Every time you pull a shot of espresso, residual grounds and coffee oils get left in the brew path (the path the water takes from the boiler to the group head and the drain tray).
Without a regular cleaning schedule, those leftovers can build up, slowly going rancid, making your espresso taste off, or eventually, bad. Continued neglect can lead to coffee goo eventually blocking the brew path and becoming an expensive repair.
Backflushing is a term used for both at home and commercial espresso machines, and is the process of forcing water (or water and a detergent) through that brew path by blocking the portafilter. This forces any built up coffee grounds and leftover oils out to the drip tray, gets your machine sparkling clean and in top working order, and keeps your coffee tasting how it should.
Only machines that have a three-way solenoid valve (also called a three way valve) can be backflushed, so check your user manual to confirm you have one before trying this. Most entry-level espresso machines use a two-way valve, AKA a group valve, which doesn’t need to be backflushed.
The three way valve ensures water goes where you want it to go: from the boiler to the group head and through the portafilter. When you stop brewing, it relieves the remaining pressure in the group head, drying out your spent puck, and allows excess water to flow into the drain tray. This means you can immediately remove the portafilter handle and tap out your espresso puck without fear of it sneezing hot coffee grounds all over you.
WATER vs. DETERGENT
A water backflush can be done every day or two, as it helps prevent build up of oils and generalized crud in the first place. A backflush with detergent/cleaner can be done every two weeks- unless you make more than 5 coffees a day with your machine, in which case every week is ideal.
Every machine is different, so refer to your manual to make sure you follow the correct method for yours.
1) Place the blind basket in your portafilter and add .5 teaspoon cleaner (such as Cafiza or Purocaf)
2) Lock in portafilter
3) Turn on the machine as though you were pulling a shot and allow the pump to run for a 3 - 5 seconds, then turn it off. (Some manufacturers recommend doing this 10-15 times, some say only 4 - 5. Some machines have an auto-function! Check your manual).
4) Remove blind basket and replace with regular basket
5) Turn on machine again to rinse out the group head
6) Repeat backflushing process with just water to fully rinse any cleaner out
A couple of minutes spent backflushing every week can go a long way in maintaining the longevity of your espresso machine, and can ensure you're making the best tasting espresso at home!