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

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Yesterday we talked about how to adjust a new coffee grinder, and today we will compare the newly purchased Huijia ZD-10 with our regular Forte-BG grinder to further confirm the grind size and calibrate the grinding.

Grinders can generally be divided into three main categories: flat burrs, conical burrs, and ghost tooth (flat steel) burrs.

Flat Burr: Grinds coffee beans into particles by cutting, so the resulting particles are mainly flake-shaped.

Conical Burr: Grinds coffee beans into particles by crushing, so the resulting particles are mainly block-shaped.

Ghost Tooth: Grinds coffee beans into particles by grinding, so the resulting particles are mainly closer to round in shape.

Today, we will focus on comparing conical burr discs and flat burr discs, calibrating the Huijia ZD-10 with conical burrs to match the grind size and brewing flavor of the Forte-BG grinder with flat burrs.

First, let's understand the differences between flat burrs and conical burrs.

Flat Burr Discs

Advantages and Disadvantages:

Flat burr grinders consist of upper and lower burr discs, with the base fixed to the motor. When the motor starts, the upper and lower burrs rotate to create a cutting action.

When coffee particles fall from the center, the rotating lower burr draws the coffee beans to the periphery, pushing them into the teeth section for grinding.

Since the upper and lower burrs of a flat burr are arranged parallel, coffee beans rely on the rotational force of the lower burr to be pushed into the burr chamber. Therefore, the weight of the coffee beans placed on top will affect the uniformity of beans entering the burr chamber, which in turn affects grinding uniformity. Additionally, the pushing action increases collisions between coffee beans, leading to a higher proportion of fine particles. This is why flat burr grinders typically have slightly less uniform particle size and more fine particles.

Coffee particles ground by flat burrs appear flake-shaped, and this shape increases the surface area of the cell walls. Therefore, using flat burrs can quickly increase coffee concentration and extraction rate, making coffee aroma significantly enhance in a short time. At the same time, because the shape is flat and elongated with smaller volume, when extraction time increases, the xylem of the particles will absorb too much water, causing increased bitterness and astringency.

Conical Burr Discs

Advantages and Disadvantages:

Conical burr grinders have a conical burr at the bottom and work with an outer ring burr for grinding. After coffee beans fall from above, they are pulled downward by the rotating conical burr, creating the grinding action. Because of the top-down approach, particles don't need to be pushed out by force, and compared to flat burr grinders, collisions between particles are greatly reduced. Therefore, conical burrs produce fewer fine particles than flat burrs, and coffee grinding uniformity is significantly improved.

Particles ground by conical burrs are closer to granular, which lengthens the water absorption path of coffee particles, requiring more time for the interior to contact water. Therefore, conical particles release fewer soluble substances in the initial stage, resulting in concentration that won't be too high in a short time. At the same time, because of their granular shape, even with longer extraction times, the xylem absorbs less water, making it less likely to produce bitterness and astringency.

Now that we understand the burr discs, I'll select the Guji Buku Able from Yirgacheffe for cupping and brewing tests comparing flat burrs and conical burrs.

Hana Guji (花魁)

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:

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

In the Huijia ZD-10 at grind size 4.2 (87% pass-through on Chinese standard #20 sieve), the cupping flavor profile shows: honey, floral notes, citrus, chocolate, and woody notes.

In cupping, the flat burr Hana Guji shows rich floral and fruit aromas with a relatively clean taste; while the conical burr performance shows sweetness with a hint of chocolate bitterness and slight woody off-flavors, which may be caused by fine particles. So how does Hana Guji perform in brewing?

Brewing Performance:

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

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

Flat Burr Disc: First bloom with 30g 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 affecting their powder-to-water ratio, I waited until all water filtered through before removing the dripper. Total brewing time was 2 minutes and 13 seconds.

Conical Burr Disc Grind Size 6: First bloom with 30g 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 brewing time was 2 minutes and 10 seconds.

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

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

After comparing both, the flat burr shows more layered flavor with more prominent nutty notes, while the conical burr has more lively acidity with less prominent nutty flavors. Therefore, I adjusted the conical burr to a finer grind to approach the flat burr flavor profile and brewed again.

Conical Burr Disc Grind Size 5: First bloom with 30g 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 brewing time was 2 minutes and 10 seconds.

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

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

Due to different burr discs and different machines, I could only get as close as possible. To summarize the entire calibration process: first, have a reference grinder (if you don't have one, I can only tactfully suggest using touch and visual observation), then compare their pass-through rates, making them as consistent as possible, followed by brewing comparisons to experience their differences, and then make fine adjustments. Just like the Huijia ZD-10 using conical burrs produces more fine particles but overall larger particles, so I adjusted the grind to 5 to make the flavor profile closer to that brewed by the flat burr Forte-BG grinder, completing the calibration process. (For daily use of the Huijia ZD-10, if you don't want too many fine particles, you can buy a flour sifter.)

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

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