How does burr sharpness affect a coffee’s particle size distribution?
Jenna Gottlieb speaks with coffee scientist and author Robert McKeon Aloe about why keeping your grinder in good working order is vital to making a better brew.
Grinding coffee is an essential stage in the journey of a coffee from seed to cup. Not only does it prepare the beans for brewing across a range of devices, it is also responsible for the irresistible smell that permeates coffee shops – something most coffee drinkers enjoy nearly as much as the coffee itself.
Key to grinding is particle size distribution (PSD): the measure of variations in grind sizes within any single “dose” of ground coffee, and the proportion of these different sizes it contains. In essence, it is what informs how coffee is extracted.
This is critically important for achieving what’s known as optimum extraction, which is the equal extraction of a coffee’s soluble components during brewing. Essentially, the smaller the particles, the more surface area there is for water to come into contact with.
As such, PSD can significantly impact the characteristics of coffee. For instance, a higher concentration of fine particles can result in an over-extracted, bitter flavour because they are easier to extract. Conversely, a higher concentration of coarse particles can result in under-extracted coffee.
A broader PSD is easier to brew as espresso, but a narrower distribution can produce a more even extraction. However, the downside to a narrow distribution is that more attention needs to be paid to puck preparation.
“The understanding of good or ideal particle distribution is still nebulous, and many of the effects on wider or narrow or finer or coarser are difficult to separate compared to other variables,” explains Dr Robert McKeon Aloe, the author of Engineering Better Espresso.
PSD is one of the most vital variables when brewing espresso, and for those truly trying to perfect their shots, it’s important to obtain an accurate measurement. This can typically be achieved with a coffee particle analyser: a machine that uses photo analysis to measure PSD.
It ultimately helps coffee professionals get closer to creating an ideally balanced, smooth shot of espresso by giving them more control over one of their brewing variables. Essentially, it’s all about data.
However, in the quest to gain ultimate control over all brewing variables, coffee professionals have encountered several major obstacles. One of them is coffee grinder burr sharpness, which evidently has a measurable effect on PSD.
How does burr sharpness actually affect PSD?
The sharpness of coffee grinder burrs is paramount when it comes to PSD. In 2015, Socratic Coffee conducted a study on burr sharpness, finding a direct correlation between the dullness of burrs and poor extraction.
Specifically, the study aimed to assess the impact of burr sharpness on total dissolved solids (TDS), a metric used to measure the amount of extracted, soluble solids in the cup.
“A difference in TDS was observed, with the sharp burr condition yielding significantly higher TDS than the dull burr condition,” the study claimed. “From this experiment, it would appear that TDS may be a useful tool to determine the sharpness of grinder burrs.”
The exact way burr sharpness influences PSD is still being investigated, and Robert explains that there are several factors to consider.
“Espresso is very complex in variables, and sometimes you can turn the dial on one variable to offset another,” he says. “I’m not saying this is the case for dull burrs, but it hasn’t been explored.”
He believes that the only way to get to the bottom of the issue is to dial-in a set of sharp burrs and a set of dull burrs to the point where they result in the same PSD.
“If the effect is merely PSD and not particle shape, then I think it’s a simpler discussion on how PSD shifts affect coffee,” he explains. “For example, I did a comparison of the Niche and Ode, analysing shapes using pattern recognition to better understand fundamental particle shapes.”
In any case, using blunt burrs is not a great idea, as cheap or dull burr grinders are more likely to chop the coffee beans into different-sized fragments, leading to a very unbalanced cup of coffee.
How to protect your coffee grinder burrs
In recent years, some baristas have attempted to compensate for this phenomenon by making other grinder adjustments or servicing their grinders more frequently.
Others yet have simply switched to modern grinders with features that reduced the toll on burrs, like the Slingshot S75 and Slingshot C68. Both can be fitted with either flat or conical burrs, but the burr adjustment mechanism is specially designed to reduce both service time and cost.
Some coffee grinder manufacturers opt to apply special coatings to the burrs, while others incorporate dual-burr systems.
“I know coatings exist, but I am not an expert on those or how frequently they go blunt,” Robert notes. “I suspect a dual-burr grinder would take longer to dull, but I’m not sure because they are so new to the market. I don’t know if anyone has even analysed which parts of the burrs become dull.”
Ultimately, whichever grinder a barista uses, proper maintenance is essential. This includes grinding away any excess beans before cleaning the machine, purging the burr blades, using lint-free cloths to wipe down the burrs, and using compressed air to dislodge coffee grime.
It’s not all about cleanliness, though – baristas need to be able to recognise when the burrs have become blunt. The easiest way to do so is to disassemble the grinder and feel the burrs with your fingers. Robert says it’ll be quite obvious if the burrs have lost their sharpness.
In the early stages of burr dulling, the grinder might start spitting out clumps of coffee grounds. If the coffee you’re grinding is getting coarser and coarser without you adjusting anything, that’s another sign of burr dullness.
Likewise, if the coffee grounds are becoming increasingly inconsistent – especially if this is visible to the naked eye – you should consider replacing your burrs entirely.
Ultimately any barista determined to perfect their brews needs to have control over PSD, and as the research suggests, dull burrs simply don’t make the cut.