After each time you “pull a shot”, the residual pressure inside the portafilter is released by a solenoid valve, which allows the waste water to flow to the drip tray and prevents you being covered in hot coffee and grounds when you unlock the portafilter.
Over time, coffee residue builds up in the group filter screen, group head, and the solenoid valve body. This does become rancid fairly quickly, and leads to off-flavours in your shots which are definitely noticeable.
The way to clear this residue is to use a cleaning powder such at Puly Caff, which you place in a special portafilter basket without holes (called a blind basket).
If you place about one teaspoon of the cleaning powder in the blind basket, and pull a “fake shot”, the hot water flows on to the cleaner which dissolves and foams up pushing cleaning foam into the screen, group and passages of the solenoid. Because the blind filter has no holes, there is nowhere for the water to go, until you stop pulling the shot when the dirty water is released into the drip tray. This process is called backflushing.
We make about 10 coffees a day with the Gaggia TS, and backflush the machine once a week. Here are some photos of the process.
First we replace our standard portafilter basket with the blind basket, and load it with about one teaspoon of the cleaner.
Next switch on the coffee switch for about 10 seconds. Hot water will enter the group and dissolve the cleaning powder which then foams up. When the switch is turned off, the cleaning solution flows to the drip tray.
Repeat the process turning the switch on for 10 seconds, then off for 10 seconds, until the water flowing to the drip tray is substantially clean.
Finally, turn the switch on and loosen the portafilter slightly allowing the cleaning water to overflow the portafilter and rinse any remaining coffee grounds into an old cloth.
Beautiful coffee flavours result !