Micro-Particle Migration in Conical Grinding Systems
Let me start with an unusual apology this time….A few years ago I announced that I was partnering with LaMarzocco to design a direct dosing, conical-burr grinder. I am very embarrassed to admit that we failed to produce any machine. The “S-Grinder” project has ground (hee-hee) to a halt. LaMarzocco and I parted company on the issue of micro-particle control in the dosing system. They took the approach of eliminating static, I favor managing the coffee powder so static cannot affect the micro-particles.
Here is some background: It is well established that grinding coffee with conical grinding burrs(or mixed-burr systems such as the DRM) produces micro-particles along with larger flakes of ground coffee, macro-particles if you will…. The heavy vanes of the conical burr compress the beans until they shatter. An “exploding” bean produces all sizes of particles to enter the fine burrs for shearing. The smaller particles, pass through the shearing and make it into the powder. Then, during extraction, some of the micro-particles actually make it into the cup, giving the espresso made from conical grounds a heavier mouth-feel and more pungent aroma/flavor characteristics. So our choice at Vivace has always been conical burrs for espresso.
However, the beneficial micro-particles present us with two broad sets of problems: controlling the flow-rate of the espresso as it oozes into your cup, and preserving the integrity of the mix of micro and macro particles through the dosing process. (In the interest of scientific rigor, I admit that all my conclusions are based on the DRM mixed burr system, but I believe the following pertains to pure conical grinding heads as well.)
When the barista hits the switch and pressurized water begins to percolate through the packed coffee, it begins to carry micro-particles with it. Towards the end of the shot these particles can form a mat across the bottom of the coffee basket and slow down the flow-rate of the espresso entering the cup. And as you know, with the high-tech, temperature stabilized espresso machines, such as the Synesso, controlling the flow-rate has become even more critical to capturing the fragrance with any fidelity. Preparing espresso with conical burrs make the control of flow rate more difficult for the barista on the bar because of the micro-particle migration within the packed coffee..
Note: The grind on the machines used for comparison was set to achieve the same flow-rate for the espresso. It is interesting just how different the particles are in each photo.
(It used to be, on the “old-tech” espresso machines where temperature wandered around a five degree range that shots from 24 to 30 seconds were all OK, never great. With precision temperature control, shots less than 23 seconds are sour-astringent, and shots over 27 seconds are hollow, almost flavorless. In our system, the best shots tend to pinch off just as the volume is reached and begin dripping between 22 and 25 seconds.. Shots that fall into the 25 to 27 second range, have dozens of flavor profiles. None are defective, hollow or sour, but within the range of acceptable there are so many different espresso coffees. Shots brewed for 27 seconds tends to be heavier on the carmelized sugars but with less varietal nuance. Shots brewing for 24 seconds feature a lighter body and perhaps more varietal nuance such as hopiness from the Brazils or blueberry from the Ethiopian. Ahh, but Vita shots that are in the bull’s eye, (25 or 26 seconds to reach our volume) taste of caramel, toast, blueberry, dark unsweetened chocolate, hops, leather and salt…each flavor competing for your attention in the aftertaste. A perfect balance of these flavors is very rare, and usually one of the varietal flavors becomes more prominent with minute differences in flow rate.
In fact, flow-rate, temperature, and pressure are a intimately connected when it comes to seducing the fragrance into a cup. If the temperature is a little low, say 201 instead of 203, and you extraction time for a shot is 25 seconds with pump pressure at nine bar, you get sour/astringent flavors from a Northern Italian roast. Similarly, if the temperature is right at 203, pressure is nine bar, but flow is fast, say 22 seconds to hit your shot volume, viola, sour astringent. And, if pump pressure is low, say seven bar, and flow rate temperature are spot on you have the same defective espresso: sour/astringent. )
Dosing the Powder
The Italian dosing hopper has within it an impeller system that forces the grounds through a small apeture into the dosing hopper. It is a series of paddles mounted on a flat disc, that spin within a circular chamber around the burrs and impact the coffee in a few milliseconds after it exits the burrs to push it towars the apeture. The effect is to grab the powder in a gentle clumping and once fixed, the distribution of micro and macro particles is preserved. (I really do not know if this is a happy accident or if Italian engineers were aware of particle migration).
However, if a grinder design is attempted to eliminate the clacking doser mechanism you must avoid “free-fall” of the powder. If the coffee is allowed to drop in a “cloud state” without being clumped, even for less than one second, the micro-particles separate and begin to migrate towards any metal surface. The resulting powder, lacking the micro-particles, makes a sickly-yellow espresso without full aroma and flavor.
To address this problem and eliminate the Italian dosing hopper you need to retain the paddle system to impell the ground coffee towards the apeture and into the doser. But that is not enough by itself. About a year ago after giving up on Marzocco I took it upon myself to modify our fleet of DRM grinders. I followed the lead of Malkoenig and Mazzer, and combined a simple polished cone with a wire mesh screen on the exit from the apeture leading to the grinding head. . I found that without the screen the coffee sprayed out of the grinder and micro particles adhered to the vertical, stainless steel wall.
(It is worth noting that micro-particles can migrate within the powder without sticking to the wall of the doser. The problem can be very subtle and requires a barista that really knows the coffee to detect when the migration problem is slight.) But with a wire grate the powder featured a very nice consistency and tumbled straight down the cone, into the porta–filter. I had to play with it a bit. At first I begin by installing a three wire grate, (two vertical and one wire horizontal in a rectangular apeture of about 1.5″w x 1″ tall) and got very cakey, hard-powder clumps. I then tried two wires and the powder became perfect. This back-pressure gate is the key to allowing the powder to tumble into the porta-filter, through a polished cone, with no particle migration or sticking to the side.. In fact you are tuning the powder for optimal density and manageability. Too much back-pressure and you get dense “caking” of the powder. Too little back-pressure and it can spray in low humidity conditions.
How Does It Work?
We have used the system for over a year with the simple two-wire gate pictured and a couple anomalies appeared. First, we learned that if the grinder burrs get dull the powder enters the hard cakey stage. Easy to fix with new burrs. But we also have a wee mystery…the grinders enter the cake stage on their own for no apparent reason. They will cake-up for 20 minutes, then resume delivering a beautiful consistent powder And this certainly bears out what I have learned in the last four years. Espresso is infinite in it’s complexity but grinding is easily the most complex part of the lovely process. And in this case, I think the machines might be accumulating micro-particles in the paddle area, due to wide manufacturing tolerances creating gaps. Then these particles break loose and clog the chute leading to the dosing cone like point release avalanches….maybe…