I also claimed that the taste of espresso systematically changed with the degree of solids extraction. This claim stands.
Last year, using the Mazzer Mini and the Versalab M3 grinder, I found that higher doses in the same basket led to a very precise decline in solids yield. Andy replicated the result with the Mini. When I started the Titan Grinder Project tests, I again replicated the result with the Mini and the M3. However, this relationship was not apparent with the conical grinders. The conicals had roughly the same solids extraction at all doses.
When I did the "Beat the Robur" part of the test, I used 13.5 and 16.5 gram doses. In these tests there was no systematic difference in solids extraction with any of the twelve grinders; in fact, the solids extraction was always so close to the 22.5% stated in Illy, that the differences could have been due to measurement error. So the relation between dose to solids extraction I had posited in the paper was clearly wrong.
I busted my head trying to explain why I had got that relation in the earlier tests, and not in the "Beat the Robur" trials, but didn't see a good reason. Fortunately Andy Schecter did, and the explanation is obvious:
- Everybody noticed how well the conical grinders distributed the grinds and how evenly they poured. I never got a change in solids extraction with these grinders
- I got the reduced solids extraction at higher doses in the early tests of the flat burr grinders, when I used simple distribution and packing. For the "Beat the Robur" test, I used WDT along with shaking and several other leveling techniques, and I did not get changes in solids extraction.
- Therefore, the lowered solids extraction is due to sub-optimal packing when using higher doses on flat burr grinders.
This result may be even more significant to barista technique than the earlier one. I am not that great a barista, but I am competent. The naked pours I get with higher doses from a flat burr grinder are not picture perfect, but they do not look like they are channeling. However, they are not extracting properly, and are probably "pseudo-channeling" internally I'm coining this name for what happens in a puck with no cracks, but with denser and less dense zones of grinds. The denser pockets get less water flow because the water preferentially moves through the less dense patches. In a low dose puck, where one gets the soupy puck at the end, the grinds absorb so much water that these density differences even out. In a higher dose puck, where one gets a firm puck at the end, density differences probably remain till the end of the pour.
I have tried some of the lighter roasted coffees used in high doses at barista competitions. With flat burr grinders I couldn't get a decent shot at anything but a lowered dose. Presumably this means that competing baristas can properly distribute at higher doses than I can, without using time consuming grind sifting and fluffing techniques. This is hardly a surprise. But it probably does indicate that almost all home baristas, even those getting very pretty bottomless pours, should either be using something like WDT or a commercial conical when using high doses of coffee which produce a dry, firm puck after the shot.
(crossposted to HB and Coffeed, the sites where I made the original claims)