Why are some coffee beans harder to grind than others?

Why are some coffee beans harder to grind than others?

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There are many issues brewers ought to contemplate when grinding coffee, together with origin, roast profile, processing methodology, and brew methodology. Each ought to inform simply how coarse or high-quality your coffee must be for optimum extraction.

Each bean behaves barely in a different way when it’s floor, for various causes. Some beans are harder to grind by hand, whereas others would possibly take longer in automated grinders, for example.

To perceive extra about why sure beans is likely to be more durable 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 seek out out what they informed me.

You can also like our article on bodily adjustments coffee beans expertise throughout roasting.

Exploring “bean brittleness”

Firstly, when taking a look at how exhausting it’s to grind sure beans, we have to differentiate between “bean density” and “bean brittleness”. These two ideas are distinct, however typically confused.

“Bean density is the load of a single coffee bean, divided by its quantity,” Monika tells me. “We typically use bulk density, the place the load of a bucket stuffed with coffee (minus the bucket) is split by the bucket’s quantity.”

Higher density coffee beans are likely to comprise increased sugar ranges, which will be defined by the extended bean growth part. 

At increased elevations and at colder temperatures, the mobile exercise of coffee slows down, offering extra time for the seed to soak up sugars from the fruit round it. Higher density is commonly seen as extra fascinating.

“However, bean density alone just isn’t accountable for how simple it’s to grind a selected coffee,” Monika provides. “That’s as a result of brittleness.”

Brittleness adjustments when a coffee bean is roasted. For instance, Monika says: “Green coffee is sort of unattainable to grind with a typical coffee grinder, whereas darker roasts typically glide by with little effort.”

Green beans have compact cell constructions. At a mobile degree, inexperienced coffee’s vacuoles (membrane-surrounded chambers that comprise vitamins and preserve water steadiness within the cell) are robust and inflexible.

However, throughout roasting, water evaporates and the cells enhance in dimension. These go away holes within the mobile construction which trigger the construction of the bean to weaken, thus changing into extra brittle.

Monika explains that density and brittleness are not often associated. “In normal, density has little to do with brittleness,” she tells me. “Glass is dense however brittle; a dishwashing sponge is mild however virtually unattainable to shatter.”

What occurs to beans once they’re floor?

“To break up the coffee beans into smaller items, the grinder burrs must introduce fractures into the bean construction,” Monika tells me.

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

Compressive drive is when the beans are pushed down or compressed, whereas tensile drive is when they’re pulled. Shear drive is when a floor is pulled in two opposing instructions directly, and torsional drive is when an object is twisted.

Burr grinders use these 4 forces on the identical time throughout a number of burrs to grind roasted coffee to a variety of sizes.

“As drive is utilized, the bean stretches or shrinks, successfully performing like a spring,” Monika says. “This occurs till it could possibly’t cope anymore and it breaks. 

“Before the breaking level, a bean [will actually] return to its unique form when the drive is eliminated, because it’s elastic.”

She additionally notes that regardless of the obvious contradiction, supplies can truly be each brittle and elastic on the identical time.

“Brittle supplies go from elastic to breaking in a short time,” Monika says. “For instance, glass bends a bit bit, however then shatters in the event you begin to compress it. The deformation is invisibly small, however it will definitely breaks.”

How does roasting have an effect on brittleness?

Roasting has the most important affect on bean brittleness, because of the huge adjustments that happen within the coffee’s cell construction through the course of.

“Roasting makes the bean cell partitions brittle, partly as a result of dehydration,” Monika tells me.

Heat and airflow are used within the roaster to scale back the water content material and dry the beans out. As water leaves the beans, they change into extra brittle.

“At first crack, steam, CO2, and volatiles contained in the cell construct up strain, inflicting cell partitions to compress towards one another and finally rupture,” Monika provides. 

“Breaking up the cell wall is important; it opens the cells, permitting water to get inside and dissolve the plant’s vitamins, which flip into attribute flavour compounds.”

Monika goes on to inform me that she has carried out analysis on the assorted totally different levels of roasting and their affect on bean brittleness. 

Her research discovered that the median particle dimension for coffee simply previous first crack – the stage when the bean first expands quickly and loses water – was 305 micrometres. In distinction, the median particle sizes for post-second crack coffee – the stage when oils start transferring in direction of the floor of the bean – was 120 micrometres. 

This exhibits {that a} longer roast will increase bean brittleness.

Monika additionally notes that roasting influences each density and brittleness, although the 2 components should not in any other case correlated.

“Beans drop pounds throughout roasting due to dehydration and the loss [of] some natural materials, however their quantity does not likely change,” she tells me. “This signifies that bean density decreases over the course of a roast, and that we are able to anticipate finding some correlation between the brittleness and density of a batch.”

So, what does this imply?

“As the roast progresses, beans preserve losing a few pounds and change into extra dehydrated,” Monika says. “The cell partitions [become] more and more weakened by fractures.”

This signifies that on the identical grind setting, darker roasts will grind barely finer than lighter roasts. 

Jordan says: “Darker roasts are extra brittle than lighter roasts, and so they fracture extra simply within the grinder because of this.”

This has an affect on how the coffee extracts, as Monika tells me: “As a darker roast can produce finer grinds than a lighter roast (on the identical grind setting), it would additionally extract extra shortly.

Darker roasts are additionally extra soluble, and have extra traits which might be historically thought-about to be “daring” than mild roasts.

“The aromatics, sugars, and acids are extra unstable in darker roasts and are subsequently simpler to extract,” Jordan says. “You will typically discover {that a} coarser grind setting works higher for darker coffees.”

How do you have to regulate your grind?

While dimension is certainly an important issue to contemplate when dialling in your grind, being attentive to a coffee’s roast profile and brittleness will aid you get the goal flavour within the cup. 

Reduced solubility and brittleness imply that lighter roasts must be floor barely finer to extend the floor space of the coffee. This will enable for extra fragrant compounds to be extracted.

As lighter roasts are much less brittle, they will even take extra time to grind (and extra effort in the event you’re utilizing a hand grinder). While the distinction is small, it could be noticeable while you’re dialling in espresso, for example.

However, Jordan tells me that this prolonged grinding timeframe isn’t actually one thing to fret about. “As lengthy because the grinding course of doesn’t take a number of minutes or longer, the aromatics shouldn’t be compromised.

“Most of the coffee’s aromatics keep intact for round 5 minutes after the coffee is floor,” he provides. “However, leaving your coffee floor for for much longer than that may trigger a number of the coffee’s liveliness and complexity to be misplaced.”

From a grinder upkeep perspective, extra brittle beans are additionally technically “higher” on your burrs, which means much less put on and tear. Lighter roasts might trigger your burrs to put on extra shortly, but it surely’s seemingly that this received’t be particularly noticeable or make an enormous distinction.

Darker roasts, nonetheless, do typically produce extra fines as a result of they’re extra brittle, and may clog grinders extra shortly. In flip, this could have an effect on particle distribution and doubtlessly make your extraction much less even. 

Ultimately, whether or not you’re utilizing mild or darkish roasts, it is best to often clear and verify your burrs to fight these points.

To conclude, each Jordan and Monika inform me that bean brittleness shouldn’t be the brewer’s largest concern. “It’s vital to make use of your palate to guage the ultimate cup earlier than deciding on any grind or brewing changes,” Jordan recommends.

“Brew recipe stays your most dependable information to a constant cup,” Monika provides.

While bean brittleness shouldn’t be an enormous space of concern for many brewers, there’s little doubt that the science behind it’s fascinating, and that it’s one other issue to tweak searching for excellent extraction.

Next time you brew a cup of coffee, you would possibly wish to consider your coffee’s roast profile and tweak the grind setting ever so barely in response. After all, brewing glorious coffee is all about being receptive to even the smallest particulars and components. It might make that little further little bit of distinction.

Enjoyed this? Then learn our article on utilizing the suitable grind dimension on your brewing methodology.

Photo credit: Baratza

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