Most things in coffee get a little easier the more you do them. Dosing consistently, understanding grind adjustment, understanding the taste of an espresso extraction, tamping simply and properly, the list goes on…
One thing that doesn’t get much easier is dealing with the build up of unpleasant flavours in an espresso machine. There are no shortcuts, you just have to deal with it regularly and to be honest it annoys me a lot. That taste is so obvious, so distracting, so unpleasant, that it ruins a lot of otherwise well made espresso for me.
I was thinking about the build up of dirt inside the group head caused by the release of pressure after you stop a shot. To better explain we have the following photo:
Photo courtesy of Dan Kehn of Home Barista
In a La Marzocco like this the route the water takes is a little confusing. It goes out of the far side of the neck of the group, through a flowmeter, back underneath the group head to a valve. When this valve is closed no water can leave the boiler, and when it opens water is pushed through and heads back inside the group head into that little tube you can see and towards the group head, until it drops down into the screw and showerscreen and over the coffee. 1 Other machines have a similar tube, of varying lengths, that run between the valve and dispersion mechanism above the coffee. (Not all LM’s have this long of a pathway outside the group – machines that don’t have flow meters and more modern models have eliminated this pretty much.)
Dirt builds up here because that last tube is a two way street. At the end of the shot the pressure shoots back up the tube until it gets to the stop valve. This prevents anything from getting back into your boiler, and this valve is a three way valve allow this pressure to exit through a drain tube, usually ending up in the drainbox under the drip tray – though some deposit straight into the drip tray in the E61 style.
The only machines that don’t have this problem are lever machines. Lever machines only release the pressure when the spring has finished expanding. This is why you can’t interrupt a lever machine shot without making a mess. (I believe the technical term is ‘portafilter sneeze’)
This tube is very hot. Any liquid in this tube will likely evaporate and leave behind whatever it had dissolved. At the end of the shot this can mean dissolved coffee gets dragged into this tube where it will quickly deposit and start to taste unpleasant. Some of that unpleasantness will be picked up when you next pull a shot and fresh water is heading down the tube towards coffee. The same unpleasantness builds up pretty quickly underneath the basket of a portafilter, and we know from sticking our noses in there how bad that would taste. 2
I was wondering if getting into the habit of flushing immediately after pulling a shot would dramatically reduce the build up of dirt in that particular area. As a flush builds up no pressure it would mean that water would only travel one way out of the group, and hopefully drag with it any coffee before it had the chance to dry out and deposit.
Some people would argue that you should leave the puck in for temperature stability. Even the WBC references this idea by no longer looking inside the portafilters at start up as you can leave pucks in as you wish. I don’t think temperature stability is a concern, but I do think dirt is. I’d rather lose a little temp at the start of the shot than have to deal with the residue of a puck sat in a machine for 10 minutes. I’m not sure I’ve ever seen any proof that having a puck in promotes thermal stability, and if it is out there then a link in the comments would be great!
Also – would using a rampdown in pump pressure tofinish the shot have a similar effect, as there would be little to no pressure to release back into the group?