The question of conical vs. flat burr is a heated debate in the specialty coffee community. Flat burrs first came to the fore in 2014, when Matt Perger opted to use one at the World Barista Championship. Since then, their popularity has risen, particularly among espresso enthusiasts.
This article will go into coffee geek mode and delve into the key differences between conical vs. flat burr grinders. While this issue could be debated until the cows come home, we would say that burr shape is far less important than build quality or price. If you invest in a quality grinder, you will find that it should produce a uniform grind size no matter what the burr shape.
What is a Burr Grinder?
So to start, a quick recap of burr grinders. Burr grinders work by forcing coffee beans through a set of burrs. One burr remains stationary while the other rotates. When coffee beans are forced through the burrs, they are crushed to your desired consistency. The grind setting will be determined by how close or far apart the burrs are. In contrast, a blade grinder uses a whizzing blade to cut the coffee beans up. How fine or coarse your ground is, is determined by the length of time the blade grinder runs.
Of the two, a burr grinder will always produce a more uniform grind setting and thus better-tasting coffee. A blade grinder produces an uneven grind size with boulders and fines, making it challenging to create a balanced, well-extracted coffee.
When it comes to burrs, the two most popular burr shapes are conical or flat. In this article, we will be digging into the differences between these two shapes. As a side note, there is also another type of burr called block burrs. Block burrs are commonly found in cheaper grinders and are not very good at producing a consistent grind (although they are a heck of a lot better than a blade grinder!). If you are looking at investing in a coffee grinder, we highly recommend opting for either a conical or flat burr grinder.
The two different burr shapes use different mechanisms to grind coffee. A conical burr consists of two cone-shaped burrs. The outer burr remains stationary while the inner burr rotates. There is a wider opening at the top through which beans feed through. They are then crushed into smaller and smaller pieces until they feed vertically down through the gap in the bottom of the burrs. The distance between the two burrs is what determines the grind size. With a conical burr, the beans pass through the burrs vertically with the help of gravity.
In contrast, a flat burr consists of two donut-shaped serrated rings that lie flat on each other. One of the burrs rotates while the other remains stationary. Coffee feeds through the center of the burr and out the burrs horizontally through the edge of the burrs. Without the help of gravity to feed the beans through the burrs, a flat burr grind has to use centrifugal force alone. This means that a flat burr grinder has to spin a lot faster and thus has a lot higher RPM (around 700 RPM) and a more powerful motor.
As a side note, this is why flat burrs are only used in electric grinders. A hand grinder wouldn’t grind quickly enough to produce the force required to feed the coffee through.
If you have been around the coffee world long, you may have heard the terms bimodal and unimodal thrown around. These terms relate to the grind size produced by the different burr shapes.
A conical burr creates what is known as a bimodal particle size distribution. This means it produces two different grind sizes: fines (tiny particles) and larger ones. While this is not obvious to the naked eye, it would soon become apparent if you were to examine the grinds under a microscope or use a sifter.
The fines restrict water flow through the coffee bed, giving time for the larger particles to extract. This slower flow rate gives rise to your conventional espresso with a thick, rich body and notes of bitterness. Given the bimodal grind size, extraction is uneven, and brewing espresso is a great balancing act, allowing time for the larger grinds to extract to that sweet spot without letting the fines over-extract.
In contrast, a flat burr grinder produces a unimodal particle size, with a single grind size. The uniform grind size gives you the flexibility to play around with espresso shots more. You can pull longer shots with higher extraction rates without having to worry about fines over extracting and producing bitter flavors. The lack of flow restriction (from the absence of fines) means that you either have to grind finer or pull longer shots to obtain the same level of extraction.
The concern with grinders producing heat is two-fold. Firstly, heat can damage a coffee bean’s delicate oils and aromatics, changing the flavor profile. Secondly, the speed of espresso extraction can be altered when a grinder heats up, requiring you to change the grind size. Needless to say, flat burrs are going to generate more heat than conical burrs. This is because flat burrs need to run at a far faster speed and create more friction, which in turn creates heat. While heat is a cause for concern, this is only an issue you would need to worry about in a commercial environment, where you are using your grinder continuously and may require you to alter your grind setting throughout the day.
Grind retention is a concern because if a grinder retains a lot of old grinds, it can contaminate your freshly ground coffee. When stale grinds are mixed with fresh grinds, it throws off the flavor. Once again, Flat burr grinders are the main culprits for grind retention. Given the design of the burr, coffee needs to feed out of the burrs horizontally, and more is retained. However, grind retention is also determined by other factors, such as the design and size of the burrs. In general, the larger the burrs, the larger the retention, and vice versa. A commercial grinder typically has large burrs and retains a lot of coffee. This is why it is essential to purge regularly to flush out any stale grounds.
Once again, you will find that a flat burr grinder will run a lot louder than a conical burr grinder. Given the high RPMs a flat burr runs at, it requires a more powerful motor. However, the noise level will vary depending on the grinder you opt for and shouldn’t factor too much into your decision.
In general, enthusiasts state that a conical burr grinder highlights the dark, roasty flavors of a coffee better, like the chocolate, nut, and caramel notes. In contrast, a flat burr grinder gives more clarity and highlights the brighter, acidic, and floral notes. However, this is somewhat subjective; and these differences are likely to only be detected by someone well-versed in coffee.
A flat burr grinder is typically more expensive than a conical burr grinder. Flat burrs are more challenging to produce and require a powerful motor to run. In contrast, a conical burr is a simple design that is easy to make and runs at lower speeds. Conical burrs are commonly found in hand grinders and domestic grinders, whereas flat burrs are typically found in commercial grinders or espresso grinders.
The Verdict: Conical vs. Flat Burr
So which is best; conical or flat burrs? Well, the answer is, it depends. The difference in the cup between the two different burr shapes is inconsequential. If you are a home barista, a conical burr grinder will most likely be sufficient for your needs. They are more affordable, run quieter and cooler, and are more forgiving to use.
However, if you are in a commercial environment or want to get into some serious espresso brewing, a flat burr will be better suited. They indisputably produce a more consistent grind and will give you more flexibility to play around with different brewing parameters.