Why Group Heads Get Dirty: Coffee Oils and Fine Grounds

There's a brown, oily film on the shower screen. Maybe grounds are stuck around the rubber ring, or the shots run slower than they used to. Here's what's behind it, in the order you'd check.

What the brown buildup is

It's mostly coffee itself. One guide says backflushing is about clearing out the coffee oils and grounds that accumulate with every shot¹. Another says coffee oils build up behind the shower screen over time on machines where no detergent flows behind it².

So the film isn't a fault in your machine. It's what's left when hot water pushes through ground coffee shot after shot, and a little oil and a few fine grounds stay behind.

Where it collects

Check three spots, in order.

1. The shower screen. A maker's manual says to wipe the group head interior and the screen with a damp cloth to remove ground coffee particles³.
2. The gasket. One guide notes that grounds collect on the rubber edge². The same guide says that can keep the portafilter from sealing well².
3. The area where the portafilter locks in. One maker says this area is exposed to a lot of coffee, and that leftovers there can degrade performance as time goes on⁴.

Behind the screen is a place you can't see without taking parts off. Whether your maker lets you do that is a question for your manual.

Signs it's building up

One guide lists these as signs a clean is overdue¹:

• A visible dark oily buildup around the group head or on the shower screen.
• Shots running slower or more unevenly than usual.

A maker's manual also says residue can detrimentally affect the taste of the coffee⁴. A change in taste alone doesn't tell you the group head is the cause, though, so look at the screen and rim first.

What makes it worse

One manual says to rinse the filter basket and portafilter under hot water straight after use to remove all residual coffee oils³. It also says to run hot water through the machine now and then with the basket and portafilter in place and no ground coffee, to rinse out residual coffee³.

Another maker says that when the machine is being turned off, the portafilter should be stored elsewhere so that the moisture can evaporate from the group head⁴.

What to do about it

Match the fix to what you see:

• Grounds on the screen or rim: wipe with a damp cloth, with the machine off and cool.
• A film you can see but can't wipe off: check your manual for the cleaning method and product the maker names.
• Slow or uneven shots: clean first, then see whether it changes. If it doesn't, the cause may be something other than dirt, and the maker or a qualified technician is the next call.

The next question is how, and whether your machine can be backflushed. What Is Backflushing explains the word and the machines it applies to. The maker's own words on how often to clean are on How Often to Clean a Group Head.