if you’ve ground too fine and made a lousy job of the distribution of the coffee grinds in the basket what you will experience is the portafilter dripping almost instantly in the pre-infusion phase, and then when you release the spring, the extraction phase taking way too long
when you are ‘doing it right’ the portafilter won’t drip for at least 10 seconds in the pre-infusion phase (longer with a light roast and/or a high dose), yet the extraction phase will still be in that 27-30s zone
what is distribution?
well even the very best grinders in the the world, regardless of cost, don’t grind the coffee to a uniform size and shape
as a result you need to ensure that the ‘mix’ of slightly different shaped and sized grinds is about the same throughout the puck
so give the coffee grinds a vigourous stir with a cocktail fork or anything similar that you have to hand
for example, you dont want all the smallest grinds in one part of the puck and all the largest grinds in another part of the puck
if this occurs the resistance presented to the water is not uniform over the surface of the puck and so the water heads for the spot where the resistance is least, and that is what is known as ‘channelling’. The bulk of the water flows through a single point in the puck at low pressure resulting in an under-extracted espresso, which will taste poor
bear in mind that channelling varies greatly from severe (water just gushes through a single point in the puck extremely quickly – obvious) to almost undetectable (less water flowing through some areas of the puck than others, but water still moving through all areas of the puck)
when you make an effort to get an even distribution of the grinds of different shapes & sizes throughout the puck you will be able to grind coarser, yet the puck will still hold back the water for a long time in the pre-infusion phase
the payoff is the extraction phase will then reduce to the 27-30s zone and you will experience whatever roast you are using at its best
rightly or wrongly i believe that the optimal extraction time is the same for all roasts (around 27s), but the pre-infusion phase needs to be varied (short for low doses &/or dark roasts – long for high doses &/or light roasts)
if your extraction phase is currently significantly longer than 30s you will be tasting a woodiness, much like sucking on a pencil as i remember from school days
improve your distribution so it doesn’t drip immediately, then you can grinder more coarsely and your extraction time will reduce and the unpleasant woodiness will disappear and your espresso will suddenly taste a lot cleaner. If the roast is meant to have fruit notes in it they will suddenly appear too as you get the extraction time down
so go on, make an effort to get the distribution right & taste the difference immediately