In an ideal puck, water spreads uniformly, extracting similar amounts from most grounds. In reality, water is opportunistic: it accelerates through the easiest routes.
When a puck has weak spots (cracks, uneven density, poor distribution), water forms channels. This creates a paradox:
The cup can taste simultaneously sharp and harsh, and increasing fineness may not “fix” it—because you’re changing resistance globally while the water is still bypassing much of the puck.
Fines can migrate and clog parts of the puck, increasing local resistance and pushing water to other areas—sometimes increasing channeling risk. This is why two grinders at the “same” average grind size can behave differently: their particle-size distributions differ.
A grind change isn’t just a speed knob; it also changes the puck’s internal plumbing.
A shot runs fast and tastes both sour and slightly harsh. You grind finer, but the next shot is still fast and now tastes even more harsh. What’s the most likely explanation?
This is a common place to get tripped up: people assume ‘finer’ always means ‘slower and more extracted.’ If channeling is the main issue, water can still find fast paths, keeping the shot quick, while the areas along those paths get even more over-extracted—adding harshness—while the rest remains under-extracted, keeping sourness. The surface-area direction is wrong in the first option, pressure changes aren’t implied by the scenario, and higher temperature doesn’t automatically make extraction more uniform.