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A not-so-well known, but highly educated coffee roaster in Italy has once advised us to leave the spent grinds in the portafilter until the next extraction. The sole purpose was to stabilize the heat dissipation in the grouphead, especially an E61 grouphead connected to a heat exchanger (not double boiler machine). The reasoning is that the grouphead would overheat less. Although we have not tested this, this is a possibility. The second reason is sweating in the portafilter basket - water vapor would build up in the basket.
There are a few reasons against leaving the spent grounds in the portafilter handle:
a) the espresso puck can stick to the dispersion screen,
b) there would be an increase of rancid coffee oils passing through the dispersion screen, and
c) in a commercial environment, it would increase wait times for cleaning.
What are your thoughts?