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 messaged FrontStreet Coffee: "I used the same grind setting for two different beans. The first brewed perfectly normally. The second stalled during brewing and over-extracted! Same setting, so why the difference?"

The answer is simple: it comes down to the "brittleness" of the beans. And to talk about brittleness, we first need to talk about density, because the two are tightly linked.

What Is Density?

Density is the amount of substance packed into a unit of volume - the more substance, the higher the density. A coffee bean's density is determined by its growing environment. At higher altitudes, temperatures are lower; the lower the temperature, the slower the coffee grows.

(FrontStreet Coffee's coffee plantation in Yunnan) Slow-grown beans have more time to absorb nutrients, and those nutrients are the "substance." More substance means a denser bean, and a denser bean is harder - and fuller in flavor.

This is why some countries grade beans by the altitude they were grown at. In Costa Rica, for example, beans grown above 1,400 meters are all classified as SHB - Super Hard Bean.

What Is Brittleness?

Brittleness describes a material's tendency to fracture under force - compression or tension. Brittleness and hardness sit on opposite ends: the harder the bean, the lower its brittleness, and vice versa. So what mainly determines brittleness?

Roast degree. Roasting is, at its core, a dehydration process: the longer the roast, the more moisture evaporates, and the more porous the bean's internal structure becomes. A porous structure means lower density and higher brittleness. In other words, the darker the roast, the higher the brittleness - and the other way around.

You can test this yourself: set a dark-roasted bean and a light-roasted bean on a table and press each one. The dark bean crushes with almost no effort - high brittleness. The light bean takes real force to break, because it is dense and low in brittleness.

(A warm reminder from FrontStreet Coffee: press with the flat side facing up, or you'll learn what it's like to squeeze a raw egg bare-handed.)

How Does Brittleness Affect Grind Size?

During grinding, beans are squeezed between the burrs and cut as the burrs rotate. At the same setting, dark-roasted beans fracture easily thanks to their high brittleness, while dense light-roasted beans resist and need more grinding to break down. Dark-roasted beans therefore produce far smaller particles - and their brittleness chips off extra fines. (Think of a cookie and its crumbs; that should make it click.)

FrontStreet Coffee ran the experiment with two beans at opposite ends of the roast spectrum, ground at the same setting. The parameters:

Beans: dark-roasted Mandheling, light-roasted Yirgacheffe.
Grind setting: 9.5 on an Ek43.
Result: the Mandheling passed 75% through a #20 sieve; the Yirgacheffe passed 65%.

So even at an identical setting, roast differences change the particle size. The bean that stalled in our friend's brew was roasted darker and more brittle, so it produced finer grounds - and those fines clogged the filter paper, causing the over-extraction.

FrontStreet Coffee

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

FrontStreet Coffee Recommendation

The two beans in our experiment are both in FrontStreet Coffee's lineup. The light-roasted Yirgacheffe is FrontStreet Coffee's Guodingding from the Gedeo Zone: washed-processed heirloom grown at 1,900-2,300 meters, with citrus, berries, lemon, sugarcane, and a green-tea finish. Dense and hard, it needs the finer end of your grinder's range.

On the dark end sits FrontStreet Coffee's Yunnan small-bean coffee from Baoshan - washed and medium roasted, showing soft nut and chocolate sweetness with a touch of plum acidity - and its 2013 natural Typica, a dense, elegant heirloom with dark berry fruit, caramel sweetness, chocolate, and a black-tea aftertaste. Roast degree changes how the same burr setting behaves, so taste across the spectrum and adjust the grind bean by bean.