Grinding for Espresso
Grinding is the fracturing of coffee bean particle cells. Its purpose is to increase the amount of coffee solids exposed to the extracting liquid.
Mahlkönig Launches Peak Grinder EK43 Barista and Grind By from Espresso Grinder, source:dailycoffeenews.com
Why Espresso Requires a Very Fine Grind
Quality espresso requires an exceptionally fine grind for numerous reasons.
- It creates particles with extremely high specific surface area, a prerequisite for rapid washing of large quantities of solids from the particle surfaces.
- It opens more particle cells, which allows more large molecular solubles and colloidal material to be transferred to the extracting liquid.
- It accelerates wetting (and diffusion, if it in fact occurs) by providing a shorter average path for water entering cells and solubles diffusing out of cells.’
- The greater specific surface area of smaller particles, along with their ability to pack together more tightly, provide the hydraulic resistance necessary for proper flow rate through the coffee bed.
I recommend you invest in the best grinder you can afford, even if it means you have to buy a cheaper espresso machine. A mediocre grinder under heavy use can damage flavor with excessive heat and can prevent even extraction by producing clumps, too many fines, or poor distribution of grounds in the basket. No espresso machine, no matter how impressive, can (yet) compensate for the problems created by poor grind quality.
The single most important feature a grinder can have is sharp burrs. This can not be overemphasized. Sharp burrs create less strain on a grinder’s motor, generate less heat, produce fewer fines, and offer better particle size distribution.
How to Evaluate a Grinder
A home barista who rarely pulls more than two or three shots in an hour will prob ably not notice much difference in the performance of various professional-quality grinders. A home barista also has the luxury of using a time-consuming method such as the Weiss Distribution Technique to compensate for poor grinder perfor mance. Therefore a home barista can achieve con sistently excellent results with any professional grinder of reasonable quality.
On the other hand, a barista who works in a cafe and frequently pulls several shots in quick succession needs to be more careful when choosing a grinder. A professional barista needs a grinder that facilitates even distribution and does not overheat the grounds when under heavy use.
The following are some important criteria for evaluating a grinder.
Minimal heating of the grounds.
Some heating of the grounds is inevitable during grinding due to friction and the breaking of molecular bonds, but additional heating of the grounds due to contact with very hot grinder surfaces is undesir able. Such heating can damage coffee flavor and accelerate the loss of aromatics. It can also cause oil to bleed to particle surfaces, creating sticky clumps of grounds,* which leads to erratic percolation.9 Clumps resist wetting and can cause large sec tions of the coffee bed to remain dry throughout percolation.
A well-designed grinder should have no small, enclosed spaces that trap and build up heat during heavy use. Sharper burrs, lower rotation speeds, and larger “functional” burr surfaces also mitigate the heating of the grounds during grind ing. I refer to functional burr surfaces because in some grinders much of the burr surface is useless, due to the burrs being too far apart to crush the beans. The larger the functional surface, the better the heat dispersion during grinding.
Appropriate particle size distribution.
Commercial espresso grinders are designed to yield bimodal (or trimodal) particle size distribution. This means the greatest frequency of particle sizes is clustered around two (or three) particular values. In such a distribution the coarser particles serve to allow proper flow, while the finer particles provide the large amount of specific surface area necessary for rapid extract ion.9 As mentioned previously, sharp burrs are necessary to optimize particle size distribution; dull burrs create a more uniform distribution of sizes.
A grinder must be able to dispense grounds without clumps. To test your grinder, dose a couple of shots’ worth of grounds onto a piece of paper and search the pile for clumps. If there are any, clean the burrs as well as the pas sageway between the burrs and the dosing chamber, and replace the burrs if they are worn. If the grinder still creates clumps, try the Weiss Distribution Technique.
Clumping is caused by excessive heat generation during grinding, a grinder de sign that forces the grounds to squeeze through a small passageway between the burrs and dosing chamber, or by particles with a lot of surface oils due to aging or dark roasting.
Ease of uniform distribution.
Many baristi have come up with ingenious ways to improve distribution while dosing, but a good grinder should not be so depen dent on a barista’s skill to achieve uniform distribution.
Some dosing mechanisms facilitate uniform distribution, while others make good distribution so difficult that even the most skilled barista has difficulty achiev ing it. Good distribution is easiest with grinders that drop grounds vertically as opposed to diagonally into the portafilter, dispense “fluffier” grounds, or have a homogenization (blending) mechanism.
Grinding Systems: Pregrinding Versus Grinding To Order
Most commercial grinders are designed to pregrind, with the dosing chamber kept full of grounds so the barista simply needs to pull the lever one or two times to dose the required amount of grounds. This system is very fast and convenient, but it has two significant flaws: first, the weight of each dose is affected by how much ground coffee is in the dosing chamber, and that amount constantly varies. Second, the ebb and flow of business causes the grounds to spend a variable amount of time degas sing after grinding and before infusion.
Degassing is the gradual release of gases, primarily CO2 plus some volatile aro matics, produced during roasting.* Once coffee is ground, degassing dramatically accelerates.
The amount of CO2 in the grounds is important because it influences flow rate during percolation. When hot water contacts the grounds, they vigorously release CO2 ,* which repels the surrounding liquid and increases flow resistance, slowing the flow rate.
The pregrinding system results in inconsistent flow rates because shots are made from grounds containing variable amounts of CO2 • Inconsistent flow rates in turn cause flavor, body, and brew strength to vary.
Grinding to order is superior to pregrinding. Grinding coffee freshly for each shot preserves more aromatics and produces more consistent flow rates because shots are made from grounds with a consistent amount of CO2 • The only disadvantage of grinding to order is that it requires more time and attention to make each shot.
Adjusting the Grind
During the normal course of business, the most important factors affecting flow rate from shot to shot are the grind and dose. Varying the dose by only 1 gram can alter the flow rate for a given shot volume by several seconds.
Therefore, a barista should not adjust the grind in response to just one shot with poor flow rate if there is a chance the dose was not identical to that of previous shots. On the other hand, whenever the flow rate has trended faster or slower over the course of several shots, the barista should feel confident the grind needs adjusting .
To achieve consistent dosing a barista should:
- Practice the same exact dosing, distribution, and grooming (leveling and refining of the distribution) techniques every shot.
- Practice until he or she can consistently create a coffee bed with a variation in mass of only about0 .5 gram.
- Periodically test his or her consistency by weighing a few doses during a busy period.
It is best to adjust the grind in small increments. If your grinder has a small tunnel between the burrs and the dosing chamber, any new grind setting should not be evaluated until the first 5 grams or so of grounds have been used or discarded . This eliminates any effect caused by “old” grounds that had been stuck in the tunnel or scattered around the dosing chamber.