Density Article Part 3 - How to check for dull grinder burrs
BY CLAY BUSH
This article is the third in a series that looks at roast density and its place in the measurement of coffee. The first article focused on what density is, and how to use it to measure green and roasted coffee in the whole-bean state. The second article looked at what happens when we grind the coffee, thereby changing coffee from its whole bean “roast density” measurement to a “puck density” measurement.
These articles are designed to explain these new metrics for coffee roasting and extraction, if you wish to read back on one and two, go ahead!
Armed with the basics of roast density and puck density we can move our focus towards the burr-geometries effect on puck density. But first, we must check our burrs are sharp and within their “grind window” If your burrs are dull, this lack of cutting efficiency will be directly transferred to your puck density. But how do we check that our burrs are sharp and in their cutting prime? The answer lies in the simple task of changing the grind.
The new standard in determining espresso extraction.
The current standard when explaining how to run any given coffee is to quote the extraction resultants. Most will say something like “20 grams in, 40mls out, in 30 seconds”. Unfortunately, you are quoting the results of the shot. That is, you are measuring the puck by means “after the fact”, which may or not be the results of a channelled shot, or bad puck prep.
What we say, is “20grams at .52 puck density”.
Puck density is a precursor to espresso extraction. So if we want accuracy in coffee we must quote the precursor, (puck density) not the resultant (time). By doing this we can gain a more accurate understanding of puck dynamics.
Changing grind means changing puck density.
Understanding puck density becomes simpler once we understand our grinder. Every time we change the grind we change our puck density. This is achieved by the pitch of the thread that opens or closes the clearance between the burrs when we change the grind setting.
Image 1. Mazzer Major Burr carrier adjusting thread being measured by a pitch gauge.
While adjustment design varies from grinder to grinder, most employ the same or similar strategy. The most common method is to close or open the clearance between burrs is by use of an adjusting thread. On the Mazzer Major, we can see that the pitch of the adjusting thread is 1.5mm. This means in one full rotation of the adjuster, it will increase or decrease the burr clearance by 1.5mm. The Mazzer adjuster has 100 increments on the adjuster which means for 1 increment adjustment, Burr clearance is increased or decreased by 0.015mm. No matter what grinder is being used, if the adjustment increments are equal, then the changes in burr clearance will be as well, per step.
We make use of this thread every time we change the grind. For instance, when we make the screw thread open up, the burrs move further apart and the resulting particles get courser and increase in size. With this increase of particle size, comes a greater air space between particles which causes extraction flow rates to increase. This greater airspace between particles has no weight, so puck density lowers. Conversely, when we make the grind finer, particle size decreases along with the airspace between particles, causing puck density to increase.
So when you change the grind you are changing the flow rate. But it’s vital to understand that the change in flow rate is a result of changing puck density.
(Image 2. This is the same burr carrier from the photo in image 1, reinstalled on the Grinder.
When adjusting the grind, the phrase, “changing the grind by a notch courser or finer” is often used. To illustrate the effect of this phrase on puck density we conducted a basic test and charted the result.
With our Mazzer major dialled in at a point we considered to be “on” we opened the grind by 1 notch and checked the puck density (1 notch course), closed it one notch and checked puck density (grind on) and closed it one more and checked puck density (1 notch fine). The results are as follows:
Grind Setting / Density Result
1 Notch finer - 0.56
Grind “On” - 0.544
1 Notch Coarser - 0.528
(graph 1. 3 grind samples were taken which represent the three notch settings in the above grinding plate. Because the pitch plate was moved consistently, one notch at a time, the resulting puck density will also move consistently.)
Using puck density to check for Burr consistency
It’s commonly understood that new burrs need a breaking-in process until they are ready to use. After the burrs have endured this break-in phase, they will go through a window of ideal use, and then slowly decline until they become dull and the grind quality is affected. But how do you determine this window of burr acceptability?
This basic change in the grind can be used to check our burr consistency. (read puck density for more on this)
Three-point test, the new standard for checking burr sharpness
A three-point test is a term we use to describe the distance between three grind changes. You can see that in graph 1 above, three different changes in grind settings will herald three different puck densities (Fine, Grind-on & Course). The important thing to look for here is that the distance in densities is consistent with the changes in the screw thread. If these three changes are consistent, and they reflect a constant density shift, it’s a sign your burrs are in a functioning state. It’s that simple.
When sharp burrs grind the coffee, this grinding will result in a consistent distribution and therefore a consistent density. When burrs are blunt, they don’t slice the coffee consistently, instead, they roll and “tear” the coffee fibre resulting in inconsistent particle distribution, and therefore puck density becomes inconsistent. So if the burrs were blunt in the above sample, what would the resulting puck density look like.
Graph 2. Our good cutter line (blue) is the results taken directly from the graph above. We can see that when the densities are graphed they are a straight line. (the blue line is actually straight. :-0) This means a consistent change in grind results in a consistent change in density. The second, red example (worn cutters) is a set of Mazzer major blades heading to the end of their grinding life. Even though the grind was changed consistently the three densities are not aligned, so in this case, the burrs are not cutting correctly and need to be replaced. It is worth noting that in comparison, worn cutters produce more fines, so that also results in a higher puck density.
Generally speaking, most cutters will provide a three-point check at some stage in their life. I haven’t found any burrs that don’t, but we suspect somewhere there will be poorly designed aftermarket cutters that will never hold a three-point check.
The life of cutters is a topic outside the scope of this article. So for the sake of this article, we shall simply state that cutters generally DO NOT hold a three-point check on installation. As they wear in they then come into their prime cutting state where a three-point check is observed. Eventually, these cutters will deteriorate and fall out of their 3 point check. At this time the cutters are either disposed of or re-sharpened. The initial “unstable” phase, followed by the consistency phase varies greatly through different cutters and cutter coatings. Each manufacture should be tested on its own merit.
How to determine a three-point test.
When providing any testing on burrs, you should first determine a three-point test. This test should be performed within the grind window of the coffee of choice.
To do this, simply dial in your grind to the best of your ability. This grind setting becomes your middle point. Then change one notch courser take a sample, make sure your blades are moving when you make grind changes. Now change back to your original setting, which is back to your middle point (your original grind on point) and take your second sample. Finally move another notch finer, to take your last sample. It is important to take your samples moving consistently finer, starting from the coarsest setting. It is also important to dump the first 2 samples after an adjustment to ensure you are testing your new grind-setting and not a mix of old and new.
If this test is inconsistent any results generated from these cutters will also be inconsistent. As a result, we test all new burrs doing the 3-point test first, should they fail this test, we grind more coffee through them until they complete a three-point test. For some grinders, this process is virtually instant. Gorilla Gear black cutters are the only cutters that I have seen that are ready out of the box. Even so, I still grind a kg through just in case. These are the exception, for others, it may be 10-15kg or even more.
Once we have determined that the burrs are within their working prime, further testing can be performed to see the “working range” of the burrs. On most grinders, this is not a necessary step as we only move our grind a few notches. But for the sake of a greater understanding, we shall look at burr geometry windows.
Working range of burr geometry.
The three-point check mentioned above is for checking if your burrs are in good working order within the range they are expected to perform in. For most grinders, this is enough. But some grinders like the Mahlkonig EK 43 are used for multiple brewing functions. The three-point check that results in a series of perfect density points, can also be extended to a four, five, six, or even a 20 point check. But sooner or later at both ends of this line, the density consistency will waver and the line will break down even if your burrs are in good working order. For example, we can grind coarser and coarser, with the blades moving consistently further apart, all the while taking density checks, in doing so the density moves consistently lower. However eventually, even though you move the grinder at consistent movements. (one notch at a time) this movement will record an inconsistent change in density. This happens because the burrs have become so far apart from one another that the burr geometry can no longer mill the coffee consistency. At this point, your burrs have reached their outer “course limit”. The same is true at the other end of our grind window. We can make the grind finer and finer but sooner or later the grind will be so fine that the burrs are outside of their effective geometry.
The breakdown of this burr geometry at the further most coarse, and fine points highlight the outer working edges of this burr geometry window.
Whilst every burr design has a “burr geometry window” this window is determined by the consistency in milling of a particular coffee over that range. It is worth noting at this point that a change in roast density could change the grind window of your burrs. As the Roast density changes, so does the fracture rate of the coffee, so the burr geometry window may change as a result. This is why we state that Roast density is the highest principle in coffee. And the grinder should be chosen based upon the roast density of the coffee you are using. But we’re getting ahead of ourselves. More on that in later articles.
The EK burr geometry window.
Our next article, Puck density and single-serve coffee, will feature the use of the EK for single-serve coffee, and what effects single-serve grinding has on puck density. This together with the fact it’s very easy to read the front dial of an EK makes it an easy way to conceive a burr geometry window. So we thought we would show the Burr geometry window of some standard EK burrs.
To date, I haven’t seen any burr/roast density configurations that remain consistent for the entire scope of the EK window. Generally, they fall short by a few numbers on the dial. Because of changing roast densities, it’s almost impossible to find a set of burrs that would hold a consistent window for all coffees. However, if the coffee industry accepts roast and puck density as an industry standard, it is conceivable that burr manufacturers stamp and calibrate their cutters for a preferred roast density. This would be a leap towards a standard measuring system in the industry, which is sorely needed.
However, for now, EK Burrs fall into two main categories.
A Burr set designed to cover the entire combined range of espresso and filter, or;
A Burr set to cover a specific purpose like espresso or filter only.
Turkish burrs for instance are engineered to be consistent at the fine range, so they can lack consistency in the course range. Some burr manufactures have chosen to tackle the entire range in one set of cutters, though this is usually unsuccessful.
Below is the working grind window of Ek factory standard burrs. This serves to highlight why they have chosen two sets of burrs to cover the range.
Graph 3. The grind window of an Ek43 Standard factory Burrs. Between notches #4 through #14 The ek holds a good consistent density, (grind window). However, under setting #4 and over #14 the grind window falls out. It means the ideal range of these burrs is between grind size # 4 and #14. This is not a slight on the factory ek burrs. In fact I imagine Malkohig are fully aware of this which is why they also have a Turkish Burr set. Turkish Burrs have much greater consistency with this lower range. They are designed for finer grinding.
These burrs will breed inconsistencies lower than a number 4, at its fine range, and above a number 14, at its course range. That means that EK standard burrs are easily used for filter coffee, but sit within an inconsistent range for espresso. It’s not to say we can’t use these burrs for espresso, but we should keep in the back of our mind that, regardless of flavor, it may be hard to keep consistent grinding. By analyzing burrs by their puck densities we can determine their grind windows for specific roast densities. In doing so we are using physics to determine consistent grinding practices.
Unaligned burrs.
Seasoned professionals, who have come up against the inconsistencies of EK alignment, will be asking if the burrs that determined the above grind window were aligned. The answer is yes. But they were not out by too much in the first place with only one small shim needed. In my history of EKs I would put this grinder in the top 20% of EK alignment scorecard. (not that such a scorecard exists), but i’m trying to basically highlight that out of the box, this as a better than average EK.
Below we have used puck density to highlight the before and after effect of alignment.
Graph #4. This graph has the grind dial number on the horizontal axis and the respective puck density for that number on the vertical axis. Blue is aligned, and red is unaligned. It is easy to see that alignment helped the Grinder perform better through the range of 1 to 6, however still failed to create a three-point check in the realm of espresso.
An unaligned EK needs a finer grind setting because basically, they cut more boulders. By aligning an EK, you are making the grinder, grind more consistently with fewer boulders and fewer fines, which raises the puck density. Basically, aligned burrs provide a more consistent cut and higher density at a finer grind. To make it clearer to understand, let’s put it in a context you may understand. Let’s assume this shot runs perfectly at a .48 puck density. That means on unaligned Burrs, your grind setting would be #1. On aligned cutters, however, the same grind (puck density) would be achieved at a grind setting of #3.1
Alignment does make a difference. However, it can’t save your grind window. The standard EK cutters won’t produce consistent results in the realm of espresso because it is out of the designed geometry window. (At least for this coffee’s particular roast density.)
We hope this Editorial has firmed your understanding of puck density in relation to determining Burr life, grind windows, and burr alignment. The physics of Puck Density can be used to determine all three. Now that we have a systematic way of determining that burrs are sharp and that our grinder is within the working range of its burr geometry. The last step before moving onto the study of burr geometry is the study of grinding practice. For that we shall look at the practice of single-serve grinding in the EK in our next article.
See you then.
Cheers
Bush & Bush Coffee Systems.
