Curiosity got me and I finally tried to use my bulk grinder (a Mahlkönig Kenia with 65 mm flat burrs, which was my dedicated brew grinder to date) to grind for espresso.

The first try was not really promising. 13.5 grams in, 28 grams out in about 4s (preinfusion) + 18s on my Europiccola. I only got to 18s because I was very gentle with the lever. If I had pressed as hard as I would normally, the shot would probably not have taken any longer than 10 - 12s. Even though that I had just produced a sink shot I tried it. To my surprise it tasted quite good and not underextracted at all!

I tried a second time with the Kenia set to the finest possible (without the burrs touching) grind setting. The shot barely took any longer than the first one. 20s with a slightly harder pull. 13,5 grams in, 28,9 grams out. This time I took the refractometer (an Atago Pal Coffee) out and measured the shots strength. The calculated EY was 19,22%. This was in line with the taste of this shot, which, for the first time for this coffee, tasted exactly like advertised. Delicious!

The big surprise for me is, that I can't even extract 19% from this same coffee using my espresso grinder (Kinu M47, hand grinder with 47 mm conical burrs). No matter how fine I grind. I just did a third shot, this time with the Kinu. Same brew ratio as the second Kenia shot: 13,5 grams in, 29 grams out. This time I had to preinfuse for 35 s and the (really hard) pull took 35s. EY: 18.86%

What really makes me think is, why is extraction from the Kenia grist so much easier? Does this have to do with burr shape? Might this be an alignment issue with the Kinu (I really don't think it has a problem in this regard)? Does grind speed matter?

Some other observations I've made:

• The grounds from the Kenia are way fluffier and even more voluminous than the grounds from the Kinu.
• The grounds from the Kenia feel way finer than those of the Kinu.

I think I'll do some more testing tomorrow. First thing that comes to my mind is a shot with grounds from the Kinu with a grind setting, that gets me to a 18s easy-pull shot. Any more ideas?

Grind too fine and the "washing" action of flow rate decreases - the diffusion action seems to take quite a bit longer and may contribute to the "double hump" of acceptable tasting extractions.
Other aspects as to why... Well maybe it is roasted for flat burrs? They produce different particle size distributions... With a broader range conicals make a packed bed with good resistance to flow (different sized particles to fill different sized gaps) but the wider range of surface area leaves an extraction quality that might not work with this particular roast.

Interesting. I didn't know there were roasts specifically for flat burrs. Now I'm even more curious, how do roasts for flat burrs and roasts for conical burrs differ?

My usage there is in regards to what the roaster themselves are using for evaluating and modifying their roast - I don't have that specific knowledge I'm afraid!
I find that when I use either the grinder type and/or pressure profile I know the roaster is calibrating from, the flavour is much closer to provided notes and usually much tastier.

This makes sense. The guy roasting the coffee is using a flat burr grinder (a Mythos) to prepare shots. This might explain why I was finally able to taste the advertised flavors in my Kenia shots.

I did run some more tests with another coffee from another roaster.

Though I was easily able to reach the (almost) same extraction yield (19,85%) with both grinders and this new coffee I could see, to a lesser extent, a similar phenomenom to the my first tests. Pull times/pressures had to be significantly longer/higher when the coffee was ground using the conical grinder.

Also the shots from the conical were on the edge to overextraction taste wise with the stated EY while the shots from the flat were just fine at that extraction. I had to go as high as 22.7% with the flat burr grinder to get similar unpleasant tasting notes as with the conical.

I also found the following quote from another_jim on the Titan Grinder Project thread:

another_jim wrote:If we can get enough particle sizer time, I'm going to try testing a very simple model:

1. The rate of flow is determined overwhelmingly by the fines -- the more fines, the slower the flow.
2. The extraction of coffee is determined by how fine the average coarse particles are.
3. The quality of grind is determined how narrow the dispersion of the coarse particles is -- the tighter the distribution, the better the taste.

The conclusion from 3 is obvious. The conclusion from 1 & 2 is more subtle. The finer one grinds, the greater the proportion of fines (more breaking of the particles). So a grinder that tends to produce more fines will have a coarser grind for the same shot speed when compared to one which produces less grinds. Therefore, a grinder that produces a higher p[roportion of fines needs to be dosed lower to get the same solids extraction and taste balance.

According to this an explanation for my findings would be that the Kinu produces more fines, thus the slower flow rate, and generally a more uneven grind, which results in the lower extraction required for a good taste. While the Kenia produces a tighter particle size distribution with less fines resulting in a fast flow and high extractions with a good taste still.

Makes sense to me. But I bet it is not that simple?

The "more uneven" is what I was referring to earlier - a broad range in particle sizes.
There's been some discussion of particle shape being different, too - but this is harder to assess on a wide scale.

aecletec wrote:[...] They [flat burrs] produce different particle size distributions... With a broader range conicals make a packed bed with good resistance to flow (different sized particles to fill different sized gaps) but the wider range of surface area leaves an extraction quality that might not work with this particular roast.

Well, somehow I totally missed that part of your initial answer

It's no worries; great you have been figuring stuff out for yourself! It's a tough topic!
Hit up Socratic Coffee on instagram if you want to see a lot more of this kinda testing, they seem to be putting out data/results of something or another every week or so!

Also have you considered any of this... how broken in is the Kinu? Burr seasoning appears to have an effect on ey when new.

Also comparing taste from a kenia to a 47 conical might not be fair. It could be similar to comparing a precisio or sette to a K30. I can only imagine as ey % goes up on these grinders the K30 bests the other two easily.

Lastly crema is bitter. If the conica makes more crema that may also play a role.