If you really love the coffee at your favorite shop, chances are you have a great barista who knows how to keep an espresso machine clean. Unfortunately, not everyone keeps their machines as clean as they should. If you have a home espresso machine, you should remember to clean yours, too.
Coffee beans contain oils. Those oils are what make your espresso so rich and delicious. However, these oils also go rancid over time. When espresso runs through the machine, the oils cling to the water screen and any brass surfaces, and the oils also can leave a film on the machine's filter. Eventually, the film will even start plugging up the holes in the filter basket and create deposits. All this affects the taste of your brew.
Some people recommend cleaning the espresso machine every 20 to 30 shots. Of course, at a busy coffee shop, bustling baristas probably don't get the opportunity to do this. But do make sure that they clean their machines on a regular basis.