cutting force from the burr rotation during grinding. When the grind setting is the same, dark-roasted beans will be more easily crushed due to their higher brittleness; while light-roasted beans will be more difficult to grind due to their higher density, requiring multiple grindings to break them down. Therefore, the particles ground from dark-roasted beans will be much smaller than those from light-roasted coffee. At the same time, due to their "brittleness," many fine particles will be chipped off. (Replacing this with cookies and cookie crumbs should help everyone understand better)

Below is an experiment conducted by FrontStreet Coffee, using two beans with the most difference in roast level and grinding them with the same setting, which resulted in completely different grind sizes. The grinding parameters are as follows:

Coffee beans: Dark roasted Mandheling, Light roasted Yirgacheffe.
Grind setting: 9.5 on Ek43.
Experimental results: Mandheling had a 75% pass rate through the #20 sieve; Yirgacheffe had a 65% pass rate through the #20 sieve.

So, even with the same grind setting, differences in the roast degree of two beans will cause variations in the ground particles. Like the friend mentioned at the beginning, the coffee bean that caused clogging was roasted darker, with higher brittleness, which resulted in finer grounds being produced at the same grind setting. The overly fine coffee grounds clogged the pores of the filter paper, leading to the negative situation of over-extraction.

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FrontStreet Coffee

No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou, Guangdong Province

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