Among the factors that determine the flavor of brewed coffee, "beans," "grind," and "machine" are hardware-related, while "technique" is software-related. Today, we’ll briefly introduce how grinding affects coffee flavor.

Currently, there are many types of coffee grinders on the market, but they can generally be divided into flat burr grinders, conical burr grinders, roller grinders, gear burr grinders, and blade grinders. Among these, roller grinders are mostly used in large factories, crushing beans like a pasta press. They are highly efficient but not suitable for cafés. Blade grinders, on the other hand, are mostly simple home grinders that chop beans using the high-speed rotation of blades. They produce extremely uneven grind sizes, generate a lot of heat, and are very detrimental to coffee extraction. Flat burr and conical burr grinders are currently the mainstream types, and their differences are as follows:

Overall, the advantages of flat burr grinders are fast speed, quick grinding, and more uniformity, but they tend to generate more heat and have a relatively shorter lifespan. Conical burr grinders, however, generate less heat, have a longer lifespan, but are slower and less uniform.

The gear burr grinder from Hario (Hario Mini Mill Slim) can produce more uniform coffee grounds while generating fewer fines, resulting in higher grinding efficiency. However, it is not suitable for espresso extraction (due to insufficient fineness).

The main factors of how grinders affect extraction are as follows:

Uniformity: Coarser coffee particles are more prone to under-extraction, while finer particles are more likely to be over-extracted. Therefore, coffee ground with a wider range of particle sizes tends to produce more murky and harsh off-flavors. When coffee grounds with similar sizes and relatively consistent shapes are extracted, the flavor becomes brighter, stronger, and the characteristics are more distinct and consistent.

Fines: Coffee grounds close to the fineness of flour are very prone to over-extraction or even dissolving in water, which affects the extraction flavor. The more fines there are, the more likely the coffee is to be over-extracted, resulting in a bitter and harsh taste. When making espresso, fines are the first to be flushed out by the water flow into the cup. If extracted properly, the dense crema will suspend the fines, keeping them floating on the surface of the crema, forming an attractive "tiger striping." However, "tiger striping" does not indicate the quality of the espresso’s flavor. Better-performing grinders or the use of sieves can reduce fines. But if there are too few fines, the brewed coffee may lack complexity. This balance can only be learned through practice, not explained in words.

Heat generation: Any grinder’s burrs generate heat during the grinding process. Heat accelerates the oxidation of coffee grounds, speeds up the evaporation of aromas, and leads to a weaker flavor in the brewed coffee. The heat generated by the burrs depends on factors such as burr size, material, blade structure, grind fineness, motor speed, bean hardness, and more. Therefore, major brands now equip grinders with cooling vents, cooling fans, use more heat-dissipating metals, or drive the burrs with motors that have higher torque and lower speed to reduce heat generation.

Static electricity: Generally speaking, the finer and faster the grind, the stronger the static electricity generated. Grinders with internal plastic components and no static-reducing design at the discharge port produce more static. Grinding beans with rich oils and darker roasts also generates more static. Espresso grinders often produce clumps of fines, affecting the evenness of distribution and leading to uneven extraction. Meanwhile, single-origin grinders use magnets at the discharge port to attract chaff and fines through static electricity, aiming to reduce over-extracted flavors and off-notes.