What's the Difference Between Conical and Flat Burr Grinders in Coffee Flavor?

Yesterday we looked at how to dial in a new coffee grinder. Today we put the newly purchased Huijia ZD-10 up against our regular Forte-BG grinder to confirm grind size and calibrate the two.

Grinders generally fall into three categories: flat burrs, conical burrs, and ghost-tooth (flat steel) burrs.

Flat Burr: cuts coffee beans into particles, so the resulting grounds are mainly flake-shaped.

Conical Burr: crushes coffee beans into particles, so the resulting grounds are mainly block-shaped.

Ghost Tooth: grinds coffee beans, so the resulting particles are closer to round.

Today we focus on conical against flat burrs, calibrating the conical-burr Huijia ZD-10 to match both the grind size and the brewed flavor of the flat-burr Forte-BG.

First, the differences between the two burr sets.

Flat Burr Discs

Advantages and Disadvantages:

A flat burr grinder consists of an upper and a lower disc, with the base fixed to the motor. When the motor starts, the two discs rotate and create a cutting action.

As coffee particles fall from the center, the rotating lower burr draws the beans toward the periphery and pushes them into the toothed section to be ground.

Because the upper and lower discs sit parallel, the beans depend on the rotational force of the lower burr to be pushed into the burr chamber. The weight of the beans sitting on top therefore affects how uniformly they enter the chamber, which in turn affects grind consistency. The pushing action also increases collisions between beans, producing a higher proportion of fines. This is why flat burr grinders typically show slightly less uniform particle size and more fines.

Grounds from a flat burr are flake-shaped, and that shape increases the surface area of the cell walls. Flat burrs therefore raise concentration and extraction rate quickly, and the aroma intensifies sharply in a short time. At the same time, because the flakes are flat, elongated, and low in volume, a longer extraction lets the xylem absorb too much water, which brings out bitterness and astringency.

Conical Burr Discs

Advantages and Disadvantages:

A conical burr grinder has a cone-shaped burr at the bottom working against an outer ring burr. Beans fall from above, are pulled downward by the rotating cone, and are ground between the two surfaces. Because the action is top-down, particles do not need to be forced out, and collisions between particles are greatly reduced compared with a flat burr. Conical burrs therefore produce fewer fines than flat burrs, and grind uniformity improves noticeably.

Grounds from a conical burr are closer to granular, which lengthens the path water must travel into each particle and requires more time for the interior to be reached. Conical grounds therefore release fewer solubles in the early stage, so concentration does not climb as fast. Because of the granular shape, even a longer extraction draws less water into the xylem, making bitterness and astringency less likely.

With the burrs understood, we selected the Guji Buku Able from Yirgacheffe for cupping and brewing comparisons between flat and conical burrs.

Hana Guji (Huakui)

Green Bean Information:

• Coffee Origin: Africa, Ethiopia
• Coffee Estate: Guji, Hambella Wamena region, Dimtu, Buku Able Estate
• Altitude: 2250~2350m
• Grade: G1
• Processing Method: Natural (African drying bed)

Cupping Performance of Hana Guji:

On the Forte-BG at grind size 5M (88% pass-through on a Chinese standard #20 sieve), the cupping profile showed rose floral notes, strawberry jam, peach flesh, and a black tea aftertaste.

On the Huijia ZD-10 at grind size 4.2 (87% pass-through on the same #20 sieve), the cupping profile showed honey, floral notes, citrus, chocolate, and woody notes.

In the cup, the flat-burr Hana Guji showed rich floral and fruit aromas with a comparatively clean taste, while the conical burr showed sweetness with a hint of chocolate bitterness and slight woody off-flavors, likely caused by fines. So how would Hana Guji perform when brewed?

Brewing Performance:

We used the Forte-BG at grind size 5R and the Huijia ZD-10 at grind size 6 to compare brews of Hana Guji.

Water temperature was 91°C, powder-to-water ratio 1:15, with a V60 dripper.

Flat Burr Disc: Bloom first with 30g of water for 30 seconds, then pour to 122g and wait until the water level drops to nearly reveal the coffee bed, then pour again to 228g. To avoid throwing off the ratio, we waited until all the water had filtered through before removing the dripper. Total brew time was 2 minutes 13 seconds.

Conical Burr Disc, Grind Size 6: Bloom with 30g of water for 30 seconds, then pour to 120g and wait until the water level drops to nearly reveal the coffee bed, then pour again to 223g. Total brew time was 2 minutes 10 seconds.

Flat burr disc flavor profile: floral and fruit aromas, cream, nuts, hazelnut, prune, and black tea, with a clean taste and a sweet aftertaste.

Conical burr disc, grind size 6, flavor profile: orange, floral notes, berries, and maple syrup, with rounded flavors and a balanced mouthfeel.

Comparing the two, the flat burr showed more layered flavor with more prominent nutty notes, while the conical burr had livelier acidity and less prominent nuttiness. We therefore adjusted the conical burr finer to move it closer to the flat burr profile and brewed again.

Conical Burr Disc, Grind Size 5: Bloom with 30g of water for 30 seconds, then pour to 120g and wait until the water level drops to nearly reveal the coffee bed, then pour again to 227g. Total brew time was 2 minutes 10 seconds.

Conical burr disc, grind size 5, flavor profile: chocolate, cream, citrus, and floral notes, with a tea-like aftertaste.

Compared with grind size 6, grind size 5 came closer to the overall flavor profile of the flat burr.

Because the burr sets and the machines differ, the best we could do was get as close as possible. To summarize the whole calibration process: start with a reference grinder (without one, the only honest advice is to rely on touch and visual observation), then compare pass-through rates and bring them as close as you can, then brew side by side to feel out the differences, and finally make fine adjustments. In this case the conical-burr Huijia ZD-10 produced more fines but also larger particles overall, so we tightened the grind to 5 to bring the profile closer to the flat-burr Forte-BG and completed the calibration. (If you use the Huijia ZD-10 daily and want fewer fines, a flour sifter helps.)

For the previous article on calibrating a new grinder, see: [FrontStreet Coffee Barista's Journal] How should the grind size of a new coffee grinder be determined?

FrontStreet Coffee Recommendation

Burr geometry matters most when the beans have something to show, so it helps to pair a new grinder with coffees that reward clarity. FrontStreet Yirgacheffe Natural Red Cherry is one of the sweetest of the light-roast classics: it is a whole-cherry natural, meaning only fully ripe, sound fruit is picked before drying on raised beds, which is why the cup opens with ripe fruit aroma and a clear ferment note, then moves to berry, citrus acidity, and a creamy texture, closing on a ripe-grape juice character. Roasted medium-light, it gives a conical burr plenty of sweetness to work with.

FrontStreet Strawberry Candy is a two-origin blend of Panama washed Geisha and a Costa Rican honey-processed coffee, roasted medium so that it keeps its floral and fruit character while picking up roasty caramel notes. Expect caramel, raisin, dried fruit, and a rich rose floral lift. FrontStreet Mozart is another good match: its raisin honey process leaves 100% of the mucilage on the bean, producing the Kyoho-grape sweetness plus pineapple and osmanthus aromas that many drinkers remember long after the cup.

For an all-round daily bean that suits hand grinding and any brewer, FrontStreet Queen Estate Yellow Bourbon from Brazil is hard to beat. The variety brings pronounced sweetness, and the medium roast shows nuts, cream, sugarcane, and a soft fruit acidity. It works as pour-over, French press, moka pot, or espresso, which makes it a good reference bean when you are calibrating a grinder and want a flavor profile you already know.