On Static
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By MICHAEL TEAHAN
Coffee, coffee, everywhere
I’ve been working the last few years on grinders. I used to poke fun at the idea of 10’s of thousands of Dyson Vacuum Cleaner prototypes—how many could you possibly need? After the first few dozen of my own projects I realize Dyson was right. I have a grinder project that does some really cool things I’ve discovered that the greatest challenge is one common to all grinders.
Static Electricity. Where does it come from and what kinds of approaches have been employed to deal with it?
Where it starts
Static electricity comes from a couple of sources. When two dissimilarity charged materials come in contact with one another, electrons tend to migrate from one to the other. This is why dragging your feet along the carpet causes a buildup of in your body and results in your getting zapped when you reach for something slightly grounded. The carpet is kind of an insulator from the dissipation of the charge (the zap) but still allows for the gradual transfer of electrons.
Another source is the coffee bean itself—sort of. When the bean is crushed, cut and torn apart, the stable bonds of electric charge in the been are broken and the coffee granules are negatively charged, loaded up and looking to cling to any grounded/positive charged surface. Even though electricity moves pretty damn fast, the charge has a lot of hurdles to clear before coffee settles down.
It is so much easier to control what happens in an espresso machine than a grinder, rather like trying to manage what’s going on inside a tornado.
Fixes, Compression
Static doesn’t always appear to be a problem. Doserless espresso grinders seem to handle things well. There a few ways to mitigate the problem of static charge. In espresso grinders it centers on compression. Compressing the grounds together has the effect of dissipating the static charge; it’s kind of like breaking it to bits then putting it back together again. The problem with compression is that it tends to lead to clumping of the grounds.
Let’s break down how doserless grinders “reassemble” the grounds and neatly unpack them again:
The geometry of the exit port of the grind chamber can be used to change the deflection of the grounds, forcing them to collide with each other in a before ejecting them.
The sweepers not only clear the coffee but also compresses the grounds against the wall of the chamber, with the angle of attack, frequency of blades and aggressiveness of the cut manipulating the compression.
For the sweepers to better compress the grounds, restricting the exit port increases retention so the sweepers have more ground coffee to push together. The restriction doesn’t have to be much. Some grinders use a mesh to allow the coffee to build up slightly (slow it down) which both compresses and breaks up the clumps as the coffee comes out. Other manufactures use a silicone gasket that is flexible and slotted to accomplish similar results. The shape of the slot seems to affect how effectively the grounds are dispensed.
This slight resistance has another advantage for doserless grinders. Using the space between the sweepers and having them retain grounds provides a predictable volume of coffee when using time to dispense doses. It also serves to blend the grounds together to aid distribution. If the coffee was just flying out of the blades, heavier particles would bias to one side of the portafilter basket then lead to uneven extraction. Early grinders tended to compress a bit less. The funnels didn’t provide a lot of blending. The uneven extraction was evident with a naked portafilter.
Old school doser grinders—the ones with levers and dispensers—didn’t really have a static problem. The ground coffee not only compressed against itself in the chamber but it also had the time to dissipate the static charge.
Sometimes you can see the effect of compression on static buildup with old doser grinders. Some grinders were terribly at compression. If you pulled the lid and ran the grinder, ground coffee flew all over the place and stuck to everything. The sweeper in the doser had to clear the walls and with time it would settle down. If you put a small spoon in front of the opening to allow the coffee to build up on the ejection port, it would begin to flow together and drop neatly into the doser. Hold it too close and it would jam—hence the importance of balancing the restriction with the flow.
Compression works but has a downside. It needs ground coffee retention to be effective. In an industry obsessed with absolutes, God forbid ground coffee sit for 60 seconds before its used to pull the next shot (its fine, BTW).
The other end of the Spectrum
Bulk grinders like Dittings or an EK43 need to clear everything between batches with zero retention, either because they are using different varietals or for different brewing methods. To accomplish this, they can’t have any real restriction in the outlet port. Since coffee is usually dosed prior to grinding, they run free and clear at the end of the cycle. The Ditting in our shop has so many fines flying around that it could shut down a city block of computer clean rooms. To be fair I probably need to renew the burrs, but none of them are good.
Large industrial grinders have blending chambers attached to the exit ports that both insure good distribution and compress the grounds to disperse static charge. The alternative is to let the ground coffee sit and dissipate the charge before packaging.
Dealing with Static without Compression
The ideal grinder would have zero retention and no static electricity issues. The mechanisms and technology to address this solution, however, is imperfect.
Dissipating static charge would ideal, but how to do it? Providing a good grounding opportunity for the coffee particles seems reasonable, but dissipation takes time. It isn’t instantaneous. You also must make sure to get to every particle of coffee uniformly in real time. When grinding espresso to order, it’s a tall order. The other problem is that when you have materials that convey a static charge , the coffee particles are drawn instantly to it, they don’t just fall away with the pull of gravity.
You could use materials that don’t convey static charge at all in the hope that the coffee particles won’t try to stick to it. While this sounds reasonable, there are two issues: the first is that even static inhibiting materials don’t convey the charge, a charge still builds on the surface of the material. It only works for a short while and then becomes ineffective. The second issue is that when you have all of these charged particles and nothing they stick to, as soon as find something they go everywhere. It’s like powder coating your portafilter.
Static dissipative materials are a good compromise. They don’t provide enough of a ground pathway to attract the coffee grounds, at least excessively, but allow the charge that does buildup on the surface to dissipate and provide a neutral surface for subsequent doses. The rate of dissipation needs to match the duty cycle of the grinder, making high volume uses more challenging.
Another approach I have seen from a manufacturer was to blast the coffee against a Teflon coated wall perpendicular to the exit and then using vibrators to knock loose any grounds that didn’t fall.
What’s the point?
This is about understanding how things work, or don’t. You can just throw away the silicone restriction gasket in Mahlkhonig Peak or forget to put the anti-static diffuser back in the Mazzer—even if it really isn’t the electronic miracle of science you might think. Grinders are better than ever, but each has a purpose. Doserless grinders are ideal for espresso while an EK43 might not be ideal, even if they have a devoted espresso following.
Lastly: speed matters. Slow grinders generate less static electricity. Conical grinders spin about a third as fast as flat burr grinders. Another reason why conical grinders rule.