Know Your Grinder
BY CLAY BUSH
When talking grinders, the focus immediately falls to particle distribution. In fact that tends to be where it starts and stops. Particle distribution and its importance, or not, to espresso, is beyond the scope of this article, and we shall address it in the future.
However, there is much more to know about grinders beyond their particle distribution curves, and some of these elements are more important than particle distribution. So the aim of this article is to give you a broader understanding of these critical elements. Hopefully, when you buy or test your next grinder, you will have a set of principles to apply in order to determine what grinder to purchase. In this way you won’t need to purchase blindly, listen to marketing spin, and world champion baristas. Grinders are designed through engineering principles and they act as a collective whole.
DUTY CYCLE
The first and most important aspect you need to know about your grinder is its duty cycle. The duty cycle is the work load that your grinder is designed to take. In most cases, Engineers have designed all of the components within its drive train to withstand both the mechanical loadings from the torque required, and simultaneously dissipate the heat generated from the application of that torque. The duty cycle is the timeframe that the user must adhere to for this engineering to be realized. Exceed this duty cycle, and not only can your coffee suffer through excessive heat, but in extreme cases, it can damage your grinder.
Figure 1.
A typical duty cycle sticker, usually found on the side of grinders. In the case of this Mazzer Major it is meant to grind for 20 seconds, before having a 30 second break. Because grinding on a Mazzer major takes about 8 seconds to grind, which means you would only need a rest of 12 seconds before grinding again. This 12 seconds would be soaked up by tamping, grooming, shot activation etc.
Understanding these duty cycles is paramount to understanding your grinder. When you do you can use these duty cycles to select a grinder appropriate to your café’s requirements. Unfortunately, there are no hard and fast rules here. Each manufacturer has different duty cycles and all of them are a reflection of the engineering that has been applied to their design. It is important to remember though that this duty cycle is to be considered in the context of the manufacturers recommended burrs. The Burr design is responsible for the grinding rate, ie How much Coffee is ground per sec. Some smaller grinders can seemingly grind for longer than larger grinders based on duty cycle alone, but these grinders are also grinding less volume of coffee per second. Understanding duty cycles with reference to the Kg/Week Café Figure can sometimes be tricky. Knowing the grind time per shot can help to clarify this.
AFTER MARKET BURRS
Lately we have been buying up burrs from across the land for testing of a calibration tool we have been working on.
by the way if you have some burrs that are not standard design we would love to hear from you. Please contact us!
When changing burr configuration the manufacturer must take into account the specifications of the original grinder. There seems to be a trend of designing burrs to get a pre-determined grind speed or particle distribution that lay outside the original engineered design of the grinder. This is a very grave trend. For example, we recently purchased some custom design burrs for the Mazzer Major grinder shown below in figure 2.
The original Mazzer Major burrs grind 20 grams of coffee (medium roast) in 7.7 seconds for a specific grind. The custom burrs ground the same quantity at the same grind in 3.2 seconds, over 100% faster! One might jump to the conclusion that this is advantageous, but after careful consideration, one will find that this is a poor decision. Grinding faster generates more heat per revolution and requires more torque per revolution. While the motor tries to maintain its set speed, linked to the 50Hz supply, it is expected to dissipate the extra heat. Generally speaking, the grinder can only dissipate heat while it is moving (internal air from motor revolutions) but because the grinding rate has increased we now have fewer revolutions and more heat. In Summary:
1. Increased loading leads to premature failure of components.
2. Increasing residual heat at peak times result in poor grind quality.
3. Faster grinding increases dosing inaccuracies.
Not all after-market cutters are evil. Some have designed new geometries but remain within the original engineered specifications for the grinder. Check them first and don’t believe what the manufacturers say. Try to understand if the claims made keep the machine within its original design specifications and build a test procedure. If they don’t match up. Don’t buy them. And please stop using Instagram and blogs to build your understanding of grinders. These are marketing forums.
Figure 2.
2 types of Mazzer Major cutters. Originals on top, custom underneath. The custom burrs can offer new exciting opportunities in grind and therefore brewing. But burr capabilities must remain within the grinder’s duty cycle. In this case the custom cutters pictured, ground coffee 100% faster, but when faced with high density coffee, they stalled the grinder and the motor could not grind, and throw the weight of the coffee past the static guard. Higher density light roasted coffee, make grinders work harder for the same result.
STATIC GUARDS
Static Guards are used on electronic grinders. (Dosser less grinders) These grinders are usually just referred to as “e grinders”. Because these “e grinders” don’t have dossers, they employ static guards to stem the flow of coffee. In doing so they can back up and fill the entire grinding chamber with coffee. This prevents air from being present and therefore prevent static. Some static guards are fixed and others move like fingers. Those that move are generally more accurate in their dosing however being made of thin metal, they move freely while grinding but can bend beyond their limit when they get overly hot. Grinders that have these “finger” static guards MUST adhere to stated duty cycles. If they bend they will cause dosing problems.
Figure 3.
Two static guard designs. The guard on the right is the “finger” type that moves, found on fiorenzato grinders. The left is a Mazzer Major Static guard that has had the centre wire cut out, to make the coffee come through faster . This can however make the dosing inaccurate.
BURR TYPE
Whilst there are several burr designs, there are only two we are concerned with. Flat blade and Conical. Flat blade burrs can be mounted both Horizontal and Vertical, and are less prone to inaccuracies from shaft vibration or alignment because the cutting surface is perpendicular to the shaft axis. This problem is more evident in conical grinders where the cutting surface is semi-parallel with the axis and as such errors will become exaggerated.
The bonus with conical burrs is they are significantly heavier in their construction and serve to dissipate heat more readily than flat blades. They can also have a greater surface area to grind coffee. As a general rule most will use flat blade cutters in cafes that use small to medium quantities of coffee. Say up to 50kg per week. Conical is used in larger accounts above 50kg. Their faster grinding and lower heat retention are the benefits of these cutters but should be maintained more stringently.
There has been a growing trend in grinders to use flat blade cutters and turn them vertical like in old deli grinder designs like the Malkohg Ek and Climax Deli grinders. Personally, I don’t believe Ek grinders are good for espresso because of their lack of fines. (for an understanding of how fines affect puck temperature see article Fines migration its effects on temperature in the coffee puck here. ) But regardless, these grinders have another feature that is worth mentioning. Because the coffee in these grinders passes down the throat and then turns horizontal. These grinders employ pre feeders (I call them worms) in order to ensure the coffee feeds into the grinders at a certain pace and also prevent “Arching”. Arching is a phenomenon known in media transfer where a product that is highly regular can randomly generate self-sustaining structural arches which will create 2 problems. An Arch will create a flow blockage and also it will alleviate weight to the cutters. These feeders will continually disrupt the beans from forming an Arch but will also regulate the flow of beans and thus the weight transfer to the cutters. These “worms” are absent in horizontal flat blade grinders and are not needed because it is more difficult for an Arch to form in a single downward flow. Gravity also assists to regulate the beans through the throat of the grinder and provides consistent weight onto the cutters. On conical grinders they will sometimes have a pre breaker on top of the burr and as the name suggests, will break beans into smaller pieces which helps regulate bean transition through the cutters and reduces “chipping”
Figure 4a.
A pre breaker on a mazzer robur grinder that serves to regulate the feed of the beans into the burrs. Controlled weight, or feed rate into burrs is paramount for even grinding.
Figure 4b.
A pre-feeder of an Ek. This worm is designed to regulate the coffee beans being fed into burrs and prevent arching. On an Ek it is important because the beans are fed horizontally into the burrs and don’t have the benefit of gravity fed coffee beans.
POPCORNING COFFEE
Something that has become a trend in the specialty industry is single serve coffee grinding. Grinders are simply not designed for this style of grinding. Not Ek’s or otherwise. Single serve grinding is the worst way to grind coffee. Without the pressure on the blades from beans in the hopper, the coffee is allowed to bounce around and as a result will make a rough and uneven particle size. Unfortunately, most people again are not familiar with the working parts of their grinders, if they we’re they would come to understand the basics of grinder design and how pre-feeders (worms) like the ones above, are used specifically to create a metered flow of beans and create consistent pressures onto blades. Understanding this should immediately make you question single serve grinding. Extensive testing in Popcorning coffee will no doubt confirm it.
As we anticipate a fair amount of conjecture on this topic, we have planned an entire upcoming article to explain in full detail, with test results to highlight why single serve coffee should be avoided at all cost. Yes… even with EK’s (come back for that one). But in the meantime, we shall continue with examples of how manufacturers have tried to eliminate this popcorning affect.
BEAN HOPPER SHAPE vs CUTTER WEIGHT
The role of hoppers is to not only store coffee ready for grinding but also provide a consistent bean weight on the blades. This constant weight ensures even cutting. At some point as the coffee quantity in the hopper declines the weight on the burrs will decrease and the quality of the cut will be jeopardized. Understanding the weakness of your hopper can be very beneficial. Fortunately, it can be very easy to determine where this weakness occurs in your hopper. The assumption that your hopper holds 1kg, so therefore the weight on the burrs is also 1kg is an easy one to make. It is however incorrect. There exists a dynamic within the hopper where the beans become, to a partial extent, self-supporting. In order to observe this you will require a hopper test rig, like the one show below. While they can’t be seen, there is a series of different size holes and we use this rig to test all hoppers we come across.
Figure 5.
Hopper Test Rig. Hopper is clear of scale by 2mm
To make this test rig, simply suspend the weight of the hopper 2 mm over the scale, ensure it’s not touching the scale. Fill the hopper with beans in 50g increments, recording the displayed weight on the scale as you go. You will observe that while the first few doses are transmitted directly to the scale, the subsequent doses gradually contribute less and less to the scale, until a point where no difference is made. In real terms, this means that there is a minimum volume, above which, bean weight will not be a contributing variable to grind quality. All hoppers will record a weight less than the amount of beans in the hopper. They do this generally by forcing pressure inwards towards the center of the hopper. Much like the forces in a tapered espresso basket. For more clarification on the issue of inward pressure with tapers, check out the article Impacts of basket (espresso portafilter) shape on extraction here.
We have tested some hoppers and displayed them in the following graph for you to see. There are many more hoppers that are not on this graph. But these three will give you an idea of how this system works
Figure 6.
Grind hopper load vrs Burr weight
On the bottom is the weight on the scale recorded in 50-gram increments. All the way up to 1kg weight (1000grams). On the left is the actual weight recorded on the scale. In a grinder application, this is the weight your cutters are receiving. As stated earlier we will do an entire article on the negativities (unfortunately there are no positives.) on single serve grinding. But amongst other things the article will highlight that there is a direct correlation between the changing weight on the burrs and changing quality of the particle size. I can tell you that ironically the drastic changes in quality happen at less than 150grams hopper weight and if you refer to the graph above, you will see that this is when the weight on the cutters decreases dramatically. Of the three hoppers tested above, the best is the Old school Gino Rossi Hopper off of a conical grinder. This hopper, for ideal results should be kept at a bean weight of between 750 grams and 1kg. In doing so consistent bean weight is applied to the burrs. While it is impractical in a Café setting to try and maintain such levels, it is worth noting where dramatic effects occur and maintain levels above that point.
Figure 7.
The Bush and Bush, Test rig. Markings are placed on the grinder hopper of a “Gino Rossi conical hopper” to indicate when the hopper empties. This then tells us when we must top up beans so as not to change the weight on the Burrs and therefore a difference in grinding. We use this hopper on all of our grinders when testing burrs. And we make sure that the bean level is between 1000gram and 750gram lines at all times. In this way we can have confidence that bean/burr weight is not affecting the particle size and distribution of the resultant ground coffee.
The hopper above has very in-depth markings, and that’s because it is a test rig. Even so, you can employ this on your own hopper, at the very least mark the spot where the burr weigh falls off dramatically. Most hoppers (except the Gino Rossi pictured above) have this “cliff” In figure 6 example above both the ek and the super jolly will record problems with hopper weight and therefore grind particle inconsistency at a weight 150grams and less. Therefore, fill your grinder (make sure the throat of your grinder is full first), to 150 grams and mark a line around it. Now you know that, if the beans in the hopper fall below this line, you may encounter inconsistency’s in brewing, ie your grind will may fly out.)
IN CLOSING
There is much to know about grinders other than their particle distribution curve. By following the above guidelines, you will come to understand your grinder better and therefore get more out of it.
Just to Recap:
- Know your duty cycle.
- If your looking for alternative cutters to put in your grinder, besides checking them for practical consistency, make sure they don’t exceed this duty cycle.
- Match your grinders duty cycle to the requirements of the café.
- Understand the weakness of your hopper. And when you do so, mark a line on it to determine when your grind inconsistencies start. (you may be able to determine this by making coffee without needing to make a test rig like ours.)
- If its an e grinder, you will have a static guard. Determine what static guard you have, as some need more care than others.
And if anyone out there has newly designed cutter geometry. We’d love to hear from you. Please contact us! :)
Cheers,
Bush and Bush Coffee Systems