The Zerno Review

After the last poll, we’ve decided to post the videos here with a little additional context. As part of this, we are going to share some thoughts from the team from time to time and with the Zerno it seemed a perfect opportunity to introduce you to Šárka!

Šárka joined a few months ago and oversees all the testing that goes on when we’re reviewing products. The Zerno is a complicated thing to test, but I’ll leave more about why to her.

From me I want to add a couple of additional thoughts that will make more sense once you’ve seen the video.

• I note that the Ali Express burrs performed worse in the retention test, and that I was surprised to see burrs have an impact on retention and exchange. I do wonder if this means that having screw holes in burrs impacts this - I know coffee can collect around the screws, but maybe this coffee is exchanged? I guess we’d need to compare a blind vs non-blind version of the same burr in an exchange/retention test to see. Or it might be some aspect of burr design we haven’t considered.

• I do worry about analysis paralysis with a consumer owning a platform like this. Would you end up worrying that you’ve got the wrong combination of burr and auger in, and that in your efforts to optimise you’re actually sub-optimising?

• I don’t think I confessed this clearly enough - but I definitely had FOMO around the release of this grinder. I got messages from people saying how much I’d like it, how good it was, and I got drawn into the hype. I say this just to share that none of us are immune from hype, or this - the upside for me is that we can build some processes around it to make sure we’re making better choices about what we pick to review.

That’s enough from me - let me hand you over to Šárka…

The Zerno Z1 is kind of a grinder of my dreams (as a coffee geek)… and nightmares (as a researcher). Testing of the Z1 (with our set up) is comparable to testing 9 different grinders at once. With the exception that testing 9 different grinders is, in a way, a bit easier on the workflow. Having 3 different pre-breakers (augers), and 3 different sets of burrs is a bit of a sport to change out repeatedly. Initially, I made the mistake of working through the data collection test by test - taking each combination through one test at a time. I quickly discovered that was not the most efficient, nor the most consistent. This approach is fine when you work with a number of different grinders and you want to compare results of each test as you go, but not in this case. In the end, to make my life a little bit easier, I had to treat each combination as one grinder and go through all of the data collection steps one combination at a time. I believe it was a more reliable approach.

The possibility of customising this grinder is one of my favourite features about it; it is also the biggest pain. The problem in testing is the inability to change burr sets and augers fast enough. Add to it the calibration step and you are potentially slightly increasing your margin of error every time a burr/auger change happens. Not to mention that side-by-side coffee tasting of different burr sets and augers is pretty much impossible. I would like to point out that this is not an issue for an every day user. There is really no need to be changing out burrs or augers multiple times a day - this is just a part of testing. I would actually recommend spending some time with each combination and trying various brewing styles with one set of burrs and one auger first. Get to know a given combination well, and then move onto switching things up - it will give you a better understanding of the differences in taste, fines production, and what brewing method might fit better for some burr/auger combinations.

One of the tests we run with every grinder is a Retention exchange test. James mentions this in the video. I start with dialling in coffee to a fixed recipe, and within a target range of extraction percentage; our usual recipe for the coffee we use is 18g dose of coffee, 42g yield, brew time of 27-30 seconds, and target extraction around 20-21%. This is the control measure (which on the graph below is a 1 on the X axis) and it essentially sets the ideal extraction that we’d like to see repeated as accurately as possible. Once the control is set, the next step is to change the grind size to a coarser setting (usually as coarse as a 1 cup pour over) and grind through a dose worth of coffee beans. Then I return to the original espresso grind size and pull 3 shots of coffee one after the other. I measure the extraction after pulling each shot. Ideally, this would be done without any knocking or bellowing out stuck grounds. That is the only way to get an accurate measure of how much coffee a grinder retains (I didn’t know this the first time I did an exchange test).

The ideal outcome of this test is for the extraction percentages to be as close as possible. As far as the espresso is concerned no grind size change happened; we’re still pulling at the same setting. However, what we ended up seeing with the Aliexpress burrs was a drop in extraction right after the change from coarse grind size back to espresso grind size. This is due to some of the larger pieces of coffee essentially staying behind somewhere in the burr chamber, which then got pushed out with the next coffee dose. Larger pieces = less resistance = usually faster extraction = lower extraction. You can see that, for the most part, the following espressos are slowly returning closer to the control measure.

It is worth noting that the SSP Aliexpress knock offs were not blind burrs, and this is a trend that can be seen in this style of burrs (with screw holes). Although both the other blind SSP burr sets have some fluctuations as well, there are not as notable as the Aliexpress set, and they do not show a significant dip in extraction after grind setting change.