grinders that retain a lot of coffee are problematic for more than one reason
there is the obvious point, which is the wastage of the coffee that goes stale in the grind path as it is discarded
but it is more of an issue that that, as the coffee lodged in the internal pathways of a grinder doesnt push out at the same rate of flow across the entire cross section of the pathway, as you might hope or expect
you can observe this very easily for yourself; the centre of the cross section begins to move first, and then flows at a higher rate than the coffee toward the outer edges of the cross section of the pathway
this causes the amount of stale grounds retained by the grinder to vary, depending on the extent to which the cells in the coffee have been fractured during roasting to release the coffee’s oils (being a function of how dark the roast is) and also the level of humidity in the air, and what you might call ‘randomness’ (by which i mean sometimes a huge wad of stale ground coffee will be displaced from around the perimeter of the cross section, and other times hardly anything at all). this is a real nuisance – in some instances you can observe the grounds in the centre of the cross section push out early and quickly, yet the stale coffee around the edges of the cross section barely moves, even after many seconds of the grinder being in operation
what impact does this have on your espresso? it means that even after you have purged your grinder for several seconds it is quite possible that around the perimeter of the cross section you will still have a significant mass of stale coffee as it is being conveyed out by the coffee in the centre of the cross section at a much slower rate than the coffee in the centre of the cross section. this means a proportion of the grounds in your basket are going to be stale (unless your purge your grinder for half a day and waste extraordinary amounts of coffee in doing so)
at this point you may be thinking the same as i used to; so what, i can live with a small proportion of stale grounds in my puck, im ok with that, im not that much of a pedant that i demand every single one of the grounds in my puck to be freshly ground
but therein lies the problem. the presence of even a fairly low percentage of stale grounds, say 10% has a secondary effect that you may not have considered: it is responsible for setting up differential flow rates in your puck, what you might more commonly call channelling
the water is going to pass through these dry, stale grounds much more quickly than it is the fresh grounds that are still moist with the volatile coffee oils and present more resistance to the water trying to pass through it. as you most probably already know, water will always take the path of least resistance, and so it heads through the stale grounds, and in doing so lowers the extraction pressure on the puck (relative to what it would be if you had 100% freshly ground coffee in the puck)
i.e. as a result you not only send the water where the stale grounds are, but you also under-extract from the fresh coffee as a result of the water heading for the path of least resistance, so all of a sudden 10% stale is having a significant negative impact on the quality of your espresso
on our Compak K10 fresh we can demonstrate this effect very easily as you can watch consecutive shots get better and better extractions, even though we have already purged two 18g double shots before we started. for this reason we are replacing it with a Compak E10
the Compak E10 has a grind pathway which closely resembles that of the K10 Pro Barista, a dosered grinder. the E10 also ditches the worm gear adjustment mechanism which i find equally infuriating on Elektra Nino and the digital 0-100 grind size display is a chocolate teapot too. The E10 gives you the best of all worlds. It gives you doserless grinding, whilst replacing the well intended but ultimately infuriating worm gear and digital display of the Fresh with the time honour screw adjustment mechanism, and best of all a greatly reduce grind retention in the order of 8g (cf soemthing close to 50g in the Fresh). less of everything, and a better grinder as a result
we are very much looking forward to the arrival of our E10 next week
we will be adding a Compak E8 to our line up of demonstration grinders in the near future too