The study rebone cited was in reference to what he apparently thought I was implying.
All things being equal, I see no way that extra tamping force could make a shot better tasting unless it lessens distribution defects hidden from view beneath the surface of the grinds.
What I empirically witnessed was the effect of additional tamping force overcoming grinds distribution defects in my portafilter basket.
Does not, tamping have a distributive effect on empty spaces within the grinds?
Here's an experiment...
Have a group of baristas do an 18 gram dose in their hidden-marked portafilter then do a blind swap between themselves, then each tamp and return the portafilter to its owner for brewing and see how the shot compares with each barista's previous shots.
If each of us subconsciously, over time, zero in on the pressure we use that is associated with good shots, the tamping force has been calibrated to what typically lies in our own personally prepared grinds in the portafilter, not someone else's.
I am starting to visualize aspects of tamping, tampers, and those who tamp... with putting, putters, and those who putt. I LOVE the game of golf and its traditions. I played golf for many years and was part of that culture but the home barista community is new and different.
Could be, I unknowingly stepped on sacred ground for ever posting this post.
That's consistent with other reports I've read over the years. Michael Teahan reported at the SCAA conference that he found no difference between a 30 and 300 pound tamp (my searches indicate it was part of his presentation "Fundamental Principles of Espresso Machine Design", but I have not found a copy; anyone know if it's online?).
To demonstrate that's the cause, prepare two identical baskets, one tamped at 30 pounds and the other at 32 pounds. Mark one of the baskets on the underside for later identification. Place the baskets on a lazy susan and spin it several times without looking. Now pull two espressos and pick your favorite. Repeat at least 3 more times. If you pick the one tamped at 32 pounds every time, you've demonstrated the cause. If you don't get it right every time, see A note on comparison tests
for how many trials with correct identification are required to demonstrate statistical significance.