We get a lot of questions here at Total Espresso concerning espresso machines and grinders which we are happy to answer. Given that the grinder is the single most important piece of espresso equipment you will purchase for quality espresso, one such often asked question concerns the difference between the two types of burr grinders. Here is our answer.
Beyond the obvious answer of shape, there lie more subtle differences which may make choosing one over the other a little more important to some customers. First of all, they both do a fairly good job of crushing coffee beans fine enough for an espresso pull. The differences mainly reside in the particle distributions, waste created per grind session, noise, heat, price, and maintenance.
Here is a picture from The Coffee Academy showing the two grinder shapes.
As you can see, conical burrs have a cone-shaped serrated ring that sits snugly inside another hollow cone-shaped serrated ring. The ridges between the burrs crush and grind the beans.
The flat burrs have two serrated rings that face each other and the beans are crushed between them. The burr teeth carry out the grinding process.
Now that we have the obvious difference out of the way, what about the others? The coffee grind is critical since it affects both extraction rate and brewing time. For most espresso drinkers, both types of grinding methods should produce a delicious shot assuming the grinder setting is dialed in for the selected bean type and conditions. However, the particle distribution, on a more microscopic scale, is a little different between the two types. A flat burr grinder produces slightly more finely ground particles with some larger particle sizes mixed in. A conical burr grinder will produce a better distribution of the two particle sizes. This type of size variation is required for a dense pack in the espresso puck that resists water flow and allows for greater saturation of the larger particles for a proper extraction. With a flat burr grinder, a barista may have to pull longer shots to get the same amount of espresso.
During the grinding process, beans are dropped from a bean hopper which sits atop the grinder motor. What happens after the beans go through the grinding process affects a lot of the other factors in the differences between the two types of burrs. In a flat burr grinder, the beans fall between the two disks, are ground between the two edges of the two plates, and are flung out to the sides into a chute. In a conical burr grinder, the beans drop vertically down through the conical burrs and the powder continues to drop vertically into a chute.
This picture from the Coffee Technicians Guild illustrates how the two grinders distribute the resulting coffee grinds after grinding. Flat burrs rotate faster creating more friction than conical burrs and require a larger motor to achieve that speed. The conical burrs operate at a slower speed which means they are less noisy and friction (heat) is reduced.
One of the disadvantages of a flat burr grinder is the amount of grinds remaining in the grinder after coffee is dosed (with a doserless grinder). The grinds tend to become trapped between the two burrs and on the edge between the chute and the burr. When the burrs first start up and when they slow down at the end of a grind, some grinds will not leave the burrs and some may not have enough momentum to continue down the chute. These trapped grounds could contaminate other batches. Because of the vertical set up of the conical burrs, grinds drop directly into the chute with fewer retained grinds. As a result, flat burrs need to be cleaned more regularly than conical burrs to help remove the trapped grounds.
Another difference between the two burr types lies in the static charge that builds up with flat burrs. This static will cause the finely ground powder to become stuck to the sides of the coffee collection boxes built into the grinder.
So now that you know the difference, which is better? In our opinion, the difference between the two is slight and other factors should drive the grinder decision, such as size of the burrs, the price, the dimensions, the noise, or the quality of construction.
Here at Total Espresso, we carry a wide range of grinders. Our conical grinders include the Ascaso I-mini and the Lelit PL-53, both of which have 38 mm conical burrs. The flat burr grinders range in size from 50 mm in the Rancilio Rocky to 65 mm in the Mahlkonig K30 Vario.