If you took one whole coffee bean and placed it in water and subjected it to heat and agitation, at some point the coffee flavoring material would be extracted. But this method would take much too long to be practical.

Now, consider taking one whole coffee bean and splitting it in half. The water now has two new surfaces to touch. If these pieces were again cut in half, four new surfaces would be available to the water.

As we continue to reduce the size of the bean particles from whole bean to fine grind, we expose more than 4,000 times the number of particles, and 16 times the surface area per unit weight of coffee immersed in hot water.

When we break down a coffee bean we expose more of the bean’s potential via surface area. CO2 gas is release and the particles are able to absorb water. Smaller particles also mean there is a shorter distance to travel from the center of the particle to the surface, which reduces the distance and time coffee flavoring materials must travel to reach the extract.

In the case of grinding for espresso, we achieve some 500,000 particles of ground coffee (20 times finer than conventional fine grind coffee).

When selecting the grind for any brewer, it is essential to match the particle size with the brewing time. Too coarse a particle size will result in grassy, underdeveloped coffee tastes. Too fine a particle size will result in bitter, over-extracted tastes.

Factors that Influence Grinding

The individual properties of the coffee bean influences grinding results. The following are key differences you may find in coffee beans. Know these differences may enable you to make grind adjustments and achieve a desired distribution of particle size.

Moisture Content

Beans are physically softer when they’ve been cooled by water quenching. They are most brittle when air-cooled without additional moisture.

Degree of Roast

Light roast coffees are more pliable and tough. They don’t break apart as easily as dark roasted beans, which tend to be hard and brittle. Consequently, dark roasted beans lead to more fines that lighter roasts.

Brittleness

Coffee is further affected by its natural origin in terms of strength, pliability and hardness. New-crop coffees give fewer fine particles than past-crop coffees, and coffee grown and varying elevations may have different grind characteristics.

Matching Grind to Brew Method

The grind must be appropriate for the coffee brewing type. For example, if you use a coarse grind setting for a filtered drip brewer with a 3 to 4 minute brewing cycle, insufficient surfaces will be exposed to the water during the short length of the brew. This will result in a weak and tasteless coffee. On the other hand, if you use a fine grind setting in a coffee urn, which typically operates on a 6 to 8 minute brewing cycle, the coffee will be over-extracted and bitter.

As a rule, the finer the grind, the greater the surface area exposed to the water; therefore, the solubles extraction is more rapid and thorough. For this reason, finer grind settings require less time in the brew cycle.

Particle Size Distribution

During grinding, coffee beans do not shear into equally-sized particles. This is due to the irregular shape of the coffee beans and the unequal distribution of forces on the structure during grinding. Therefore, after grinding, we are left with a majority of coffee mass within a narrow range of the grind setting. This mass also contains coffee particles that are larger, called boulders, and those that are smaller, or fines.

If the particle size distribution includes too many boulders and fines, then the resulting brew will have too many overextracted and underextracted tastes. This is where quality of grinder and grinder maintenance play an important role in narrowing particle size distribution. Importantly, a good grinder will feature:

• Sharp burrs
• Longer burr grinding paths
• Less heat generation during grinding