Kaffee Bitte wrote:The harder the tamp and the firmer the puck the more likely that air being pulled up into the puck on an upstroke(downstroke for springs) will force its way through in the form of a crack in the puck. This crack WILL become a channel when you start the second pull. My experience with light tamps have been that I rarely if ever see the evidence of this in the puck after the shot is pulled. So do I have evidence, yes.
Makes sense. I haven't seen cracked pucks, but I quit doing multiple pulls fairly early on, when it became clear there was little to gain and potentially something to lose. La Pavoni is perfectly capable of achieving a normale double brew ratio with a single pull.
I'll question this conventional wisdom one last time, and then shut up. As I understand it, tamp pressure drops off drastically as depth increases in the puck. So the light vs. hard tamp really only impacts the top of the puck. After the grounds are infused with water from the first pull, they swell. At that point, how much difference does it make, whether the tamp was hard or light? It's possible that tamp pressure affects the first pull more than subsequent pulls.
Regardless, I repeated the previous experiment with a light tamp. This time I weighed the dose and each pull.
14g dose in double basket
Light tamp (weight of tamper), around 6mm below rim.
First pull, with preinfusion: yum! Brew ratio is 60%, straddling the normale-ristretto interface.
Second pull: yuck! Bitter dishwater.
The slightly greater volume in the second pull is probably due to fully infused grounds plus a few extra drops between pulls.
Tastewise, any subtle differences between hard and light tamps were subsumed by the gross differences between the first and second pulls. My conclusion: for La Pavoni, overextraction is a far bigger factor than tamp pressure when it comes to multiple pulls. I do not recommend multiple pulls on this machine.