May 10, 2021

Why are some coffee beans harder to grind than others?


There are many things brewers should consider when grinding coffee, including origin, roast profile, processing method, and brew method. Each should inform just how coarse or fine your coffee should be for optimal extraction.

Each bean behaves slightly differently when it is ground, for a number of reasons. Some beans are more difficult to grind by hand, while others might take longer in automatic grinders, for instance.

To understand more about why certain beans might be harder to grind than others, I spoke to Monika Fekete, Senior Chemist and Coffee Expert at Breville, and Jordan Markezich, Support Specialist at Baratza. Read on to find out what they told me.

You may also like our article on physical changes coffee beans experience during roasting.

Exploring “bean brittleness”

Firstly, when looking at how hard it is to grind certain beans, we need to differentiate between “bean density” and “bean brittleness”. These two concepts are distinct, but often confused.

“Bean density is the weight of a single coffee bean, divided by its volume,” Monika tells me. “We often use bulk density, where the weight of a bucket full of coffee (minus the bucket) is divided by the bucket’s volume.”

Higher density coffee beans tend to contain higher sugar levels, which can be explained by the prolonged bean development phase. 

At higher elevations and at colder temperatures, the cellular activity of coffee slows down, providing more time for the seed to absorb sugars from the fruit around it. Higher density is often viewed as more desirable.

“However, bean density alone is not responsible for how easy it is to grind a particular coffee,” Monika adds. “That’s due to brittleness.”

Brittleness changes when a coffee bean is roasted. For example, Monika says: “Green coffee is nearly impossible to grind with a typical coffee grinder, while darker roasts often glide through with little effort.”

Green beans have compact cell structures. At a cellular level, green coffee’s vacuoles (membrane-surrounded chambers that contain nutrients and maintain water balance in the cell) are strong and rigid.

However, during roasting, water evaporates and the cells increase in size. These leave holes in the cellular structure which cause the structure of the bean to weaken, thus becoming more brittle.

Monika explains that density and brittleness are rarely related. “In general, density has little to do with brittleness,” she tells me. “Glass is dense but brittle; a dishwashing sponge is light but almost impossible to shatter.”

What happens to beans when they’re ground?

“To break up the coffee beans into smaller pieces, the grinder burrs need to introduce fractures into the bean structure,” Monika tells me.

According to Dr David Hoxley, a physics lecturer at La Trobe University, grinder burrs apply “two types of force to the bean: compressive forces crush the beans, while tensile, shear, and torsional forces tear them apart”.

Compressive force is when the beans are pushed down or compressed, while tensile force is when they are pulled. Shear force is when a surface is pulled in two opposing directions at once, and torsional force is when an object is twisted.

Burr grinders use these four forces at the same time across multiple burrs to grind roasted coffee to a range of sizes.

“As force is applied, the bean stretches or shrinks, effectively acting like a spring,” Monika says. “This happens until it can’t cope anymore and it breaks. 

“Before the breaking point, a bean [will actually] go back to its original shape when the force is removed, as it’s elastic.”

She also notes that despite the apparent contradiction, materials can actually be both brittle and elastic at the same time.

“Brittle materials go from elastic to breaking very quickly,” Monika says. “For example, glass bends a little bit, but then shatters if you start to compress it. The deformation is invisibly small, but it eventually breaks.”

How does roasting affect brittleness?

Roasting has the biggest impact on bean brittleness, thanks to the vast changes that occur in the coffee’s cell structure during the process.

“Roasting makes the bean cell walls brittle, partly due to dehydration,” Monika tells me.

Heat and airflow are used in the roaster to reduce the water content and dry the beans out. As water leaves the beans, they become more brittle.

“At first crack, steam, CO2, and volatiles inside the cell build up pressure, causing cell walls to compress against each other and eventually rupture,” Monika adds. 

“Breaking up the cell wall is essential; it opens the cells, allowing water to get inside and dissolve the plant’s nutrients, which turn into characteristic flavour compounds.”

Monika goes on to tell me that she has conducted research on the various different stages of roasting and their impact on bean brittleness

Her studies found that the median particle size for coffee just past first crack – the stage when the bean first expands rapidly and loses water – was 305 micrometres. In contrast, the median particle sizes for post-second crack coffee – the stage when oils begin moving towards the surface of the bean – was 120 micrometres. 

This shows that a longer roast increases bean brittleness.

Monika also notes that roasting influences both density and brittleness, despite the fact that the two factors are not otherwise correlated.

“Beans lose weight during roasting because of dehydration and the loss [of] some organic material, but their volume does not really change,” she tells me. “This means that bean density decreases over the course of a roast, and that we can expect to find some correlation between the brittleness and density of a batch.”

So, what does this mean?

“As the roast progresses, beans keep losing weight and become more dehydrated,” Monika says. “The cell walls [become] increasingly weakened by fractures.”

This means that at the same grind setting, darker roasts will grind slightly finer than lighter roasts. 

Jordan says: “Darker roasts are more brittle than lighter roasts, and they fracture more easily in the grinder as a result.”

This has an impact on how the coffee extracts, as Monika tells me: “As a darker roast can produce finer grinds than a lighter roast (at the same grind setting), it will also extract more quickly.

Darker roasts are also more soluble, and have more characteristics that are traditionally considered to be “bold” than light roasts.

“The aromatics, sugars, and acids are more volatile in darker roasts and are therefore easier to extract,” Jordan says. “You will often find that a coarser grind setting works better for darker coffees.”

How should you adjust your grind?

While size is definitely the most important factor to consider when dialling in your grind, taking note of a coffee’s roast profile and brittleness will help you get the target flavour in the cup. 

Reduced solubility and brittleness mean that lighter roasts should be ground slightly finer to increase the surface area of the coffee. This will allow for more aromatic compounds to be extracted.

As lighter roasts are less brittle, they will also take more time to grind (and more effort if you’re using a hand grinder). While the difference is small, it may be noticeable when you’re dialling in espresso, for instance.

However, Jordan tells me that this extended grinding timeframe isn’t really something to worry about. “As long as the grinding process doesn’t take several minutes or longer, the aromatics should not be compromised.

“Most of the coffee’s aromatics stay intact for around five minutes after the coffee is ground,” he adds. “However, leaving your coffee ground for much longer than that will cause some of the coffee’s liveliness and complexity to be lost.”

From a grinder maintenance perspective, more brittle beans are also technically “better” for your burrs, meaning less wear and tear. Lighter roasts may cause your burrs to wear more quickly, but it’s likely that this won’t be especially noticeable or make a huge difference.

Darker roasts, however, do generally produce more fines because they are more brittle, and can clog grinders more quickly. In turn, this can affect particle distribution and potentially make your extraction less even. 

Ultimately, whether you’re using light or dark roasts, you should regularly clean and check your burrs to combat these issues.

To conclude, both Jordan and Monika tell me that bean brittleness should not be the brewer’s biggest concern. “It’s important to use your palate to evaluate the final cup before deciding on any grind or brewing adjustments,” Jordan recommends.

“Brew recipe remains your most reliable guide to a consistent cup,” Monika adds.

While bean brittleness should not be a huge area of concern for most brewers, there’s no doubt that the science behind it is interesting, and that it is another factor to tweak in search of perfect extraction.

Next time you brew a cup of coffee, you might want to evaluate your coffee’s roast profile and tweak the grind setting ever so slightly in response. After all, brewing excellent coffee is all about being receptive to even the smallest details and factors. It could make that little extra bit of difference.

Enjoyed this? Then read our article on using the right grind size for your brewing method.

Photo credits: Baratza

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