Espresso machines are expensive pieces of equipment. They can cost as little as $5000 (for a two-group machine) and as much as $20,000. This outlay is a significant portion of the budget for building out an espresso bar. The machine is also a critical part of the operation in that, if it goes down, the cafe cannot effectively do business that day.
The espresso grinder, unfortunately, is often the last piece of equipment purchased. In many cases, little start-up capital remains, so the owner of a new cafe will resort to a cheap grinder that will keep the build-out from going over budget.
Despite the huge difference in cost, though, the grinder is FAR more important than the espresso machine. In short: While the machine may cost 10x as much as a grinder, the grinder is 10x more important.
In a basic sense, the espresso machine does only three things: it heats the water, it dispenses a pre-measured amount of water, and it delivers the water under pressure. The grinder, in contrast, performs a much more complicated task: it prepares the coffee bean for proper extraction.
1. Role of the Grinder
The primary function of the grinder is to “powder” the coffee. By increasing the surface area of the coffee, this greatly facilitates the transfer of soluble material and emulsifiable oils from the ground coffee into the liquid brew.
At the microscopic level, the coffee bean is made up of different-shaped cells, each filled with a mass of sugars, proteins, chlorogenic acids, and lipids and each cell enclosed in polysacharide walls. When one properly grinds a roasted coffee bean, the sharp grinding burrs do not simply separate the cells but rather shave through the cell walls. This exposes the cell contents thereby allowing for their extraction by hot water.
As discussed in an earlier issue of this series, coffee chemistry shows us that the sweet, desirable components of coffee are extremely soluble in water. These components are completely extracted in the time in takes for an ounce of water to pass though the coffee. The undesirable components (i.e. caffeine, chlorogenic acids), in contrast, are less soluble. Only a small fraction is extracted by each ounce of water and during each second of contact.
Since the goal of extraction is to draw out all of the desirable components but few of the undesirables, the optimal grind faces two apparently contradictory requirements.
On the one hand, keeping the percolation time short will limit the extraction of undesirable components, such as caffeine and chlorogenic acids. This suggests that the grind has to be coarse to permit rapid flow of water through the ground coffee.
On the other hand, one must achieve a high concentration of solids to prepare a flavorful cup of espresso in this short period of time. That requires the grind to be fine, thereby exposing a larger surface area.
Extensive testing had demonstrated that a contact time of around 30 seconds between the hot water and ground coffee provides the best balance between the water soluble sweet components and the less desirable (and less water soluble) bitter components.
2. Uniform Grind Is Not the Goal
One common misconception is that the ideal grinder will provide a highly uniform grind (i.e. all particles are the same or a very similar size). That is just not correct.
If coffee particles are uniformly large enough to allow the water to work its way through the coffee bed in 30 seconds, one will produce a very weak espresso. The problem, in this instance, is that there is not enough surface area to extract a sufficient amount of solids and oils.
On the other hand, if the particles are uniformly small enough to provide proper extraction, the water will not pass through the coffee fast enough. The contact time would be way too long, resulting in a very bitter espresso.
The way we meet the opposing demands of comparatively fast percolation (i.e. 30 seconds) and high extraction is through a design compromise whereby the ideal grinder produces a non-uniform distribution of particle sizes. The larger particles control the flow rate of the water but contribute less to extraction. The smaller particles, in contrast, contribute much of the dissolved solid extraction but do little to control the water flow rate.
The exact shape of the particle distribution curve is still the subject of much debate among espresso scientists.
3. Grinder Types
In industrial applications, the most common type of grinder is the “ball mill.” Since ball mill grinders require the use of water for wet milling, this renders ball mills impractical for grinding coffee. This is unfortunate. Because the recycling of material is adjustable in wet milling, a ball mill could produce the optimal particle distribution for espresso extraction.
Among dry grinders, the most economical devices are “impact grinders.” These grind by having a series of blades rotate at high speeds and strike the objects in their paths. The blades apply a shock impact to the bean which then crumbles because of the compression. These “whirly bird” type of grinders (i.e. “blade grinders”) are often used in the home setting to grind coffee. But, since they produce too many fine particles and tend to crush (rather than shave) the beans, they are unsuitable for espresso grinding.
That leaves “gap grinders”- for which the average particle size depends on the spacing between a pair of cutting tools- as the preferable method for grinding espresso beans. The cutting tool may be (A) a pair of rollers with parallel axes (i.e. high production industrial grinders), (B) a pair of truncated cones rotating co-axially (i.e. high performance professional grinder or “conical” burrs), or (C) a pair of flat disks with truncated cone cavities almost touching at their bases that are also rotating co-axially (i.e. professional grinding or “flat” burrs).
Some high-end grinders combine two of the grinder designs in stages. For example, the espresso grinder may use the conical burrs to do the initial breaking of the beans and then feed the ground coffee to a second stage of flat burrs that accomplish the finishing.
4. Grinder Burrs Have Finite Lives
Regardless of which type of grinder you use, it is important to note that grinder burrs have finite lives.
When the burrs become blunt, they tend to crush the beans. Blunt burrs cleave beans along the cell walls, rather than shave through cell walls. This leaves the polysacharide walls in place and protects the coffee cell contents from coming into contact with hot water, thereby preventing proper extraction.
As a rule of thumb, flat burrs can grind roughly 500 pounds of beans, after which they will likely need to be changed. Conical burrs can typically grind up to 700 pounds before needing replacement.
Keep in mind that these are rough rules of thumb; your actual “mileage” may vary. In general, the more coffee you grind in a week, the longer the burrs will last.
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