Another word for backflushing could be "polishing the ruby" (much more poetic)

Today our Espresso Wizard (Dennis Skitzki) conducted an informal seminar on the basics of espresso machine mechanics. We disrobed an Astoria single group machine (recently de-throned from its perch at the Yours Truly Chagrin Falls) and had a look at its innards.

We traced the path that water in the coffee brewing circuit takes as it goes through an espresso machine. This water is separate from the water that enters the boiler and is eventually used for steaming milk. The water in the coffee brewing circuit never enters the boiler. Instead, it goes through a heat exchanger, which is a pipe that runs through the boiler.

1. Water for coffee brewing first goes through the flow meter, which measures 4 cc's of water for each rotation. This flow meter is only present on machines that have the portion control feature (we call these machines "automatics").
2. Then the water proceeds through the heat exchanger, which is located in the boiler. This is how the water gets up to temperature.
3. Then the water enters a copper line that goes from the heat exchanger to the brew head.
4. Then I think the water gets pressurized somehow. This particular machine had an internal pump that is used to pressurize.
5. Inside the brew head, the water passes through a screen and a giggler that actually slows the water down before the water hits the screen and shower.
6. The flow of water in the brew head is controlled by a three way valve (the "brew valve") that turns the flow of pressurized water on and off.
7. The water is disbursed by the shower and finally passes through the screen before it comes in contact with the coffee in the portafilter.

We spent quite a bit of time on the features and characteristics of the three way valve, or brew valve. We learned that this is the valve that discharges the pressure that is generated in the portafilter during the brewing process. It has an armature in it that is made of synthetic ruby. This ruby is actually grown from pure oxides of aluminum in a "furnace" that reaches approximately 2000 degrees Centigrade. The reason a ruby is used is so that the coffee oils and/or other substances in the water do not stick to the armature and therefore impede the smooth operation of the valve. Apparently synthetic ruby is a very inert, hard, strong substance.

More information on synthetic rubies from the US Geological Survey

We learned that when we backflush an espresso machine, using espresso machine cleaning soap, we are actually cleaning this 3 way valve, and the ruby armature inside. So another word for backflush might be "polishing the ruby". That's much more poetic and noble. Backflushing (or polishing the ruby) removes any coffee oils or residues from this 3 way valve. The armature inside has very close tolerances and it won't seal positively if it has too much coffee oil or residue on it. Dennis recommends backflushing without soap in the blind portafilter throughout the course of the day, and certainly after a busy period of espresso machine usage. Then, backflush with soap once a day, and be sure to brew a single espresso and discard after use. This reseasons the shower, screen, and the 3 way valve. If you don't reseason, your espresso will tend to have a metallic, stripped down taste.

A few other tidbits...

• Water in the boiler is pressurized by the heating element, not the pump.
• Lighter roast coffee (like our blonde espresso) will actually expand more when it is brewed, as compared to darker roast coffee. Lighter roast coffee retains more cellular structure.
• Some machines come with an automatic backflush program, but they don't actually seem to help that much, because people still don't follow the directions.
• Espresso machines should indeed be left on overnight. This is because of the following reasons:

1. Espresso machines that are left on overnight have been shown to use 30% less power than machines that are turned off every night.
2. Turning off your machine stresses the heating element unnecessarily, since the most stress is incurred when you turn the machine back on.
3. Espresso machines are made of dissimilar metals, so the temperature flux is inherently bad for the machine.

Astra espresso machines and some other brands keep the group head warm and keep the coffee in the water circuit at a constant temperature by utilizing thermo siphon or thermo circulation concept. There is a closed circuit of water between the heat exchanger in the boiler and the brew head. As water in the brew head cools, it sinks, and recycles to the heat exchanger to reheat. As it warms, it rises, and circulates back to the brew head.

La Marzocco espresso machines, considered by many to be a top of the line espresso machine, are still manufactured with double boilers. Dennis considers this old technology. There are two boilers, one for the water for brewing coffee, and one for the water for steam.

Our Espresso Wizard really outdid himself, with the endless fountain of facts and information. Thank you, Dennis!